{"id":4971,"date":"2019-06-24T16:39:56","date_gmt":"2019-06-24T16:39:56","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/13-8-case-study-conclusion-a-pain-in-the-foot-3\/"},"modified":"2023-11-30T18:58:19","modified_gmt":"2023-11-30T18:58:19","slug":"13-8-case-study-conclusion-a-pain-in-the-foot-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/13-8-case-study-conclusion-a-pain-in-the-foot-3\/","title":{"raw":"11.8\u00a0Case Study Conclusion: A Pain in the Foot","rendered":"11.8\u00a0Case Study Conclusion: A Pain in the Foot"},"content":{"raw":"&nbsp;\r\n<div>\r\n\r\n[caption id=\"attachment_4138\" align=\"aligncenter\" width=\"400\"]<img class=\"wp-image-4138\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Running-Shoes-by-bruno-nascimento-PHIgYUGQPvU-unsplash-scaled-3.jpg\" alt=\"11.8.1 Running Shoes\" width=\"400\" height=\"267\" \/> <em>Figure 11.8.1 Wear supportive shoes \u2014 your feet will thank you!<\/em>[\/caption]\r\n<h1>Case Study Conclusion: A Pain in the Foot<\/h1>\r\n<\/div>\r\nAs Sophia discovered in the beginning of the chapter, wearing high heels can result in a condition called metatarsalgia. Metatarsalgia is named for the metatarsal bones, which are the five bones that run through the ball of the foot\u00a0 just behind the toes (highlighted in Figures 11.8.2 and 11.8.3). Wearing high heels causes excessive pressure on the ball of the foot, as described in the beginning of this chapter. Additionally, the toes are forced to pull upward in high heels, which moves the fleshy padding away from the ball of the foot and adds to the overall pressure placed on this region. Over time, this can cause inflammation and direct stress on the bones, resulting in the pain in the ball of the foot known as metatarsalgia. The pain occurs especially in weight-bearing positions, such as standing, walking, or running \u2014 which is what Sophia was experiencing. There may also be pain, numbness, or tingling in the toes associated with metatarsalgia.\r\n<table class=\"no-lines\" style=\"border-collapse: collapse; width: 100%; height: 79px;\" border=\"0\">\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 50%;\">\r\n\r\n[caption id=\"attachment_4140\" align=\"aligncenter\" width=\"367\"]<img class=\"wp-image-4140\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Metatarsalgia-by-Esther-Max-on-flickr-2.jpg\" alt=\"11.7.2 Metatarsalgia pain\" width=\"367\" height=\"203\" \/> <em>Figure 11.8.2 Metatarsalgia is a painful and even debilitating condition characterized by pain in the ball of your foot with worsens when you stand, run, walk or flex your foot.<\/em>[\/caption]<\/td>\r\n<td style=\"width: 50%;\">\r\n\r\n[caption id=\"attachment_4141\" align=\"aligncenter\" width=\"500\"]<img class=\"wp-image-4141 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gray290_-_Mratatarsus-1-2.png\" alt=\"11.7.3 Metatarsals\" width=\"500\" height=\"189\" \/> <em>Figure 11.8.3 Illustration of the bones of the foot, with the metatarsal bones highlighted in pink.<\/em>[\/caption]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nWearing high heels can also cause stress fractures in the feet, which are tiny breaks in bone that occur due to repeated mechanical stress. This is caused by the excessive pressure that high heels put on some of the bones of the feet. These fractures are somewhat similar to what occurs in [pb_glossary id=\"4122\"]osteoporosis [\/pb_glossary]when the bone mass decreases to the point where bones can fracture easily as a person goes about their daily activities. In both cases, a major noticeable injury is not necessary to create the tiny fractures. As you have learned, tiny fractures that accrue over time are the cause of dowager\u2019s hump (or [pb_glossary id=\"4143\"]kyphosis[\/pb_glossary]), which is often seen in women with osteoporosis.\r\n\r\nDon\u2019t think you are immune to stress fractures just because you don\u2019t wear high heels! This injury also commonly occurs in people who participate in sports involving repetitive striking of the foot on the ground, such as running, tennis, basketball, or gymnastics. They may be avoided by taking preventative measures. You should ramp up any increase in activity slowly, cross-train by engaging in a variety of different sports or activities, rest if you experience pain, and wear well-cushioned and supportive running shoes.\u00a0 It is important to know that your cardiovascular and muscular systems adapt to an increase in physical activity much more quickly than the skeletal system.\r\n\r\n[caption id=\"attachment_4144\" align=\"alignright\" width=\"411\"]<img class=\"wp-image-4144\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Heels-by-gavin-allanwood-ndpX28miBtE-unsplash-scaled-3.jpg\" alt=\"11.8.4 Heels\" width=\"411\" height=\"411\" \/> <em>Figure 11.8.4 High heels with a narrow, pointed toe box and thin stiletto heels<\/em>[\/caption]\r\n\r\nSophia learned through her online research that wearing high heels can also lead to foot deformities, such as bunions and <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839\">hammertoes<\/a>. As you learned in an earlier chapter, a bunion is a protrusion on the side of the foot, most often at the base of the big toe. It can be caused by wearing shoes with a narrow, pointed toe box \u2014 a common shape for high heels (see Figure 11.8.4). The pressure of the shoes on the side of the foot causes an enlargement of bone or inflammation of other tissues in the region, which pushes the big toe toward the other toes.\r\n\r\n<span style=\"font-size: 1em; text-align: initial;\">Hammertoes are an abnormal bend in the middle joint of the second, third, or fourth toe (with the big toe being the first toe), causing the toe to be shaped similarly to a hammer. The narrow, pointed toe box of many high heels, combined with the way the toes are squished into the front of the shoe as a result of the height of the heel, can cause the toes to become deformed this way. Treatments for bunions and hammertoe include wearing shoes with a roomy toe box, padding or taping the toes, and toe exercises and stretches. If the bunion or hammertoe does not respond to these treatments, surgery may be necessary to correct the deformity.<\/span>\r\n\r\nBecause the bones of the skeleton are connected and work together with other systems to support the body, wearing high heels can also cause physical problems in areas other than the feet. Wearing high heels shifts a person\u2019s posture and alignment, and can put strain on tendons, muscles, and other joints in the body. Research published in 2014 from a team at Stanford University suggests that wearing high heels, particularly if the person is overweight or the heels are very high, may increase the risk of osteoarthritis (OA) in the knee, due to added stress on the knee joint as the person walks. As you have learned, OA results from the breakdown of cartilage and bone at the joint. Because it can only be treated to minimize symptoms \u2014 and not for a cure \u2014 OA could be an unfortunate long-term consequence of wearing high heels.\r\n\r\nSophia has decided that wearing high heels regularly is not worth the pain and potential long-term damage to her body. After consulting with her doctor, who confirmed she had metatarsalgia, she was able to successfully treat it with ice, rest, and wearing comfortable, supportive shoes instead of heels.\r\n\r\nHigh heels are not the only kind of shoes that can cause problems. Flip-flops, worn-out sneakers, and shoes that are too tight can all cause foot issues. To prevent future problems from her shoe choices, Sophia is following guidelines recommended by medical experts. The guidelines include:\r\n<ul>\r\n \t<li>Wearing shoes that fit well, have plenty of room in the toes, are supportive, and are comfortable right away. There should be no \u201cbreak-in\u201d period needed for shoes.<\/li>\r\n \t<li>Avoiding high heels, especially those with heels over two inches high, or those that have narrow, pointed toe boxes or very thin heels. The heels pictured in Figure 11.8.4 are an example of a type of shoe that should be avoided.<\/li>\r\n \t<li>If high heels\u00a0<em>must<\/em>\u00a0be worn, it\u00a0should only be for a limited period of time.<\/li>\r\n<\/ul>\r\nAs you have learned in this chapter, your skeletal system carries out a variety of important functions in your body, including physical support. But even though it is strong, your skeletal system can become damaged and deformed \u2014 even through such a seemingly innocuous act as wearing a certain type of shoe. Taking good care of your skeletal system is necessary to help it continue to take good care of the rest of you.\r\n<div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">Chapter 11 Summary<\/span><\/h1>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nIn this chapter, you learned about the skeletal system. Specifically, you learned that:\r\n<ul>\r\n \t<li>The [pb_glossary id=\"2906\"]skeletal system[\/pb_glossary] is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.<\/li>\r\n \t<li>[pb_glossary id=\"5913\"]Bones[\/pb_glossary] are organs made of supportive connective tissues, mainly the tough protein [pb_glossary id=\"3970\"]collagen[\/pb_glossary]. Bones also contain blood vessels, nerves, and other tissues. Bones are hard and rigid, due to deposits of calcium and other mineral salts within their living tissues. Besides bones, the skeletal system includes cartilage and ligaments.<\/li>\r\n \t<li>The skeletal system has many different functions, including supporting the body and giving it shape, protecting internal organs, providing attachment surfaces for skeletal muscles, allowing body movements, producing blood cells, storing minerals, helping to maintain mineral [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary], and producing endocrine hormones.<\/li>\r\n \t<li>There is relatively little sexual dimorphism in the human skeleton, although the female skeleton tends to be smaller and less robust than the male skeleton. The greatest sex difference is in the pelvis, which is adapted for childbirth in females.<\/li>\r\n \t<li>The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton.<\/li>\r\n \t<li>The [pb_glossary id=\"3953\"]axial skeleton[\/pb_glossary] consists of a total of 80 bones. It includes the skull, vertebral column, and rib cage. It also includes the three tiny ossicles in the middle ear and the hyoid bone in the throat.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>The [pb_glossary id=\"3957\"]skull[\/pb_glossary] provides a bony framework for the head. It consists of 22 different bones: eight in the [pb_glossary id=\"3975\"]cranium[\/pb_glossary], which encloses the brain, and 14 in the face, which includes the upper and lower jaw.<\/li>\r\n \t<li>The vertebral column is a flexible, S-shaped column of 33 [pb_glossary id=\"3964\"]vertebrae[\/pb_glossary] that connects the trunk with the skull and encloses the spinal cord. The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal regions. The S shape of the vertebral column allows it to absorb shocks and distribute the weight of the body.<\/li>\r\n \t<li>The [pb_glossary id=\"4002\"]rib cage[\/pb_glossary] holds and protects the organs of the upper part of the trunk, including the heart and lungs. It includes the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>The [pb_glossary id=\"3954\"]appendicular skeleton[\/pb_glossary] consists of a total of 126 bones. It includes the bones of the four limbs, shoulder girdle, and pelvic girdle. The girdles attach the appendages to the axial skeleton.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>Each upper limb consists of 30 bones. There is one bone (called the humerus) in the upper arm, and two bones (called the ulna and radius) in the lower arm. The wrist contains eight\u00a0carpal bones, the hand contains\u00a0five\u00a0metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.<\/li>\r\n \t<li>Each lower limb also consists of 30 bones. There is one bone (called the femur) in the upper leg, and two bones (called the tibia and fibula) in the lower leg. The patella covers the knee joint. The ankle contains\u00a0seven\u00a0tarsal bones, and the foot contains five\u00a0metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.<\/li>\r\n \t<li>The shoulder girdle attaches the upper limbs to the trunk of the body. It is connected to the axial skeleton only by muscles, allowing mobility of the upper limbs. Bones of the shoulder girdle include a right and left clavicle, and a right and left scapula.<\/li>\r\n \t<li>The pelvic girdle attaches the legs to the trunk of the body and supports the organs of the abdomen. It is connected to the axial skeleton by ligaments. The pelvic girdle consists of two halves that are fused together in adults. Each half consists of three bones: the ilium, pubis, and ischium.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>Bones are organs that consist mainly of bone (or osseous) tissue. Osseous tissue is a type of supportive connective tissue consisting of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard, without making it brittle.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>There are two types of osseous tissues: [pb_glossary id=\"4030\"]compact bone tissue[\/pb_glossary] and [pb_glossary id=\"4031\"]spongy bone tissue[\/pb_glossary]. Compact bone tissue is smooth and dense. It forms the outer layer of bones. Spongy bone tissue is porous and light, and it is found inside many bones.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>Besides osseous tissues, bones also contain nerves, blood vessels, [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary], and [pb_glossary id=\"4039\"]periosteum[\/pb_glossary].<\/li>\r\n \t<li>Bone tissue is composed of four different types of bone cells: [pb_glossary id=\"4040\"]osteoblasts[\/pb_glossary], [pb_glossary id=\"4041\"]osteocytes[\/pb_glossary], [pb_glossary id=\"4042\"]osteoclasts[\/pb_glossary], and [pb_glossary id=\"4043\"]osteogenic cells[\/pb_glossary]. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Bone is a very active tissue, constantly being remodeled by the work of osteoblasts and osteoclasts.<\/li>\r\n \t<li>There are six types of bones in the human body: [pb_glossary id=\"4049\"]long bones[\/pb_glossary]\u00a0(such as the limb bones), [pb_glossary id=\"4050\"]short bones[\/pb_glossary] (such as the wrist bones), [pb_glossary id=\"4053\"]sesamoid bones[\/pb_glossary] (such as the patella), [pb_glossary id=\"4054\"]sutural bones[\/pb_glossary] in the skull, and [pb_glossary id=\"4055\"]irregular bones[\/pb_glossary] (such as the vertebrae).<\/li>\r\n \t<li>Early in the development of a human fetus, the skeleton is made almost entirely of [pb_glossary id=\"3951\"]cartilage[\/pb_glossary]. The relatively soft cartilage gradually turns into hard bone \u2014 a process that is called [pb_glossary id=\"4077\"]ossification[\/pb_glossary]. It begins at a primary ossification center in the middle of bone, and later also occurs at secondary ossification centers in the ends of bone. The bone can no longer grow in length after the areas of ossification meet and fuse at the time of skeletal maturity.<\/li>\r\n \t<li>Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, osteoclasts resorb bone and osteoblasts make new bone to replace it. [pb_glossary id=\"4109\"]Bone remodeling[\/pb_glossary] shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral homeostasis in the blood.<\/li>\r\n \t<li>Bone repair is the natural process in which a bone repairs itself following a bone fracture. This process may take several weeks. In the process, the periosteum produces cells that develop into osteoblasts, and the osteoblasts form new bone matrix to heal the fracture. Bone repair may be affected by diet, age, pre-existing bone disease, or other factors.<\/li>\r\n \t<li>[pb_glossary id=\"3949\"]Joints[\/pb_glossary]\u00a0are locations at which bones of the skeleton connect with one another.<\/li>\r\n \t<li>Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.<\/li>\r\n \t<li>The structural classification of joints depends on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.<\/li>\r\n \t<li>The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: [pb_glossary id=\"4094\"]pivot[\/pb_glossary], [pb_glossary id=\"4096\"]hinge[\/pb_glossary], [pb_glossary id=\"4098\"]saddle[\/pb_glossary], [pb_glossary id=\"4100\"]plane[\/pb_glossary], [pb_glossary id=\"5483\"]condyloid[\/pb_glossary], and [pb_glossary id=\"4093\"]ball-and-socket[\/pb_glossary] joints.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>A number of disorders affect the skeletal system, including bone fractures and bone cancers. The two most common disorders of the skeletal system are osteoporosis and osteoarthritis.<\/li>\r\n \t<li>[pb_glossary id=\"4122\"]Osteoporosis[\/pb_glossary] is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation and bone resorption in bone remodeling. Osteoporosis may also occur as a side effect of other disorders or certain medications.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>Osteoporosis is diagnosed by measuring a patient\u2019s bone density and comparing it with the normal level of peak bone density. Fractures are the most dangerous aspect of osteoporosis. Osteoporosis is rarely fatal, but complications of fractures often are.<\/li>\r\n \t<li>Risk factors for osteoporosis include older age, female sex, European or Asian ancestry, family history of osteoporosis, short stature and small bones, smoking, alcohol consumption, lack of exercise, vitamin D deficiency, poor nutrition, and consumption of soft drinks.<\/li>\r\n \t<li>Osteoporosis is often treated with medications (such as bisphosphonates) that may slow or even reverse bone loss. Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior, such as undertaking weight-bearing exercise.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>[pb_glossary id=\"4102\"]Osteoarthritis[\/pb_glossary] (OA) is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage, coupled with low-grade inflammation of the joints.<\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li style=\"list-style-type: none;\">\r\n<ul>\r\n \t<li>Diagnosis of OA is typically made on the basis of signs and symptoms, such as joint deformities, pain, and stiffness. X-rays or other tests are sometimes used to either support the diagnosis or rule out other disorders. Age is the chief risk factor for OA. Other risk factors include joint injury, excess body weight, and a family history of OA.<\/li>\r\n \t<li>OA cannot be cured, but the symptoms can often be treated successfully. Treatments may include exercise, efforts to decrease stress on joints, pain medications, and surgery to replace affected hip or knee joints.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\nAs you have learned in this chapter, one of the important functions of the skeletal system is to allow movement of the body. But it doesn\u2019t do it alone. Movement is caused by the contraction of muscles, which pull on the bones, causing them to move. Read the next chapter to learn about this and other important functions of the muscular system.\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">Chapter 11 Review<\/span><\/h1>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<ol>\r\n \t<li style=\"list-style-type: none;\">\r\n<ol>\r\n \t<li>[h5p id=\"599\"]<\/li>\r\n \t<li>Why does the rib cage need to be flexible? Why can it\u00a0be flexible?<\/li>\r\n \t<li>In general, what do \u201cgirdles\u201d in the skeletal system do?<\/li>\r\n \t<li>Would swimming be more effective as an exercise for preventing osteoporosis or as a treatment for osteoarthritis? Explain your answer.<\/li>\r\n \t<li>Explain why some of the vertebrae become misshapen in the condition called dowager\u2019s hump (or kyphosis).<\/li>\r\n \t<li>Explain why osteoarthritis often involves inflammation in the joints.<\/li>\r\n \t<li>Osteoporosis can involve excess bone resorption, as well as insufficient production of new bone tissue. What are the two main bone cell types that carry out these processes, respectively?<\/li>\r\n \t<li>Describe two roles that calcium in bones play in the body.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<h2>Attributions<\/h2>\r\n<strong>Figure 11.8.1<\/strong>\r\n\r\n<a href=\"https:\/\/unsplash.com\/photos\/PHIgYUGQPvU\" rel=\"cc:attributionURL\">Running Shoes by bruno-nascimento-PHIgYUGQPvU<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@bruno_nascimento\">Bruno Nascimento<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\n<strong>Figure 11.8.2<\/strong>\r\n\r\n<a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/26430974194\">Metatarsalgia\/ Best Shoes for Metatarsalgia<\/a> by <a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/\" rel=\"dc:creator\">Esther Max<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\n\r\n<\/div>\r\n<strong>Figure 11.8.3<\/strong>\r\n\r\n<a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Gray290_-_Mratatarsus.png\" rel=\"cc:attributionURL\">Gray290_-_Mratatarsus (1)<\/a> by\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Henry_Vandyke_Carter\">Henry Vandyke Carter<\/a> (1831-1897) (Revised by Warren H. Lewis, coloured by <a class=\"mw-userlink\" title=\"User:Was a bee\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Was_a_bee\">Was a bee<\/a>) on Wikimedia Commons is in the <a class=\"extiw\" title=\"en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain. (<a class=\"extiw\" title=\"en:Bartleby.com\" href=\"https:\/\/en.wikipedia.org\/wiki\/Bartleby.com\">Bartleby.com<\/a>:\u00a0<a class=\"external text\" href=\"http:\/\/www.bartleby.com\/107\/\" rel=\"nofollow\">Gray\u2019s Anatomy<\/a>,\u00a0<a class=\"external text\" href=\"https:\/\/www.bartleby.com\/107\/illus290.html\" rel=\"nofollow\">Plate 290<\/a>)\r\n\r\n<strong>Figure 11.8.4<\/strong>\r\n\r\n<a href=\"https:\/\/unsplash.com\/photos\/ndpX28miBtE\" rel=\"cc:attributionURL\">Heels by gavin-allanwood-ndpX28miBtE-unsplash<\/a> by Photo by <a href=\"https:\/\/unsplash.com\/@gavla\">Gavin Allanwood<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n<h2>References<\/h2>\r\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Hammertoe and mallet toe [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839<\/p>\r\n<p class=\"hanging-indent\">VanDyke Carter, H. (1858). Illustration plate 290. In H. Gray,\u00a0 <i>Anatomy of the Human Body.<\/i> Lea &amp; Febiger. Bartleby.com, 2000. www.bartleby.com\/107\/.<\/p>","rendered":"<p>&nbsp;<\/p>\n<div>\n<figure id=\"attachment_4138\" aria-describedby=\"caption-attachment-4138\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4138\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Running-Shoes-by-bruno-nascimento-PHIgYUGQPvU-unsplash-scaled-3.jpg\" alt=\"11.8.1 Running Shoes\" width=\"400\" height=\"267\" \/><figcaption id=\"caption-attachment-4138\" class=\"wp-caption-text\"><em>Figure 11.8.1 Wear supportive shoes \u2014 your feet will thank you!<\/em><\/figcaption><\/figure>\n<h1>Case Study Conclusion: A Pain in the Foot<\/h1>\n<\/div>\n<p>As Sophia discovered in the beginning of the chapter, wearing high heels can result in a condition called metatarsalgia. Metatarsalgia is named for the metatarsal bones, which are the five bones that run through the ball of the foot\u00a0 just behind the toes (highlighted in Figures 11.8.2 and 11.8.3). Wearing high heels causes excessive pressure on the ball of the foot, as described in the beginning of this chapter. Additionally, the toes are forced to pull upward in high heels, which moves the fleshy padding away from the ball of the foot and adds to the overall pressure placed on this region. Over time, this can cause inflammation and direct stress on the bones, resulting in the pain in the ball of the foot known as metatarsalgia. The pain occurs especially in weight-bearing positions, such as standing, walking, or running \u2014 which is what Sophia was experiencing. There may also be pain, numbness, or tingling in the toes associated with metatarsalgia.<\/p>\n<table class=\"no-lines\" style=\"border-collapse: collapse; width: 100%; height: 79px;\">\n<tbody>\n<tr>\n<td style=\"width: 50%;\">\n<figure id=\"attachment_4140\" aria-describedby=\"caption-attachment-4140\" style=\"width: 367px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4140\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Metatarsalgia-by-Esther-Max-on-flickr-2.jpg\" alt=\"11.7.2 Metatarsalgia pain\" width=\"367\" height=\"203\" \/><figcaption id=\"caption-attachment-4140\" class=\"wp-caption-text\"><em>Figure 11.8.2 Metatarsalgia is a painful and even debilitating condition characterized by pain in the ball of your foot with worsens when you stand, run, walk or flex your foot.<\/em><\/figcaption><\/figure>\n<\/td>\n<td style=\"width: 50%;\">\n<figure id=\"attachment_4141\" aria-describedby=\"caption-attachment-4141\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4141 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gray290_-_Mratatarsus-1-2.png\" alt=\"11.7.3 Metatarsals\" width=\"500\" height=\"189\" \/><figcaption id=\"caption-attachment-4141\" class=\"wp-caption-text\"><em>Figure 11.8.3 Illustration of the bones of the foot, with the metatarsal bones highlighted in pink.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Wearing high heels can also cause stress fractures in the feet, which are tiny breaks in bone that occur due to repeated mechanical stress. This is caused by the excessive pressure that high heels put on some of the bones of the feet. These fractures are somewhat similar to what occurs in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4122\">osteoporosis <\/a>when the bone mass decreases to the point where bones can fracture easily as a person goes about their daily activities. In both cases, a major noticeable injury is not necessary to create the tiny fractures. As you have learned, tiny fractures that accrue over time are the cause of dowager\u2019s hump (or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4143\">kyphosis<\/a>), which is often seen in women with osteoporosis.<\/p>\n<p>Don\u2019t think you are immune to stress fractures just because you don\u2019t wear high heels! This injury also commonly occurs in people who participate in sports involving repetitive striking of the foot on the ground, such as running, tennis, basketball, or gymnastics. They may be avoided by taking preventative measures. You should ramp up any increase in activity slowly, cross-train by engaging in a variety of different sports or activities, rest if you experience pain, and wear well-cushioned and supportive running shoes.\u00a0 It is important to know that your cardiovascular and muscular systems adapt to an increase in physical activity much more quickly than the skeletal system.<\/p>\n<figure id=\"attachment_4144\" aria-describedby=\"caption-attachment-4144\" style=\"width: 411px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4144\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Heels-by-gavin-allanwood-ndpX28miBtE-unsplash-scaled-3.jpg\" alt=\"11.8.4 Heels\" width=\"411\" height=\"411\" \/><figcaption id=\"caption-attachment-4144\" class=\"wp-caption-text\"><em>Figure 11.8.4 High heels with a narrow, pointed toe box and thin stiletto heels<\/em><\/figcaption><\/figure>\n<p>Sophia learned through her online research that wearing high heels can also lead to foot deformities, such as bunions and <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839\">hammertoes<\/a>. As you learned in an earlier chapter, a bunion is a protrusion on the side of the foot, most often at the base of the big toe. It can be caused by wearing shoes with a narrow, pointed toe box \u2014 a common shape for high heels (see Figure 11.8.4). The pressure of the shoes on the side of the foot causes an enlargement of bone or inflammation of other tissues in the region, which pushes the big toe toward the other toes.<\/p>\n<p><span style=\"font-size: 1em; text-align: initial;\">Hammertoes are an abnormal bend in the middle joint of the second, third, or fourth toe (with the big toe being the first toe), causing the toe to be shaped similarly to a hammer. The narrow, pointed toe box of many high heels, combined with the way the toes are squished into the front of the shoe as a result of the height of the heel, can cause the toes to become deformed this way. Treatments for bunions and hammertoe include wearing shoes with a roomy toe box, padding or taping the toes, and toe exercises and stretches. If the bunion or hammertoe does not respond to these treatments, surgery may be necessary to correct the deformity.<\/span><\/p>\n<p>Because the bones of the skeleton are connected and work together with other systems to support the body, wearing high heels can also cause physical problems in areas other than the feet. Wearing high heels shifts a person\u2019s posture and alignment, and can put strain on tendons, muscles, and other joints in the body. Research published in 2014 from a team at Stanford University suggests that wearing high heels, particularly if the person is overweight or the heels are very high, may increase the risk of osteoarthritis (OA) in the knee, due to added stress on the knee joint as the person walks. As you have learned, OA results from the breakdown of cartilage and bone at the joint. Because it can only be treated to minimize symptoms \u2014 and not for a cure \u2014 OA could be an unfortunate long-term consequence of wearing high heels.<\/p>\n<p>Sophia has decided that wearing high heels regularly is not worth the pain and potential long-term damage to her body. After consulting with her doctor, who confirmed she had metatarsalgia, she was able to successfully treat it with ice, rest, and wearing comfortable, supportive shoes instead of heels.<\/p>\n<p>High heels are not the only kind of shoes that can cause problems. Flip-flops, worn-out sneakers, and shoes that are too tight can all cause foot issues. To prevent future problems from her shoe choices, Sophia is following guidelines recommended by medical experts. The guidelines include:<\/p>\n<ul>\n<li>Wearing shoes that fit well, have plenty of room in the toes, are supportive, and are comfortable right away. There should be no \u201cbreak-in\u201d period needed for shoes.<\/li>\n<li>Avoiding high heels, especially those with heels over two inches high, or those that have narrow, pointed toe boxes or very thin heels. The heels pictured in Figure 11.8.4 are an example of a type of shoe that should be avoided.<\/li>\n<li>If high heels\u00a0<em>must<\/em>\u00a0be worn, it\u00a0should only be for a limited period of time.<\/li>\n<\/ul>\n<p>As you have learned in this chapter, your skeletal system carries out a variety of important functions in your body, including physical support. But even though it is strong, your skeletal system can become damaged and deformed \u2014 even through such a seemingly innocuous act as wearing a certain type of shoe. Taking good care of your skeletal system is necessary to help it continue to take good care of the rest of you.<\/p>\n<div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">Chapter 11 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>In this chapter, you learned about the skeletal system. Specifically, you learned that:<\/p>\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2906\">skeletal system<\/a> is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5913\">Bones<\/a> are organs made of supportive connective tissues, mainly the tough protein <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3970\">collagen<\/a>. Bones also contain blood vessels, nerves, and other tissues. Bones are hard and rigid, due to deposits of calcium and other mineral salts within their living tissues. Besides bones, the skeletal system includes cartilage and ligaments.<\/li>\n<li>The skeletal system has many different functions, including supporting the body and giving it shape, protecting internal organs, providing attachment surfaces for skeletal muscles, allowing body movements, producing blood cells, storing minerals, helping to maintain mineral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5761\">homeostasis<\/a>, and producing endocrine hormones.<\/li>\n<li>There is relatively little sexual dimorphism in the human skeleton, although the female skeleton tends to be smaller and less robust than the male skeleton. The greatest sex difference is in the pelvis, which is adapted for childbirth in females.<\/li>\n<li>The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3953\">axial skeleton<\/a> consists of a total of 80 bones. It includes the skull, vertebral column, and rib cage. It also includes the three tiny ossicles in the middle ear and the hyoid bone in the throat.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3957\">skull<\/a> provides a bony framework for the head. It consists of 22 different bones: eight in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3975\">cranium<\/a>, which encloses the brain, and 14 in the face, which includes the upper and lower jaw.<\/li>\n<li>The vertebral column is a flexible, S-shaped column of 33 <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3964\">vertebrae<\/a> that connects the trunk with the skull and encloses the spinal cord. The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal regions. The S shape of the vertebral column allows it to absorb shocks and distribute the weight of the body.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4002\">rib cage<\/a> holds and protects the organs of the upper part of the trunk, including the heart and lungs. It includes the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs.<\/li>\n<\/ul>\n<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3954\">appendicular skeleton<\/a> consists of a total of 126 bones. It includes the bones of the four limbs, shoulder girdle, and pelvic girdle. The girdles attach the appendages to the axial skeleton.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>Each upper limb consists of 30 bones. There is one bone (called the humerus) in the upper arm, and two bones (called the ulna and radius) in the lower arm. The wrist contains eight\u00a0carpal bones, the hand contains\u00a0five\u00a0metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.<\/li>\n<li>Each lower limb also consists of 30 bones. There is one bone (called the femur) in the upper leg, and two bones (called the tibia and fibula) in the lower leg. The patella covers the knee joint. The ankle contains\u00a0seven\u00a0tarsal bones, and the foot contains five\u00a0metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.<\/li>\n<li>The shoulder girdle attaches the upper limbs to the trunk of the body. It is connected to the axial skeleton only by muscles, allowing mobility of the upper limbs. Bones of the shoulder girdle include a right and left clavicle, and a right and left scapula.<\/li>\n<li>The pelvic girdle attaches the legs to the trunk of the body and supports the organs of the abdomen. It is connected to the axial skeleton by ligaments. The pelvic girdle consists of two halves that are fused together in adults. Each half consists of three bones: the ilium, pubis, and ischium.<\/li>\n<\/ul>\n<\/li>\n<li>Bones are organs that consist mainly of bone (or osseous) tissue. Osseous tissue is a type of supportive connective tissue consisting of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard, without making it brittle.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>There are two types of osseous tissues: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4030\">compact bone tissue<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4031\">spongy bone tissue<\/a>. Compact bone tissue is smooth and dense. It forms the outer layer of bones. Spongy bone tissue is porous and light, and it is found inside many bones.<\/li>\n<\/ul>\n<\/li>\n<li>Besides osseous tissues, bones also contain nerves, blood vessels, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4039\">periosteum<\/a>.<\/li>\n<li>Bone tissue is composed of four different types of bone cells: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4043\">osteogenic cells<\/a>. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Bone is a very active tissue, constantly being remodeled by the work of osteoblasts and osteoclasts.<\/li>\n<li>There are six types of bones in the human body: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4055\">irregular bones<\/a> (such as the vertebrae).<\/li>\n<li>Early in the development of a human fetus, the skeleton is made almost entirely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3951\">cartilage<\/a>. The relatively soft cartilage gradually turns into hard bone \u2014 a process that is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4077\">ossification<\/a>. It begins at a primary ossification center in the middle of bone, and later also occurs at secondary ossification centers in the ends of bone. The bone can no longer grow in length after the areas of ossification meet and fuse at the time of skeletal maturity.<\/li>\n<li>Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, osteoclasts resorb bone and osteoblasts make new bone to replace it. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4109\">Bone remodeling<\/a> shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral homeostasis in the blood.<\/li>\n<li>Bone repair is the natural process in which a bone repairs itself following a bone fracture. This process may take several weeks. In the process, the periosteum produces cells that develop into osteoblasts, and the osteoblasts form new bone matrix to heal the fracture. Bone repair may be affected by diet, age, pre-existing bone disease, or other factors.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3949\">Joints<\/a>\u00a0are locations at which bones of the skeleton connect with one another.<\/li>\n<li>Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.<\/li>\n<li>The structural classification of joints depends on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.<\/li>\n<li>The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4094\">pivot<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4096\">hinge<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4098\">saddle<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4100\">plane<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5483\">condyloid<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4093\">ball-and-socket<\/a> joints.<\/li>\n<\/ul>\n<\/li>\n<li>A number of disorders affect the skeletal system, including bone fractures and bone cancers. The two most common disorders of the skeletal system are osteoporosis and osteoarthritis.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4122\">Osteoporosis<\/a> is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation and bone resorption in bone remodeling. Osteoporosis may also occur as a side effect of other disorders or certain medications.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>Osteoporosis is diagnosed by measuring a patient\u2019s bone density and comparing it with the normal level of peak bone density. Fractures are the most dangerous aspect of osteoporosis. Osteoporosis is rarely fatal, but complications of fractures often are.<\/li>\n<li>Risk factors for osteoporosis include older age, female sex, European or Asian ancestry, family history of osteoporosis, short stature and small bones, smoking, alcohol consumption, lack of exercise, vitamin D deficiency, poor nutrition, and consumption of soft drinks.<\/li>\n<li>Osteoporosis is often treated with medications (such as bisphosphonates) that may slow or even reverse bone loss. Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior, such as undertaking weight-bearing exercise.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4102\">Osteoarthritis<\/a> (OA) is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage, coupled with low-grade inflammation of the joints.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul>\n<li>Diagnosis of OA is typically made on the basis of signs and symptoms, such as joint deformities, pain, and stiffness. X-rays or other tests are sometimes used to either support the diagnosis or rule out other disorders. Age is the chief risk factor for OA. Other risk factors include joint injury, excess body weight, and a family history of OA.<\/li>\n<li>OA cannot be cured, but the symptoms can often be treated successfully. Treatments may include exercise, efforts to decrease stress on joints, pain medications, and surgery to replace affected hip or knee joints.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p>As you have learned in this chapter, one of the important functions of the skeletal system is to allow movement of the body. But it doesn\u2019t do it alone. Movement is caused by the contraction of muscles, which pull on the bones, causing them to move. Read the next chapter to learn about this and other important functions of the muscular system.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">Chapter 11 Review<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li style=\"list-style-type: none;\">\n<ol>\n<li>\n<div id=\"h5p-599\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-599\" class=\"h5p-iframe\" data-content-id=\"599\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 11 Review Quiz\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Why does the rib cage need to be flexible? Why can it\u00a0be flexible?<\/li>\n<li>In general, what do \u201cgirdles\u201d in the skeletal system do?<\/li>\n<li>Would swimming be more effective as an exercise for preventing osteoporosis or as a treatment for osteoarthritis? Explain your answer.<\/li>\n<li>Explain why some of the vertebrae become misshapen in the condition called dowager\u2019s hump (or kyphosis).<\/li>\n<li>Explain why osteoarthritis often involves inflammation in the joints.<\/li>\n<li>Osteoporosis can involve excess bone resorption, as well as insufficient production of new bone tissue. What are the two main bone cell types that carry out these processes, respectively?<\/li>\n<li>Describe two roles that calcium in bones play in the body.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 11.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/PHIgYUGQPvU\" rel=\"cc:attributionURL\">Running Shoes by bruno-nascimento-PHIgYUGQPvU<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@bruno_nascimento\">Bruno Nascimento<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 11.8.2<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/26430974194\">Metatarsalgia\/ Best Shoes for Metatarsalgia<\/a> by <a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/\" rel=\"dc:creator\">Esther Max<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/p>\n<\/div>\n<p><strong>Figure 11.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Gray290_-_Mratatarsus.png\" rel=\"cc:attributionURL\">Gray290_-_Mratatarsus (1)<\/a> by\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Henry_Vandyke_Carter\">Henry Vandyke Carter<\/a> (1831-1897) (Revised by Warren H. Lewis, coloured by <a class=\"mw-userlink\" title=\"User:Was a bee\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Was_a_bee\">Was a bee<\/a>) on Wikimedia Commons is in the <a class=\"extiw\" title=\"en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain. (<a class=\"extiw\" title=\"en:Bartleby.com\" href=\"https:\/\/en.wikipedia.org\/wiki\/Bartleby.com\">Bartleby.com<\/a>:\u00a0<a class=\"external text\" href=\"http:\/\/www.bartleby.com\/107\/\" rel=\"nofollow\">Gray\u2019s Anatomy<\/a>,\u00a0<a class=\"external text\" href=\"https:\/\/www.bartleby.com\/107\/illus290.html\" rel=\"nofollow\">Plate 290<\/a>)<\/p>\n<p><strong>Figure 11.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/ndpX28miBtE\" rel=\"cc:attributionURL\">Heels by gavin-allanwood-ndpX28miBtE-unsplash<\/a> by Photo by <a href=\"https:\/\/unsplash.com\/@gavla\">Gavin Allanwood<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Hammertoe and mallet toe [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839<\/p>\n<p class=\"hanging-indent\">VanDyke Carter, H. (1858). Illustration plate 290. In H. Gray,\u00a0 <i>Anatomy of the Human Body.<\/i> Lea &amp; Febiger. Bartleby.com, 2000. www.bartleby.com\/107\/.<\/p>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_4971_4122\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4122\"><div tabindex=\"-1\"><div>\n<figure id=\"attachment_1179\" aria-describedby=\"caption-attachment-1179\" style=\"width: 165px\" class=\"wp-caption alignright\"><img class=\" wp-image-1179\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Leaky-battery-1.jpg\" alt=\"Image shows the end of a battery which has leaked its acidic contents. The leak looks like a thick crust of a whitish substance.\" width=\"165\" height=\"220\"><figcaption id=\"caption-attachment-1179\" class=\"wp-caption-text\"><em>Figure 3.12.1. Batteries contain strong acids which should not come into contact with skin or eyes.<\/em><\/figcaption><\/figure>\n<p><span style=\"font-size: 1em;font-weight: normal\">Created by:\u00a0CK-12\/Adapted by Christine Miller<\/span><\/p>\n<h1>Danger!\u00a0 Acid!<\/h1>\n<\/div>\n<p>You probably know that\u00a0\u00a0batteries\u00a0contain dangerous chemicals,\u00a0including\u00a0strong <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1183\">acids<\/a>. Strong acids can hurt you if they come into contact with your skin or\u00a0eyes. Therefore, it may surprise you to learn that your life\u00a0<em>depends<\/em>\u00a0on acids. There are many acids inside your body, and some of them are as strong as battery\u00a0acid. Acids are needed for\u00a0digestion and some\u00a0forms of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3661\">energy<\/a>\u00a0production. Genes are made of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3383\">nucleic acids<\/a>,\u00a0proteins\u00a0of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3615\">amino acids<\/a>, and\u00a0lipids\u00a0of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3565\">fatty acids<\/a>.<\/p>\n<div>\n<h1>Water\u00a0and\u00a0Solutions<\/h1>\n<\/div>\n<p>Acids (such as battery acid) are\u00a0solutions. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3719\">solution<\/a><\/strong>\u00a0is a\u00a0mixture\u00a0of two or more substances that has the same\u00a0composition\u00a0throughout. Many solutions are a mixture of\u00a0water\u00a0and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.<\/p>\n<p>In pure\u00a0water\u00a0(such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An\u00a0ion\u00a0is an electrically charged\u00a0atom or molecule. The breakdown of water is represented by the chemical equation:<\/p>\n<p>2 H<sub>2<\/sub>O \u2192 <span style=\"color: #ff0000\">H<sub>3<\/sub>O<\/span><sup>+<\/sup>\u00a0+ <span style=\"color: #3366ff\">OH<\/span><sup>-<\/sup><\/p>\n<p>The products of this reaction are a hydronium\u00a0ion\u00a0(<span style=\"color: #ff0000\">H3O<sup>+<\/sup><\/span>) and a hydroxide\u00a0ion\u00a0(<span style=\"color: #3366ff\">OH<sup>-<\/sup><\/span>). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (<span style=\"color: #ff0000\">H<sup>+<\/sup><\/span>). The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3679\">hydronium ion<\/a>, which has a positive charge, forms when another water molecule accepts the hydrogen ion.<\/p>\n<div>\n<h1>Acidity and\u00a0pH<\/h1>\n<\/div>\n<p>The\u00a0concentration\u00a0of hydronium ions in a\u00a0solution\u00a0is known as\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3375\">acidity<\/a>.<\/strong>\u00a0In pure water, the\u00a0concentration\u00a0of hydronium ions is very low; only about one in ten million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3375\">Acidity<\/a> is measured on a scale called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3709\">pH<\/a><\/strong>, as shown in Figure 3.12.2. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.<\/p>\n<figure id=\"attachment_1196\" aria-describedby=\"caption-attachment-1196\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-1196\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/PH_Scale-1.png\" alt=\"Image shows a pH scale. 0-6.9 is acidic, 7 is neutral, and 7.1-14 is basic.\" width=\"600\" height=\"180\"><figcaption id=\"caption-attachment-1196\" class=\"wp-caption-text\"><em>Figure 3.12.2. The pH scale measures acidity. It ranges from 1-14.<\/em><\/figcaption><\/figure>\n<div>\n<p>This pH scale shows the acidity of many common substances. The lower the pH value, the more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3599\">acidic<\/a> a substance is.<\/p>\n<figure id=\"attachment_2303\" aria-describedby=\"caption-attachment-2303\" style=\"width: 472px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2303\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ph-scale-with-examples-1.jpg\" alt=\"Image of the pH scale and examples of substances for each of the numbers on the scale.\" width=\"472\" height=\"809\"><figcaption id=\"caption-attachment-2303\" class=\"wp-caption-text\"><em>Figure 3.12.3. Examples of solutions for various pH levels.<\/em><\/figcaption><\/figure>\n<\/div>\n<h2>Acids<\/h2>\n<p>If a\u00a0solution\u00a0has a higher\u00a0concentration\u00a0of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3679\">hydronium ion<\/a>s than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1183\">acid<\/a><\/strong>. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is.<\/p>\n<p>Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms.\u00a0Even stomach\u00a0acid\u00a0would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.<\/p>\n<h2>Bases<\/h2>\n<p>If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3623\">base<\/a><\/strong>. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the colour from clothing.<\/p>\n<div>\n<h1>Buffers<\/h1>\n<p>A buffer is a solution that can resist changes in pH.\u00a0 Buffers are able to maintain a certain pH by by absorbing any H+ or OH- ions added to the solution.\u00a0 Buffers are extremely important in biological systems in order to maintain a pH conducive to life.\u00a0 Bicarbonate is an example of a buffer which is used to maintain pH of the blood.\u00a0 In this buffering system, if blood becomes too acidic, carbonic acid will convert to carbon dioxide and water.\u00a0 If the blood becomes too basic, carbonic acid will convert to bicarbonate and H+ ions:<\/p>\n<p style=\"text-align: center\"><strong>\u00a0CO<sub>2<\/sub> + H<sub>2<\/sub>O \u2194 H<sub>2<\/sub>CO<sub>3<\/sub> \u2194 HCO<sub>3<\/sub><sup>-<\/sup> + H<sup>+<\/sup><\/strong><\/p>\n<h1>Acids, Bases, and\u00a0Enzymes<\/h1>\n<\/div>\n<p>Many acids and bases in living things provide the pH that\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3665\">enzymes<\/a>\u00a0need. Enzymes are biological catalysts that must work effectively for\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3581\">biochemical reactions<\/a>\u00a0to occur.\u00a0Most enzymes can do their job only at a certain level of acidity.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3573\">Cells<\/a>\u00a0secrete <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1183\">acids<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3623\">base<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0to maintain the proper pH for enzymes to do their work.<\/span><\/p>\n<p>Every time you digest food, acids and bases are at work in your\u00a0digestive system. Consider the\u00a0enzyme\u00a0pepsin, which helps break down\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">proteins\u00a0<\/a>in the stomach. Pepsin needs an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3599\">acidic<\/a> environment to do its job. The stomach secretes\u00a0a\u00a0strong <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1183\">acid<\/a> called hydrochloric acid that allows pepsin to work. When stomach contents enter the\u00a0small intestine, the acid must be neutralized, because enzymes in the small intestine need a basic environment in order to work. An organ called the\u00a0pancreas\u00a0secretes a\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3623\">base<\/a>\u00a0named bicarbonate into the small intestine, and this base neutralizes the acid.<\/p>\n<div>\n<h1>Feature: My\u00a0Human Body<\/h1>\n<\/div>\n<p>Do you ever have heartburn? The answer is probably \"yes.\" More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it's time to seek medical treatment.<\/p>\n<figure id=\"attachment_1355\" aria-describedby=\"caption-attachment-1355\" style=\"width: 303px\" class=\"wp-caption alignright\"><img class=\"wp-image-1355\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/GERD-1.png\" alt=\"Image shows two diagrams of the stomach and esophagus. In the first diagram, the esophageal sphincter is tightly closed, preventing contents of the stomach from re-entering the esophagus. In the second diagram, the esophageal sphincter is relaxed, open, and the stomach contents are able to re-enter the esophagus.\" width=\"303\" height=\"363\"><figcaption id=\"caption-attachment-1355\" class=\"wp-caption-text\"><em>Figure 3.12.4. Acid reflux results when the esophageal sphincter doesn't close completely.<\/em><\/figcaption><\/figure>\n<p>Heartburn doesn't have anything to do with the\u00a0heart, but it does cause a burning sensation in the vicinity of the chest.\u00a0Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting\u00a0proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from backflowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.<\/p>\n<p>GERD\u00a0\u2014 as well as occasional heartburn\u00a0\u2014\u00a0often can be improved by dietary and other lifestyle changes that decrease the amount and acidity of reflux from the stomach into the esophagus.<\/p>\n<ul>\n<li>Some foods and beverages seem to contribute to GERD, so these should be avoided.\u00a0Problematic foods include\u00a0chocolate, fatty foods, peppermint, coffee, and alcoholic beverages.<\/li>\n<li>Decreasing portion size and eating the last meal of the day at least a couple of hours before bedtime may reduce the risk of reflux occurring.<\/li>\n<li>Smoking tends to weaken the lower esophageal sphincter, so quitting the habit may help control reflux.<\/li>\n<li>GERD is often associated with being overweight. Losing\u00a0weight\u00a0often brings improvement.<\/li>\n<li>Some people are helped by sleeping with the head of the bed elevated. This allows gravity to help control the backflow of acids into the esophagus from the stomach.<\/li>\n<\/ul>\n<p>If you have frequent heartburn and lifestyle changes don't help, you may need medication to control the condition.\u00a0Over-the-counter (OTC) antacids may be all that you need to control the occasional heartburn attack. OTC medications are usually bases that neutralize stomach acids. They may also create bubbles\u00a0that help block stomach contents from entering the esophagus. For some people, OTC medications are not enough, and prescription medications are instead required for the control of\u00a0GERD. These prescription medications generally work by inhibiting acid secretion in the stomach.<\/p>\n<p>Be sure to see a doctor if you can't control your heartburn, or you have it often. Untreated GERD not only interferes with quality of life, it may also lead to more serious complications, ranging from esophageal bleeding to esophageal\u00a0cancer.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.12 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3719\">solution<\/a> is a\u00a0mixture\u00a0of two or more substances that has the same\u00a0composition\u00a0throughout. Many solutions consist of water and one or more dissolved substances.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3375\">Acidity<\/a> is a measure of the hydronium ion concentration in a solution.\u00a0Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3709\">pH scale<\/a>.<\/li>\n<li>Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.<\/li>\n<li>Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly. Enzymes are the biological catalysts (like pepsin) needed to digest\u00a0protein\u00a0in the stomach.\u00a0Pepsin\u00a0requires an acidic environment.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.12 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-170\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-170\" class=\"h5p-iframe\" data-content-id=\"170\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Acids and Bases\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is a solution?<\/li>\n<li>Define acidity.<\/li>\n<li>Explain how acidity is measured.<\/li>\n<li>Compare and contrast acids and bases.<\/li>\n<li>Hydrochloric acid is secreted by the stomach to provide an acidic environment for the\u00a0enzyme pepsin. What is the pH of this acid? How strong of an acid is it compared with other acids?<\/li>\n<li>Define an ion. Identify the ions in the equation below, and explain\u00a0what makes them ions:\n<ul>\n<li>2 H<sub>2<\/sub>O \u2192 H<sub>3<\/sub>O<sup>+<\/sup>\u00a0+ OH<sup>-<\/sup><\/li>\n<\/ul>\n<\/li>\n<li>Explain why the\u00a0pancreas\u00a0secretes bicarbonate into the\u00a0small intestine.<\/li>\n<li>Do you think pepsin would work in the\u00a0small intestine? Why or why not?<\/li>\n<li>You may have mixed vinegar and baking soda and noticed that they bubble and react with each other. Explain why this happens. Explain also what happens to the pH of this solution after you mix the vinegar and baking soda.<\/li>\n<li>Pregnancy\u00a0hormones can cause the lower esophageal sphincter to relax. What effect do you think this has on pregnant women? Explain your answer.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.12 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=rIvEvwViJGk&amp;feature=youtu.be<\/p>\n<p style=\"text-align: center\">pH and Buffers by Bozeman Science, 2014.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=DupXDD87oHc&amp;feature=youtu.be<\/p>\n<p class=\"title style-scope ytd-video-primary-info-renderer\" style=\"text-align: center\">The strengths and weaknesses of acids and bases - George Zaidan and Charles Morton, TED-Ed, 2013.<\/p>\n<\/div>\n<\/div>\n<div>\n<h2>Attributions<\/h2>\n<\/div>\n<p><strong>Figure 3.12.1<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/41002268@N03\/42971519835\" rel=\"cc:attributionURL\">Leaky battery<\/a>\u00a0by\u00a0<a href=\"https:\/\/www.flickr.com\/photos\/41002268@N03\/\" rel=\"dc:creator\">Carbon Arc<\/a> on <a href=\"https:\/\/www.flickr.com\/\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/\">CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license. \u200b<\/p>\n<p><strong>Figure 3.12.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:PH_Scale.png#filelinks\" rel=\"cc:attributionURL\">PH_Scale<\/a> by <a title=\"User:Christinelmiller\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Christinelmiller\">Christinelmiller<\/a> on Wikimedia Commons is used under a\u00a0 \u00a9 <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" rel=\"license\">CC0 1.0 <\/a>(https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) public domain dedication license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 3.12.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:2713_pH_Scale-01.jpg\" rel=\"cc:attributionURL\">Ph scale with examples<\/a> by <a href=\"http:\/\/cnx.org\/content\/col11496\/1.6\/\">OpenStax College<\/a>, on Wikimedia Commons, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 3.12.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:GERD.png\" rel=\"cc:attributionURL\">GERD<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Betts, J.G.,\u00a0 Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.,\u00a0 Womble, M., DeSaix, P. (2013, April 25). Figure 26.15 The pH Scale [digital image]. In <em>Anatomy and Physiology<\/em>. OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-4-acid-base-balance<\/p>\n<p class=\"hanging-indent\">Bozeman Science. (2014, February 22). pH and buffers. YouTube. https:\/\/www.youtube.com\/watch?v=rIvEvwViJGk&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2013, October 24). The strengths and weaknesses of acids and bases - George Zaidan and Charles Morton. YouTube. https:\/\/www.youtube.com\/watch?v=DupXDD87oHc&amp;feature=youtu.be<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4143\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4143\"><div tabindex=\"-1\"><p>Created by: CK-12\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_2399\" aria-describedby=\"caption-attachment-2399\" style=\"width: 997px\" class=\"wp-caption alignnone\"><img class=\"wp-image-2399\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Stem_cell_differentiation.svg_-1.png\" alt=\"Shows differentiation pathways a stem cell can take, based on gene regulation: Sex cell, muscle cell, fat cell, bone cell, blood cell, nervous cell, epithelial cell or immune cell. .\" width=\"997\" height=\"704\"><figcaption id=\"caption-attachment-2399\" class=\"wp-caption-text\"><em>Figure 5.9.1 Differentiation pathways for a stem cell based on gene regulation. <\/em><\/figcaption><\/figure>\n<div>\n<h1>Express Yourself<\/h1>\n<\/div>\n<p>This sketch illustrates some of the variability in human\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3573\">cells<\/a>. The shape and other\u00a0characteristics\u00a0that make each type of cell unique depend mainly on the\u00a0specific\u00a0proteins\u00a0that particular cell type makes. Proteins are encoded in genes. All the\u00a0cells\u00a0in an organism have the same genes, so they all have genetic instructions for the same proteins. Obviously, different types of cells must use (or express) different genes to make different proteins.<\/p>\n<div>\n<h1>What Is Gene Expression?<\/h1>\n<\/div>\n<p>Using a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3429\">gene<\/a> to make a\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">protein<\/a>\u00a0is called\u00a0<strong>gene expression<\/strong>. It includes the synthesis of the protein by the processes of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2241\">transcription<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a> into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2212\">mRNA<\/a>,\u00a0 and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2242\">translation<\/a> of mRNA into a protein. It may also include further processing of the protein after synthesis.<\/p>\n<p>Gene expression is regulated to ensure that the correct\u00a0proteins\u00a0are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the\u00a0transcription\u00a0phase of\u00a0protein synthesis\u00a0to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1573\">eukaryotic<\/a>\u00a0cells, and it is the focus of this concept.<\/p>\n<div>\n<h1>Regulation of Transcription<\/h1>\n<\/div>\n<figure id=\"attachment_2407\" aria-describedby=\"caption-attachment-2407\" style=\"width: 718px\" class=\"wp-caption alignright\"><img class=\"wp-image-2407\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Activators-and-Repressors-1.png\" alt=\"\" width=\"718\" height=\"455\"><figcaption id=\"caption-attachment-2407\" class=\"wp-caption-text\"><em>Figure 5.9.2 Regulation of Transcription. Regulatory proteins bind to their corresponding regulatory elements in order to control transcription.<\/em><\/figcaption><\/figure>\n<p>As shown in Figure 5.9.2, transcription is controlled by <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">regulatory proteins<\/a><\/strong>. These proteins bind to regions of\u00a0DNA, called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2403\">regulatory elements<\/a><\/strong>, which are located near promoters. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2404\">promoter<\/a> is the region of a gene where\u00a0RNA\u00a0polymerase binds to initiate transcription of the\u00a0DNA to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2212\">mRNA<\/a>. After regulatory proteins bind to regulatory elements, the proteins can interact with RNA polymerase. Regulatory proteins are typically either activators or repressors.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2405\">Activators<\/a><\/strong>\u00a0are regulatory proteins that promote transcription by enhancing the interaction of\u00a0RNA polymerase with the promoter.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2406\">Repressors<\/a><\/strong>\u00a0are regulatory proteins that prevent transcription by impeding the progress of\u00a0RNA\u00a0polymerase along the DNA strand, so the DNA cannot be transcribed to mRNA.<\/p>\n<p>&nbsp;<\/p>\n<div>\n<p><span style=\"font-size: 1.424em;font-weight: bold\">Enhancers<\/span><\/p>\n<\/div>\n<p>Although regulatory proteins and elements are typically the key players in the regulation of transcription, other factors may also be involved.\u00a0Regulation of transcription may also involve enhancers.\u00a0<strong>Enhancers<\/strong>\u00a0are distant regions of DNA that can loop back to interact with a gene's promoter. They can also\u00a0increase the likelihood that transcription of the gene will occur.<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2409\">Enhancers<\/a><\/p>\n<h2>The TATA Box<\/h2>\n<p>Different types of cells have unique patterns of regulatory elements that result in\u00a0<em>only<\/em>\u00a0the necessary genes being transcribed. That\u2019s why a\u00a0blood cell and nerve cell, for example, are so different from each other. Some regulatory elements, however, are common to virtually\u00a0<em>all<\/em>\u00a0genes, regardless of the cells in which they occur. An example is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2410\">TATA box<\/a><\/strong>, which\u00a0is a regulatory\u00a0element\u00a0that is part of the promoter of almost every eukaryotic gene. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter so transcription can begin.<\/p>\n<figure id=\"attachment_2450\" aria-describedby=\"caption-attachment-2450\" style=\"width: 886px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2450 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/TATA_box_description-1.png\" alt=\"Components of DNA regulating transcription: upstream enhancer, promoter sequences, TATA box: TATAWAW, Exons and Introns.\" width=\"886\" height=\"203\"><figcaption id=\"caption-attachment-2450\" class=\"wp-caption-text\"><em>Figure 5.9.3 Components of DNA Regulating Transcription. W in the TATA box sequence can be either A or T.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Regulation During\u00a0Development<\/h1>\n<\/div>\n<p>The\u00a0regulation of gene expression\u00a0is extremely important\u00a0in\u00a0an organism's early\u00a0development. Regulatory proteins must \"turn on\" certain genes in particular cells at just the right time, so the individual develops normal organs and organ systems. Homeobox genes are important genes that regulate development.<\/p>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2411\">Homeobox genes<\/a><\/strong>\u00a0are a large group of similar genes that direct the formation of many body structures during the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3891\">embryonic<\/a> stage. In humans, there are an estimated 235 functional homeobox genes. They are present on every chromosome and generally grouped in clusters. Homeobox genes contain instructions for making chains of 60\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3615\">amino acids<\/a>, called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2413\">homeodomains<\/a><\/strong>. Proteins containing homeodomains are\u00a0transcription factors\u00a0that bind to and control the activities of other genes. The homeodomain is the part of the protein that binds to the target gene and controls its expression.<\/p>\n<div>\n<h1>Gene Expression and\u00a0Cancer<\/h1>\n<\/div>\n<figure id=\"attachment_2416\" aria-describedby=\"caption-attachment-2416\" style=\"width: 315px\" class=\"wp-caption alignleft\"><img class=\"wp-image-2416 size-full\" style=\"font-size: 1em\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/pathways-to-cancer-1.png\" alt=\"This flow chart shows how a series of mutations in tumor-suppressor genes and proto-oncogenes leads to cancer.\" width=\"315\" height=\"500\"><figcaption id=\"caption-attachment-2416\" class=\"wp-caption-text\"><em>Figure 5.9.4 This flow chart shows how a series of mutations in tumor-suppressor genes and proto-oncogenes leads to cancer.<\/em><\/figcaption><\/figure>\n<p>Some types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancer <\/a>occur because of mutations in the genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes: proto-oncogenes and tumor-suppressor genes. Both are shown in Figure 5.9.4.<\/p>\n<ul>\n<li><strong>Proto-oncogenes <\/strong>are genes that normally help cells divide. When a proto-oncogene mutates to become an oncogene, it is continuously expressed, even when it is not supposed to be. This is like a car's accelerator pedal being stuck at full throttle. The car keeps racing at top\u00a0speed. A cell, in this case,\u00a0keeps dividing out of control, which can lead to cancer.<\/li>\n<li><strong>Tumor suppressor genes <\/strong>are genes that normally slow down or stop\u00a0cell division. When a\u00a0mutation\u00a0occurs in a tumor suppressor gene, it can no longer control cell division. This is like a car without brakes. The car can't be slowed or stopped.\u00a0A cell, in this case,\u00a0keeps dividing out of control, which can lead to cancer.<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">5.9 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Using a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3429\">gene<\/a> to make a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">protein<\/a> is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3939\">gene expression<\/a>. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any stage of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2210\">protein synthesis<\/a>\u00a0or processing.<\/li>\n<li>The regulation of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2241\">transcription<\/a> is controlled by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">regulatory proteins <\/a>that bind to regions of DNA called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2403\">regulatory elements<\/a>, which are usually located near <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2404\">promoters<\/a>. Most regulatory proteins are either <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2405\">activators<\/a> that promote transcription, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2406\">repressors<\/a> that impede transcription.<\/li>\n<li>A regulatory\u00a0element\u00a0common to almost all eukaryotic genes is the TATA box. A number of regulatory proteins must bind to the TATA box in the promoter before transcription can proceed.<\/li>\n<li>Regulation of gene expression\u00a0is extremely important during an organism's early development. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2411\">Homeobox genes<\/a> \u2014 which encode for chains of\u00a0amino acids\u00a0called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2413\">homeodomains<\/a> \u2014 are important genes that regulate development.<\/li>\n<li>Some types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancer<\/a> occur because of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2381\">mutations<\/a>\u00a0in the genes that control the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3551\">cell cycle<\/a>. Cancer-causing mutations most often occur in two types of regulatory genes: tumor-suppressor genes and proto-oncogenes.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">5.9 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Define gene expression.<\/li>\n<li>Why must gene expression be regulated?<\/li>\n<li>Explain how regulatory proteins may activate or repress transcription.<\/li>\n<li>\n<div id=\"h5p-236\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-236\" class=\"h5p-iframe\" data-content-id=\"236\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Regulation of Gene Expression\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is the TATA box, and how does it work?<\/li>\n<li>Describe homeobox genes and their role in\u00a0an organism's development.<\/li>\n<li>Discuss the role of regulatory gene mutations in cancer.<\/li>\n<li>Explain the relationship between proto-oncogenes and oncogenes.<\/li>\n<li>If a newly fertilized egg contained a\u00a0mutation\u00a0in a homeobox gene,\u00a0how do you think this would affect the developing embryo? Explain your answer.<\/li>\n<li>Compare and contrast enhancers and activators.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\"><strong>5.9 Explore More<\/strong><\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?time_continue=3&amp;v=vi-zWoobt_Q&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Regulated Transcription, ndsuvirtualcell, 2008.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=BmFEoCFDi-w<\/p>\n<p style=\"text-align: center\">How do cancer cells behave differently from healthy ones? - George Zaidan,<br \/>\nTED-Ed, 2012.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=Z3B-AaqjyjE<\/p>\n<p style=\"text-align: center\">What is leukemia? - Danilo Allegra and Dania Puggioni, 2015.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 5.9.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Stem_cell_differentiation.svg\" rel=\"cc:attributionURL\">Stem_cell_differentiation.svg<\/a> by <a class=\"new\" title=\"User:Haileyfournier (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Haileyfournier&amp;action=edit&amp;redlink=1\">Haileyfournier<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 5.9.2<\/strong><\/p>\n<p>Activators and Repressors by Christine Miller is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 5.9.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:TATA_box_description.png\" rel=\"cc:attributionURL\">TATA_box_description<\/a> by <a class=\"new\" title=\"User:Luttysar (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Luttysar&amp;action=edit&amp;redlink=1\">Luttysar<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 5.9.4<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/5.8\/\" rel=\"cc:attributionURL\">Pathways to cancer<\/a> by <a href=\"https:\/\/www.ck12.org\/student\/?_ga=2.233647837.2120627790.1594492874-1392032960.1592870652\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\">CC BY-NC 3.0<\/a> (http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<\/p>\n<p><img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><\/p>\n<p>&nbsp;<\/p>\n<p><span style=\"font-size: 1.424em;font-weight: bold\">References<\/span><\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2012). Figure 3 Flow chart (series of mutations leading to cancer) [digital image]. In <em>CK-12 College Human Biology <\/em>(Section 5.8) [online Flexbook]<span style=\"text-align: initial;font-size: 1em\">. CK12.org. https:\/\/www.ck12.org\/c\/physical-science\/concentration\/?referrer=crossref<\/span><\/p>\n<p class=\"hanging-indent\">ndsuvirtualcell.(2008). Regulated transcription. YouTube. https:\/\/www.youtube.com\/watch?v=vi-zWoobt_Q&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2012, December 5). How do cancer cells behave differently from healthy ones? - George Zaidan. YouTube. https:\/\/www.youtube.com\/watch?v=BmFEoCFDi-w&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, April 15). What is leukemia? - Danilo Allegra and Dania Puggioni. YouTube. https:\/\/www.youtube.com\/watch?v=Z3B-AaqjyjE&amp;feature=youtu.be<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_2906\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_2906\"><div tabindex=\"-1\"><p>Created by:\u00a0CK-12\/Adapted by Christine Miller<\/p>\n<div>\n<figure id=\"attachment_231\" aria-describedby=\"caption-attachment-231\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-231\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Borrelia_hermsii_Bacteria_13758011613.jpg\" alt=\"Image shows a large red blood cell, with a filamentous green bacterium resting on its surface.\" width=\"400\" height=\"300\"><figcaption id=\"caption-attachment-231\" class=\"wp-caption-text\"><em>Figure 4.3.1 A bacterium attacks a human erythrocyte. Both are cells.<\/em><\/figcaption><\/figure>\n<h1>Bacteria\u00a0Attack!<\/h1>\n<\/div>\n<p>The colourful image in Figure 4.3.1 shows a bacterial cell (in green) attacking human red blood cells. The bacterium causes a disease called relapsing fever. The bacterial and human cells look very different in size and shape. Although all living cells have certain things in common \u2014 such as a plasma membrane and cytoplasm \u2014 different types of cells, even within the same organism, may have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary not only in shape, but also in size, as this example shows. In most organisms, however, even the largest cells are no bigger than the period at the end of this sentence. Why are cells so small?<\/p>\n<div>\n<h1>Explaining Cell Size<\/h1>\n<\/div>\n<p>Most organisms, even very large ones, have microscopic cells. Why don't cells get bigger instead of remaining tiny and multiplying? Why aren't you one giant cell rolling around school? What limits cell size?<\/p>\n<p>Once you know how a cell functions, the answers to these questions are clear. To carry out life processes, a cell must be able to quickly pass substances in and out of the cell. For example, it must be able to pass\u00a0nutrients\u00a0and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. The size of a cell is limited by its need to pass substances across that outer surface.<\/p>\n<p>Look at the three cubes in Figure 4.3.2. A<span style=\"text-align: initial;font-size: 1em\"> larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells \u2014 a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen, and produces more waste. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The <\/span><em style=\"text-align: initial;font-size: 1em\">larger<\/em><span style=\"text-align: initial;font-size: 1em\"> the cell is, the <\/span><em style=\"text-align: initial;font-size: 1em\">smaller<\/em><span style=\"text-align: initial;font-size: 1em\"> its ratio of surface area to volume, and the more difficult it will be for the cell to get rid of its waste and take in necessary substances. This is what limits the size of the cell.<\/span><\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_234\" aria-describedby=\"caption-attachment-234\" style=\"width: 741px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-232\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cell-Size.png\" alt=\"Image shows three cubes: a small, a medium and a large. The cube with length of 1 has a surface area to volume ratio of 6:1. The cube with a length of 2 has a surface area to volume ratio of 3:1 and the cube with the length of 3 has a surface area to volume ratio of 2:1.\" width=\"741\" height=\"556\"><figcaption id=\"caption-attachment-234\" class=\"wp-caption-text\"><em>Figure 4.3.2 Surface area to volume ratio.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Cell Form and Function<\/h1>\n<\/div>\n<p>Cells with different functions often have varying shapes. The cells pictured below (Figure 4.3.3) are just a few examples of the many different shapes that human cells may have. Each type of cell\u00a0 has characteristics that help it do its job. The job of the nerve cell, for example, is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail of each tiny sperm cell? Its tail helps a sperm cell \"swim\" through fluids in the female reproductive tract in order to reach an egg cell. The white blood cell has the job of destroying bacteria and other pathogens. It is a large cell that can engulf foreign invaders.<\/p>\n<div>\n<div id=\"h5p-35\">\n<div class=\"h5p-content\" data-content-id=\"35\"><\/div>\n<\/div>\n<p><em>Figure 4.3.3 Human cells may have many different shapes that help them to do their jobs.<\/em><\/p>\n<\/div>\n<div>\n<h1>Cells With and Without a\u00a0Nucleus<\/h1>\n<\/div>\n<p>The\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">nucleus<\/a>\u00a0is a basic\u00a0cell structure\u00a0present in many \u2014 but not\u00a0<em>all \u2014<\/em>\u00a0living cells. The\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">nucleus<\/a>\u00a0of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a>. Based on whether or not they have a nucleus, there are two basic types of cells: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1572\">prokaryotic<\/a> cells and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1573\">eukaryotic<\/a> cells.<\/p>\n<h2>Prokaryotic Cells<\/h2>\n<figure id=\"attachment_234\" aria-describedby=\"caption-attachment-234\" style=\"width: 392px\" class=\"wp-caption alignright\"><img class=\"wp-image-233\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/2000px-Average_prokaryote_cell-_en.svg_.png\" alt=\"Image shows a diagram of a bacterium. The bacterium is smaller than a typical eukaryotic cell, has fewer organelles and contains no membrane-bound organelles.\" width=\"392\" height=\"319\"><figcaption id=\"caption-attachment-234\" class=\"wp-caption-text\"><em>Figure 4.3.3 Bacteria are prokaryotes, meaning they do not have a nucleus. Their DNA is contained in a region called the nucleoid.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1572\">Prokaryotic<\/a> cells<\/strong> are cells without a nucleus. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a> in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane.\u00a0 In addition, these cells are typically smaller than eukaryotic cells and contain fewer organelles.\u00a0 Prokaryotic cells are found in single-celled organisms, such as the bacterium represented by the model in Figure 4.3.3. Organisms with prokaryotic cells are called <strong>prokaryotes<\/strong>. They were the first type of organisms to evolve, and they are still the most common organisms today.<\/p>\n<div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Eukaryotic Cells<\/h2>\n<figure id=\"attachment_234\" aria-describedby=\"caption-attachment-234\" style=\"width: 2500px\" class=\"wp-caption alignnone\"><img class=\"size-full wp-image-234\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Animal-Cell-1.png\" alt=\"Image shows a diagram of a eukaryotic cell. The cell has many organelles labelled, including: nucleus, nucleolus, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi body, vesicles, mitochondria and centrioles.\" width=\"2500\" height=\"1000\"><figcaption id=\"caption-attachment-234\" class=\"wp-caption-text\"><em>Figure 4.3.4 Eukaryotic cells, like this animal cell, contain a nucleus and many other membrane-bound organelles.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1573\">Eukaryotic<\/a> cells<\/strong> are cells that contain a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">nucleus<\/a>. A typical eukaryotic cell is represented by the model in Figure 4.3.4. Eukaryotic cells are usually larger than prokaryotic cells. They are found in some single-celled and all multicellular organisms. Organisms with eukaryotic cells are called <strong>eukaryotes<\/strong>, and they range from\u00a0fungi\u00a0to\u00a0humans.<\/p>\n<p>Besides a nucleus, eukaryotic cells also contain other\u00a0organelles. An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2083\">organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm that performs a specific job in the cell.\u00a0Organelles\u00a0called\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2383\">mitochondria<\/a>, for example, provide\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2340\">energy<\/a>\u00a0to the cell, and organelles called vesicles store substances in the cell. Organelles allow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1573\">eukaryotic<\/a> cells to carry out more functions than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1572\">prokaryotic<\/a> cells can.<\/p>\n<p>Interestingly, scientists think that mitochondria were once free-living prokaryotes that infected (or were engulfed by) larger cells. The two organisms developed a symbiotic relationship that was beneficial to both of them, resulting in the smaller prokaryote becoming an organelle within the larger cell. This is called endosymbiotic theory, and it is supported by a lot of evidence, including the fact that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2383\">mitochondria<\/a> have their own <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a> separate from the DNA in the nucleus of the eukaryotic cell. Endosymbiotic theory will be described in more detail in later sections, and it's also discussed in the video below.<\/p>\n<div>\n<p>https:\/\/www.youtube.com\/watch?v=FGnS-Xk0ZqU<\/p>\n<p class=\"title style-scope ytd-video-primary-info-renderer\" style=\"text-align: center\">Endosymbiotic Theory, Amoeba Sisters, 2017.<\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">4.3 Summary<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Cells must be very small so they have a large enough surface area-to-volume ratio to maintain normal cell processes.<\/li>\n<li>Cells with different functions often have different shapes.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1572\">Prokaryotic<\/a> cells do not have a nucleus. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1573\">Eukaryotic <\/a>cells\u00a0<em>do<\/em>\u00a0have a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2402\">nucleus<\/a>, along with other\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2083\">organelles<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">4.3 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Explain why most cells are very small.<\/li>\n<li>Discuss variations in the form and function of cells.<\/li>\n<li>\n<div id=\"h5p-36\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-36\" class=\"h5p-iframe\" data-content-id=\"36\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Cell Variation\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>\n<div id=\"h5p-37\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-37\" class=\"h5p-iframe\" data-content-id=\"37\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Prokaryote vs. Eukaryote T\/F\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Do human cells have organelles? Explain your answer.<\/li>\n<li>Which are usually larger \u2013 prokaryotic or eukaryotic cells? What do you think this means for their relative ability to take in needed substances and release wastes? Discuss your answer.<\/li>\n<li>DNA in eukaryotes is enclosed within the _______ \u00a0________.<\/li>\n<li>Name three different types of cells in humans.<\/li>\n<li>Which organelle provides energy in eukaryotic cells?<\/li>\n<li>What is a function of a vesicle in a cell?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">4.3 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?time_continue=1&amp;v=9i7kAt97XYU&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">How we think complex cells evolved - Adam Jacobson, TED-Ed, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=Pxujitlv8wc<\/p>\n<p style=\"text-align: center\">Prokaryotic vs. Eukaryotic Cells (updated), Amoeba Sisters, 2018.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 4.3.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Borrelia_hermsii_Bacteria_(13758011613).jpg\" rel=\"cc:attributionURL\">Borrelia_hermsii_Bacteria_(13758011613)<\/a> by <a href=\"https:\/\/www.flickr.com\/people\/54591706@N02\">NAID<\/a> on Wikimedia Commons is released into the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 4.3.2<\/strong><\/p>\n<p>Cell Size by Christine Miller is released into the <a href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" rel=\"license\">Public Domain<\/a> (https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/).<\/p>\n<p><strong>Figure 4.3.3<\/strong><\/p>\n<ul>\n<li><a style=\"font-size: 1em\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:BioTek-Wikipedia-Image.tif\">Chondrocyte. BioTek-Wikipedia-Image<\/a><span style=\"font-size: 1em\"> by BioTek Instruments, Inc. on Wikimedia Commons is used under a <\/span><a style=\"font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en\">CC BY-SA 3.0<\/a><span style=\"font-size: 1em\"> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en) license. <\/span><\/li>\n<li><a style=\"font-size: 1em\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Neutrophil_with_anthrax_copy.jpg\">Neutrophil with anthrax copy<\/a><span style=\"font-size: 1em\"> by Volker Brinkmann from <\/span><a style=\"font-size: 1em\" href=\"https:\/\/journals.plos.org\/plospathogens\/issue?id=10.1371\/issue.ppat.v01.i03\">PLOS Pathogens<\/a><span style=\"font-size: 1em\"> on Wikimedia Commons is used under a\u00a0<\/span><a style=\"font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en\">CC BY 2.5<\/a><span style=\"font-size: 1em\"> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\u00a0<\/span><\/li>\n<li><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:PLoSBiol4.e126.Fig6fNeuron.jpg\"><em>PLoSBio<\/em>l4.e126.Fig6fNeuron<\/a> by Lee, et al. from <a href=\"https:\/\/journals.plos.org\/plosbiology\/article?id=10.1371\/journal.pbio.0040029\">PLOS Biology<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en\">CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.<\/li>\n<li><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sperm_(265_33)_human.jpg\">Sperm (265 33) human<\/a> by <a class=\"extiw\" title=\"cs:Josef Reischig\" href=\"https:\/\/cs.wikipedia.org\/wiki\/Josef_Reischig\">Doc. RNDr. Josef Reischig, CSc.<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/li>\n<\/ul>\n<\/div>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 4.3.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Average_prokaryote_cell-_en.svg\" rel=\"cc:attributionURL\">Model of a prokaryotic cell: bacterium<\/a> by Mariana Ruiz Villarreal [<a title=\"User:LadyofHats\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:LadyofHats\">LadyofHats]<\/a> on Wikimedia Commons is released into the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 4.3.5<\/strong><\/p>\n<p>Animal Cell adapted by Christine Miller is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" rel=\"license\">CC0 1.0<\/a> (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en) public domain dedication license. (Original image, <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Animal_Cell_Unannotated.svg\">Animal Cell Unannotated,<\/a> is by <a href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Kelvin13\">Kelvin Song<\/a> on Wikimedia Commons.)<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Amoeba Sisters. (2017, May 3). Endosymbiotic theory. YouTube. https:\/\/www.youtube.com\/watch?v=FGnS-Xk0ZqU&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Amoeba Sisters. (2018, July 30). Prokaryotic vs. eukaryotic cells (updated). YouTube. https:\/\/www.youtube.com\/watch?v=Pxujitlv8wc&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Brinkmann, V. (November 2005). Neutrophil engulfing Bacillus anthracis. <em>PLoS Pathogens 1<\/em> (3): Cover page [digital image]. DOI:10.1371. https:\/\/journals.plos.org\/plospathogens\/issue?id=10.1371\/issue.ppat.v01.i03<\/p>\n<p class=\"hanging-indent\">Lee, W.C.A., Huang, H., Feng, G., Sanes, J.R., Brown, E.N., et al. (2005, December 27) Figure 6f, slightly altered (plus scalebar, minus letter \"f\".) [digital image]. Dynamic Remodeling of Dendritic Arbors in GABAergic Interneurons of Adult Visual Cortex. <em>PLoS Biology, 4<\/em>(2), e29. doi:10.1371\/journal.pbio.0040029. https:\/\/journals.plos.org\/plosbiology\/article?id=10.1371\/journal.pbio.0040029<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, February 17). How we think complex cells evolved - Adam Jacobson. https:\/\/www.youtube.com\/watch?v=9i7kAt97XYU&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_5913\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_5913\"><div tabindex=\"-1\"><p>A rigid organ that constitutes part of the vertebrate skeleton in animals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3970\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3970\"><div tabindex=\"-1\"><p>Created by:\u00a0CK-12\/Adapted by Christine Miller<\/p>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\">Is It Magic?<\/h1>\n<figure id=\"attachment_2296\" aria-describedby=\"caption-attachment-2296\" style=\"width: 267px\" class=\"wp-caption alignleft\"><img class=\"wp-image-2296\" style=\"color: #373d3f;font-weight: bold;font-size: 1em\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Chlorine_gas_in_high_concentration-1.jpg\" alt=\"Chlorine gas in high concentration in a Florence flask \" width=\"267\" height=\"267\"><figcaption id=\"caption-attachment-2296\" class=\"wp-caption-text\"><em>Figure 3.8.1 Chlorine gas in high concentration.<\/em><\/figcaption><\/figure>\n<p>The harmless-looking bottle in Figure 3.8.1 contains a greenish-yellow, poisonous gas. The gas is chlorine, which is also used as bleach and to keep the water in pools and hot tubs free of germs. Chlorine can kill just about anything. Would you breathe in chlorine gas or drink liquid chlorine? Of course not, but you often eat a compound containing chlorine. You probably eat this chlorine compound just about every day. Can you guess what it is? It's table salt.<\/p>\n<figure id=\"attachment_534\" aria-describedby=\"caption-attachment-534\" style=\"width: 283px\" class=\"wp-caption alignright\"><img class=\"wp-image-534\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/salt-4160306_1280-1.jpg\" alt=\"Image shows a salt shaker filled with salt sitting on a wooden counter.\" width=\"283\" height=\"189\"><figcaption id=\"caption-attachment-534\" class=\"wp-caption-text\"><em>Figure 3.8.2 Table salt contains the elements sodium and chloride.<\/em><\/figcaption><\/figure>\n<p>Table salt is actually sodium chloride (NaCl), which forms when chlorine and sodium (Na) combine in certain proportions. How does the toxic green chemical chlorine change into the harmless white\u00a0compound\u00a0we know as table salt? It isn't magic \u2014 it's chemistry, and it happens in a chemical reaction.<\/p>\n<div>\n<h1>What Is a Chemical Reaction?<\/h1>\n<\/div>\n<p>A\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3637\"><strong>chemical<\/strong>\u00a0<strong>reaction<\/strong><\/a>\u00a0is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">reactant<\/a>,<\/strong>\u00a0and a substance that forms as a result of a chemical reaction is called a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">product<\/a>.<\/strong>\u00a0During the reaction, the\u00a0reactants\u00a0are used up to create the products.<\/p>\n<p>The burning of methane\u00a0gas, as\u00a0shown in the picture\u00a0below, is a\u00a0chemical reaction. In this reaction, the reactants are methane (CH<sub>4<\/sub>) and oxygen (O<sub>2<\/sub>), and the products are carbon dioxide (CO<sub>2<\/sub>) and\u00a0water\u00a0(H<sub>2<\/sub>O). As this example shows, a chemical reaction involves the breaking and forming of\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3413\">chemical bonds<\/a><\/strong>, which\u00a0are forces that hold together the atoms of a molecule. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and\u00a0water.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_581\" aria-describedby=\"caption-attachment-581\" style=\"width: 508px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-581\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gas-flame-1.jpg\" alt=\"Image shows a lit gas stove burner. The flames are blue and there is a pot on the burner.\" width=\"508\" height=\"327\"><figcaption id=\"caption-attachment-581\" class=\"wp-caption-text\"><em>Figure 3.8.3 Flames from methane burning.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Chemical Equations<\/h1>\n<\/div>\n<p>Chemical reactions\u00a0can be represented by chemical equations. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3415\">chemical equation<\/a><\/strong>\u00a0is a symbolic way of showing what happens during a chemical reaction. The burning of methane, for example, can be represented by the chemical equation:<\/p>\n<p><strong>CH<span style=\"color: #008000\"><sub>4<\/sub><\/span>\u00a0+ <span style=\"color: #000080\">2<\/span>O<span style=\"color: #008000\"><sub>2<\/sub><\/span>\u00a0\u2192 CO<span style=\"color: #008000\"><sub>2<\/sub><\/span>\u00a0+ <span style=\"color: #000080\">2<\/span>H<span style=\"color: #008000\"><sub>2<\/sub><\/span>O<\/strong><\/p>\n<p>The arrow in a chemical equation separates the reactants from the products, and shows the direction in which the reaction proceeds. If the reaction could occur in the opposite direction as well, two arrows pointing in opposite directions would be used. The number <span style=\"color: #000080\"><strong>2<\/strong><\/span> in front of O<sub>2<\/sub>\u00a0and H<sub>2<\/sub>O, called the <em>coefficient<\/em>, shows that <strong><span style=\"color: #000080\">two<\/span><\/strong> oxygen molecules and <span style=\"color: #000080\"><strong>two<\/strong><\/span> water molecules are involved in the reaction. If just one molecule is involved, no number is placed in front of the chemical symbol.\u00a0Note the <em>subscript<\/em> of <span style=\"color: #008000\"><strong>2<\/strong><\/span> for the oxygen (O) and hydrogen (H) atoms in the oxygen and water molecules, respectively. That tells you that each oxygen molecule is made up of <span style=\"color: #008000\"><strong>two<\/strong><\/span> oxygen atoms. If there is no subscript, then\u00a0there is a single\u00a0atom. Thus, one water molecule is made up of two hydrogen atoms and one oxygen\u00a0atom. In order for this\u00a0chemical reaction to take place, one methane molecule reacts with two oxygen molecules to form one carbon dioxide molecule and two water molecules.<\/p>\n<div>\n<figure id=\"attachment_977\" aria-describedby=\"caption-attachment-977\" style=\"width: 290px\" class=\"wp-caption alignright\"><img class=\" wp-image-977\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Antoine_lavoisier-1.jpg\" alt=\"Shows a black and white caricature of Antoine Lavoisier with a thought bubble above his head containing the words &quot; All the reactants must end up in the product - they can't just disappear&quot;.\" width=\"290\" height=\"304\"><figcaption id=\"caption-attachment-977\" class=\"wp-caption-text\"><em>Figure 3.8.4 Antoine Lavoisier is known as \"the father of modern chemistry.\"<\/em><\/figcaption><\/figure>\n<h1>Conservation of Mass<\/h1>\n<\/div>\n<p>In a chemical reaction, the quantity of each\u00a0element\u00a0does not change. There is the same amount of each element in the products as there was in the reactants. Mass is always conserved. According to the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3417\">law of conservation of mass<\/a><\/strong>\u00a0\u2014<strong>\u00a0<\/strong>which was first demonstrated convincingly by French chemist <a href=\"https:\/\/en.wikipedia.org\/wiki\/Antoine_Lavoisier\">Antoine Lavoisier<\/a> in 1785 \u2014 mass is neither created nor destroyed during a chemical reaction. Therefore, during a chemical reaction, the total mass of products is equal to the total mass of reactants.\u00a0The\u00a0conservation of mass\u00a0is reflected in a reaction's chemical equation. The same number of atoms of each\u00a0element\u00a0appears on each side of the arrow. In the chemical equation above, there are four hydrogen atoms on each side of the arrow. Can you find all four of them on each side of the equation?<\/p>\n<div>\n<h1>Chemical vs. Physical Changes<\/h1>\n<\/div>\n<p>Many processes that happen all around us on a daily basis involve <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3637\">chemical reactions<\/a>. Not every change, however, is a chemical change. Some changes are simply physical and do not involve chemical reactions. Physical changes include change in size of pieces and change in state.\u00a0 If you break an eggshell and pour out the egg into a pan, its chemical makeup and properties do not change. This is just a physical change. No chemical reactions have occurred, and no chemical bonds have broken or formed. Other examples of physical changes are cutting paper into smaller pieces and letting an ice cube melt. What if you put the egg in the pan over a hot flame? The egg turns to a rubbery solid and changes colour. The properties of the egg have changed because its chemical makeup has changed. Cooking the egg is a chemical change that involves chemical reactions.<\/p>\n<p>Other common examples of chemical changes include a cake baking, metal rusting, and a candle burning.\u00a0More practice is below.<\/p>\n<div>\n<div id=\"h5p-19\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-19\" class=\"h5p-iframe\" data-content-id=\"19\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Intro to Human Biology\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 3.8.5 Chemical changes often involve chemical reactions as well.\u00a0<\/em><\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">3.8 Summary<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3637\">chemical reaction<\/a> is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">reactant<\/a>, and a substance that forms during a chemical reaction is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">product<\/a>. During the chemical reaction, bonds break in reactants and new bonds form in products.<\/li>\n<li>Chemical reactions can be represented by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3415\">chemical equations<\/a>. According to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3417\">law of conservation of mass<\/a>, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.<\/li>\n<li>Many chemical reactions \u2014 such as iron rusting and organic matter rotting \u2014 occur all around us each day, but not <em>all<\/em> changes are chemical processes. Some changes \u2014 like ice melting or paper being torn into smaller pieces \u2014 are physical processes that do not involve chemical reactions and the formation of new substances.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">3.8 Review Questions<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is a chemical reaction?<\/li>\n<li>Define the reactants and products in a chemical reaction.<\/li>\n<li>List three examples of common changes that involve chemical reactions.<\/li>\n<li>Define a chemical bond.<\/li>\n<li>What is a chemical equation? Give an example.<\/li>\n<li>What does it mean for a chemical equation to be balanced? Why must a chemical equation be balanced?<\/li>\n<li>Our cells use glucose (C<sub>6<\/sub>H<sub>12<\/sub>O<sub>6<\/sub>) to obtain energy in a chemical reaction called cellular respiration. In this reaction, six oxygen molecules (O<sub>2<\/sub>) react with one glucose molecule. Answer the following questions about this reaction:\n<ul>\n<li>How many oxygen atoms are in one molecule of glucose?<\/li>\n<li>Write out what the reactant side of this equation would look like.<\/li>\n<li>In total, how many oxygen atoms are in the reactants? Explain how you calculated your answer.<\/li>\n<li>In total, how many oxygen atoms are in the products? Is it possible to answer this question without knowing what the products are? Why or why not?<\/li>\n<\/ul>\n<\/li>\n<li>Answer the following questions about the following equation: CH<sub>4<\/sub>+ 2O<sub>2<\/sub> \u2192 CO<sub>2<\/sub> + 2H<sub>2<\/sub>O\n<ul>\n<li>Can carbon dioxide (CO<sub>2<\/sub>)transform into methane (CH<sub>4<\/sub>) and oxygen (O<sub>2<\/sub>) in this reaction? Why or why not?<\/li>\n<li>How many molecules of carbon dioxide (CO<sub>2<\/sub>) are produced in this reaction?<\/li>\n<\/ul>\n<\/li>\n<li>Is the evaporation of liquid water into water vapor a chemical reaction? Why or why not?<\/li>\n<li>Why do bonds break in the reactants during a chemical reaction?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.8 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=2S6e11NBwiw&amp;feature=emb_logo<\/p>\n<p class=\"title style-scope ytd-video-primary-info-renderer\" style=\"text-align: center\">The law of conservation of mass - Todd Ramsey, TED-Ed, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=37pir0ej_SE<\/p>\n<p style=\"text-align: center\">Chemical Changes: Crash Course Kids #19.2, by Crash Course Kids, 2015.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 3.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Chlorine_gas_in_high_concentration.jpg\" rel=\"cc:attributionURL\">Chlorine_gas_in_high_concentration<\/a> by <a class=\"new\" title=\"User:Larenmclane (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Larenmclane&amp;action=edit&amp;redlink=1\">Larenmclane<\/a> on Wikimedia Commons, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en) license.<\/p>\n<p><strong>Figure 3.8.2<\/strong><\/p>\n<p><a href=\"https:\/\/pixabay.com\/photos\/salt-salt-shaker-spices-kitchen-4160306\/\" rel=\"cc:attributionURL\">Tags: Salt Salt Shaker Spices Kitchen Spice Component; salt-4160306_1280<\/a> by <a href=\"https:\/\/pixabay.com\/users\/katie175-1833633\/?utm_source=link-attribution&amp;utm_medium=referral&amp;utm_campaign=image&amp;utm_content=4160306\" rel=\"dc:creator\">katie175<\/a> <span style=\"font-size: 1em\">from <a href=\"https:\/\/pixabay.com\/\">Pixabay<\/a><\/span><span style=\"font-size: 1em\"> is used under the <a href=\"https:\/\/pixabay.com\/service\/license\/\">Pixabay License<\/a> (https:\/\/pixabay.com\/de\/service\/license\/).<\/span><\/p>\n<p><strong>Figure 3.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/pixabay.com\/photos\/gas-flame-gas-stove-italy-gas-cook-1452999\/\" rel=\"cc:attributionURL\">Tags: Gas Flame Gas Stove Italy Gas Cook Kitchen<\/a> by\u00a0<a href=\"https:\/\/pixabay.com\/users\/moerschy-127417\/?utm_source=link-attribution&amp;utm_medium=referral&amp;utm_campaign=image&amp;utm_content=1452999\" rel=\"dc:creator\">moerschy<\/a> <span style=\"font-size: 1em\">from <a href=\"https:\/\/pixabay.com\/\">Pixabay<\/a><\/span><span style=\"font-size: 1em\"> is used under the <a href=\"https:\/\/pixabay.com\/service\/license\/\">Pixabay License<\/a> (https:\/\/pixabay.com\/de\/service\/license\/).<\/span><\/p>\n<p><strong>Figure 3.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Antoine_lavoisier.jpg\" rel=\"cc:attributionURL\">Antoine_lavoisier<\/a> by unknown on Wikimedia Commons has been adapted by Christine Miller. The orginal work, believed to be from http:\/\/www.schuster-ingolstadt.de\/Chemie.htm has been released into the \u00a0<a class=\"extiw\" title=\"en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 3.8.5<\/strong><\/p>\n<div class=\"h5p-media-copyright\">\n<ul>\n<li><a href=\"https:\/\/unsplash.com\/photos\/4-cUMjy98N8\">Ice cream melting<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@aronvisuals\">Aron Visuals<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the \u00a0<a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><span style=\"font-size: 1em\"><a href=\"https:\/\/unsplash.com\/photos\/5cFqO92t7pM\">Kombucha<\/a> [photo] <\/span><span style=\"font-size: 1em\">by <\/span><a style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/@curryandlove\">Klara Avsenik<\/a> <span style=\"font-size: 1em\">on <\/span><a style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/\">Unsplash<\/a><span style=\"font-size: 1em\"> is used under the \u00a0<\/span><a class=\"ICezk _2GAZm _2WvKc\" style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a><span style=\"font-size: 1em\"> (https:\/\/unsplash.com\/license). <\/span><\/li>\n<li><a style=\"font-size: 1em\" href=\"https:\/\/www.publicdomainpictures.net\/en\/view-image.php?image=130897&amp;picture=grated-cheese\">Grated cheese<\/a><span style=\"font-size: 1em\"> by Steve Buissinne\u00a0on <\/span><a style=\"font-size: 1em\" href=\"https:\/\/www.publicdomainpictures.net\/en\/index.php\">PublicDomainPictures<\/a><span style=\"font-size: 1em\"> is used under the \u00a0<\/span><a class=\"black_white_text\" style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" target=\"_blank\" rel=\"license noopener noreferrer\">CC0 1.0 <\/a><span style=\"font-size: 1em\">Universal <\/span><span style=\"font-size: 1em\">Public Domain Dedication license (<\/span><span style=\"font-size: 1em\">https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/).<\/span><\/li>\n<\/ul>\n<\/div>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Crash Course Kids. (2015, July 16). <span style=\"font-size: 1em;text-align: initial\">Chemical changes: Crash Course Kids #19.2. YouTube. https:\/\/www.youtube.com\/watch?v=37pir0ej_SE<\/span>\u200b<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, February 26 ). The law of conservation of mass - Todd Ramsey. YouTube. https:\/\/www.youtube.com\/watch?v=2S6e11NBwiw&amp;feature=emb_logo<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, June 15). Antoine Lavoisier. <i>Wikipedia. <\/i>https:\/\/en.wikipedia.org\/w\/index.php?title=Antoine_Lavoisier&amp;oldid=962631283<\/p>\n<\/div>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_5761\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_5761\"><div tabindex=\"-1\"><p>The ability of an organism to maintain constant internal conditions despite external changes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3953\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3953\"><div tabindex=\"-1\"><p style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\">Created by:\u00a0CK-12\/Adapted by: Christine Miller<\/p>\n<figure id=\"attachment_37\" aria-describedby=\"caption-attachment-37\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-37\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/vaccine-1-scaled.jpg\" alt=\"1.2.1 Vaccine\" width=\"400\" height=\"269\"><figcaption id=\"caption-attachment-37\" class=\"wp-caption-text\"><em>Figure 1.2.1 Getting vaccinated.<\/em><\/figcaption><\/figure>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\"><strong>Ouch!<\/strong><\/h1>\n<p>The person in Figure 1.2.1 is getting a flu vaccine. You probably know that getting a vaccine can hurt \u2014 but it's usually worth it. A vaccine contains dead or altered forms of germs that normally cause a disease, such as flu or measles. The germs in vaccines have been inactivated or weakened so they can no longer cause illness, but are still \"noticed\" by the immune system.<\/p>\n<p>They stimulate the immune system to produce chemicals that can kill the\u00a0<em>actual<\/em>\u00a0germs if they enter the body, thus preventing future disease. How was such an ingenious way to prevent disease discovered? The short answer is more than two centuries of science.<\/p>\n<div>\n<figure id=\"attachment_40\" aria-describedby=\"caption-attachment-40\" style=\"width: 236px\" class=\"wp-caption alignright\"><img class=\"wp-image-40\" title=\"Photo in the Public Domain, from the Centre for Disease Control and Prevention, ID#3265\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Smallpox-1.jpg\" alt=\"A young child in Bangladesh is covered with skin lesions from smallpox. The scarring covers the child's face, including lips and eyelids, as well as the torso and arms.\" width=\"236\" height=\"359\"><figcaption id=\"caption-attachment-40\" class=\"wp-caption-text\"><em>Figure 1.2.2 A young child in Bangladesh is covered with skin lesions from smallpox. Until it was eradicated, this highly contagious infection caused many deaths, and those that survived were often severely scarred for life.<\/em><\/figcaption><\/figure>\n<h1>Science as Process<\/h1>\n<\/div>\n<p>You may think of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3717\">science<\/a><\/strong> as a large and detailed body of knowledge, but science is also the process by which this knowledge is obtained.\u00a0Science\u00a0uses\u00a0experimentation, evidence, and logic to continuously test ideas. Over time and through repeated experimentation\u00a0and testing, scientific knowledge advances.<\/p>\n<p>We've been accumulating knowledge of vaccines for more than two centuries.\u00a0The discovery of the first vaccine, as well as the process of vaccination, dates back to 1796. An English doctor named <a href=\"https:\/\/en.wikipedia.org\/wiki\/Edward_Jenner\" target=\"_blank\" rel=\"noopener noreferrer\">Edward Jenner<\/a> observed that people who became infected with cowpox did not get sick from smallpox, a similar but much more severe disease (Figure 1.2.2). Jenner decided to transmit cowpox to a young boy to see if it would protect him from smallpox. He gave the boy cowpox by scratching liquid from cowpox sores into the boy's skin. Then, six weeks later, he scratched liquid from smallpox sores into the boy's skin. As Jenner predicted, the boy did not get sick from smallpox. Jenner had discovered the first vaccine, although additional testing was needed to show that it really was effective.<\/p>\n<p>Almost a century passed before the next vaccine was discovered, a vaccine for cholera in 1879. Around the same time, French chemist <a href=\"https:\/\/en.wikipedia.org\/wiki\/Louis_Pasteur\" target=\"_blank\" rel=\"noopener noreferrer\">Louis Pasteur<\/a> found evidence that many human diseases are caused by germs, which earned\u00a0him\u00a0the title of \"father of germ theory.\" Since Pasteur's time, vaccines have been discovered for scores of additional diseases caused by germs, and scientists are currently researching vaccines for many others.<\/p>\n<div>\n<h1>Benefits of Science<\/h1>\n<\/div>\n<p>Medical advances such as the discovery of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors. Science is needed to design safe cars, predict storms, control\u00a0global warming, develop new technologies of many kinds, help couples have children, and put humans on the moon. Clearly, the diversity of applications of scientific knowledge is vast!<\/p>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.2 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Science is a large and detailed body of knowledge. It is also the process by which this knowledge is obtained.<\/li>\n<li>Science\u00a0uses\u00a0experimentation, evidence, and logic to continuously test ideas. Over time and\u00a0through\u00a0repeated experimentation\u00a0and testing, scientific knowledge advances.<\/li>\n<li>Medical advances such as the\u00a0development\u00a0of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.2 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Explain why science is considered both a process\u00a0and\u00a0a body of knowledge.<\/li>\n<li>State three specific examples of human endeavors that are based on scientific knowledge.<\/li>\n<li>How does science influence your daily life?<\/li>\n<li>Jenner used a young boy as a\u00a0research\u00a0subject in his smallpox vaccine research. Today, scientists must follow strict guidelines when using human subjects in their research. What unique concerns do you think might arise when human beings are used as research subjects?<\/li>\n<li>What gave Jenner the idea to develop a vaccine for smallpox?<\/li>\n<li>Why do you think almost a century passed between the\u00a0development\u00a0of the first vaccine (for smallpox) and the development of the next vaccine (for cholera)<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.2 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?time_continue=1&amp;amp;v=yqUFy-t4MlQ&amp;amp;feature=emb_logohttps:\/\/www.ted.com\/talks\/uri_alon_why_science_demands_a_leap_into_the_unknown?language=en<\/p>\n<p style=\"text-align: center\">How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\n<p class=\"textbox__title\">\n<\/div>\n<\/div>\n<h2 id=\"x-ck12-YzhkZDM1NjIyZWY2OTkxNDVlODQ3NzVmZGJkNzkyYmY.-wlj\"><strong>Attributions<\/strong><\/h2>\n<p><strong>Figure 1.2.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/a1p0Z7RSkL8\">Vacina, centro de vacina\u00e7\u00e3o<\/a>, by <span class=\"_2aPXR\"><a href=\"https:\/\/unsplash.com\/@hyttalosouza\">Hyttalo Souza<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a>, used under the <a href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/p>\n<p><strong>Figure 1.2.2<\/strong><\/p>\n<p><a href=\"https:\/\/phil.cdc.gov\/details.aspx?pid=3265\">Child with Smallpox\/ID#3265<\/a>, by CDC\/ James Hicks, from the\u00a0Centre for Disease Control and Prevention, is in the <a href=\"https:\/\/commons.wikimedia.org\/wiki\/Commons:Licensing#Material_in_the_public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<h2><strong>References<\/strong><\/h2>\n<p class=\"hanging-indent\">TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi.\u00a0 YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, August 9). Edward Jenner. In\u00a0<i>Wikipedia. <\/i>https:\/\/en.wikipedia.org\/w\/index.php?title=Edward_Jenner&amp;oldid=971970576<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, August 5). Louis Pasteur. In\u00a0<i>Wikipedia. <\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Louis_Pasteur&amp;oldid=971330056<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3957\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3957\"><div tabindex=\"-1\"><p>Created by:\u00a0CK-12\/Adapted by: Christine Miller<\/p>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\">What Is Pseudoscience?<\/h1>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3497\">Pseudoscience<\/a><\/strong>\u00a0is a claim, belief, or practice that is presented as scientific but does not adhere to the standards and methods of science. True science is based on repeated evidence-gathering and testing of falsifiable hypotheses. Pseudoscience does not adhere to these criteria.\u00a0In addition to\u00a0phrenology, some other examples of pseudoscience include astrology, extrasensory perception (ESP), reflexology, reincarnation, and Scientology,<\/p>\n<h2>Characteristics of Pseudoscience<\/h2>\n<p>Whether a field is actually science or just pseudoscience is not always clear. However, pseudoscience generally exhibits certain common characteristics. Indicators of pseudoscience include:<\/p>\n<ul>\n<li><strong>The use of vague, exaggerated, or untestable claims:<\/strong>\u00a0Many claims made by pseudoscience cannot be tested with evidence. As a result, they cannot be falsified, even if they are not true.<\/li>\n<li><strong>An over-reliance on confirmation rather than refutation:<\/strong>\u00a0Any incident that appears to justify a pseudoscience claim is treated as proof of the claim. Claims are assumed true until proven otherwise, and\u00a0the burden of disproof is placed on skeptics of the claim.<\/li>\n<li><strong>A lack of openness to testing by other experts:<\/strong>\u00a0Practitioners of pseudoscience avoid subjecting their ideas to peer review. They may refuse to share their data and justify the need for secrecy\u00a0with\u00a0claims\u00a0of proprietary or privacy.<\/li>\n<li><strong>An\u00a0absence of progress in advancing knowledge:<\/strong>\u00a0In pseudoscience, ideas are not subjected to repeated testing followed by rejection or refinement, as hypotheses are in true science. Ideas in pseudoscience\u00a0may remain unchanged for hundreds \u2014 or even thousands \u2014 of years. In fact, the older an idea is, the more it tends to be trusted in pseudoscience.<\/li>\n<li><strong>Personalization of issues:<\/strong>\u00a0Proponents of pseudoscience adopt beliefs that have little or no rational basis, so they may try to confirm their beliefs by treating critics as enemies. Instead of arguing to support their own beliefs, they attack the motives and character of their critics.<\/li>\n<li><strong>The use of misleading language:<\/strong>\u00a0Followers of pseudoscience may use scientific-sounding terms to make their ideas sound more convincing. For example, they may use the formal name dihydrogen monoxide to refer to\u00a0plain old water.<\/li>\n<\/ul>\n<h2>Persistence of Pseudoscience<\/h2>\n<p>Despite failing to meet scientific standards, many pseudosciences survive. Some pseudosciences remain very popular with large numbers of believers. A good example is astrology.<\/p>\n<p>Astrology is\u00a0the study of the movements and relative positions of celestial objects as a means for divining information about human affairs and terrestrial events. Many ancient cultures attached importance to astronomical events, and some developed elaborate systems for predicting terrestrial events from celestial observations. Throughout most of its history in the West, astrology was considered a scholarly tradition and was common in academic circles. With the advent of modern Western science, astrology was called into question. It was challenged on both theoretical and experimental grounds, and it was eventually shown to have no scientific validity or explanatory power.<\/p>\n<figure id=\"attachment_58\" aria-describedby=\"caption-attachment-58\" style=\"width: 300px\" class=\"wp-caption alignleft\"><img class=\"wp-image-58 size-medium\" title=\"Image in the public domain.\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/zodiac-29420_1280-1.png\" alt=\"\" width=\"300\" height=\"212\"><figcaption id=\"caption-attachment-58\" class=\"wp-caption-text\"><em>Figure 1.7.1 Zodiac signs.<\/em><\/figcaption><\/figure>\n<p>Today, astrology is considered a pseudoscience, yet it continues to have many devotees. Most people know their astrological sign, and many people are familiar with the personality traits supposedly associated with their sign. Astrological readings and horoscopes are readily available online and in print media, and a lot of people read them,\u00a0even if only occasionally. About a third of all adult Americans actually believe that astrology is scientific. Studies suggest that the\u00a0persistent popularity of pseudosciences such as astrology reflects a high level of scientific illiteracy. It seems that many Americans do not have an accurate understanding of scientific principles and methodology. They are not convinced by scientific arguments against their beliefs.<\/p>\n<h2>Dangers of Pseudoscience<\/h2>\n<p>Belief in astrology\u00a0is unlikely to\u00a0cause a person harm, but belief in some other pseudosciences might \u2014 especially in\u00a0health care-related areas. Treatments that seem scientific but are not may be ineffective, expensive, and even dangerous to patients. Seeking out pseudoscientific treatments may also delay or preclude patients from seeking scientifically-based medical treatments that have been tested and found safe and effective. In short, irrational health care may not be harmless.<\/p>\n<div>\n<h1>Scientific Hoaxes, Frauds, and Fallacies<\/h1>\n<\/div>\n<p>Pseudoscience is not the only way that science may be misused. Scientific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3499\"><strong>hoaxes<\/strong><\/a>, frauds, and fallacies may misdirect the pursuit of science, put patients at risk, or mislead and confuse the public. An example of each of these misuses of science and its negative effects is described below.<\/p>\n<h2>The Piltdown Hoax<\/h2>\n<figure id=\"attachment_59\" aria-describedby=\"caption-attachment-59\" style=\"width: 233px\" class=\"wp-caption alignleft\"><img class=\"wp-image-59\" title=\"Image by By James Howard McGregor [Public domain], via Wikimedia Commons\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Piltdown-Man-1.jpg\" alt=\"A side profile view of an artists rendition of what the Piltdown Man may have looked like, had he been real.\" width=\"233\" height=\"221\"><figcaption id=\"caption-attachment-59\" class=\"wp-caption-text\"><em>Figure 1.7.2 This reconstruction of Piltdown Man's head was based on jaw and skull bone fragments.<\/em><\/figcaption><\/figure>\n<p>Piltdown Man (see picture left) was a paleontological hoax in which bone fragments were presented as the fossilized remains of a previously unknown early human. These fragments consisted of parts of a skull and jawbone, reported to have been found in 1908 in a gravel pit at Piltdown, East Sussex, England. The significance of the specimen remained the subject of controversy until it was exposed in 1953 as a hoax. It eventually came to light that the specimen consisted of the lower jawbone of an orangutan deliberately combined with skull bones of a modern human. The Piltdown hoax is perhaps the most infamous paleontological hoax ever perpetrated, both for its impact on the direction of research on human evolution and for the length of time\u00a0between\u00a0its \"discovery\"\u00a0and\u00a0its full exposure as a forgery.<\/p>\n<figure id=\"attachment_60\" aria-describedby=\"caption-attachment-60\" style=\"width: 247px\" class=\"wp-caption alignright\"><img class=\"wp-image-60\" title=\"Photo by Anrie [CC BY-SA 3.0 (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0)], from Wikimedia Commons\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Sterkfontein_Piltdown_man-1.jpg\" alt=\"A replica of the infamous Piltdown skull. The skull is encased in a glass sphere. The replica shows portions of the skull which were bone in white, and the portions of the skull which were inferred in black.\" width=\"247\" height=\"242\"><figcaption id=\"caption-attachment-60\" class=\"wp-caption-text\"><em>Figure 1.7.3 A replica of the infamous Piltdown skull.<\/em><\/figcaption><\/figure>\n<p>In 1912, the head of the geological department at the British Museum proposed that Piltdown man represented an evolutionary missing link between apes and humans. With its human-like cranium and ape-like jaw, it seemed to support the idea then prevailing in England that human evolution began with the brain. The Piltdown specimen led scientists down a blind alley in the belief that the human brain increased in size before the jaw underwent size reductions to become more like the modern human jaw. This belief confused and misdirected the study of human evolution for decades, and actual fossils of early humans were ignored because they didn't support the accepted paradigm.<\/p>\n<h2>The Vaccine-Autism Fraud<\/h2>\n<p>You may have heard that certain vaccines put the health of young children at risk. This persistent idea is not supported by scientific evidence or accepted by the vast majority of experts in the field. It stems largely from an elaborate medical research fraud that was reported in a 1998 article published in the respected British medical journal,\u00a0<em>The Lancet.<\/em>\u00a0The main author of the article was a British physician named <a href=\"https:\/\/en.wikipedia.org\/wiki\/Andrew_Wakefield\">Andrew Wakefield<\/a>. In the article, Wakefield and his colleagues described case histories of 12 children, most of whom were reported to have developed autism soon after the administration of the MMR (measles,\u00a0mumps,\u00a0rubella) vaccine.<\/p>\n<p>Several subsequent peer-reviewed studies failed to show any association between the MMR vaccine and autism. It also later emerged that Wakefield had received research funding from a group of people who were suing vaccine manufacturers.\u00a0In 2004, ten of Wakefield's 12 coauthors formally retracted the conclusions\u00a0in\u00a0their paper. In 2010, editors of\u00a0<em>The Lancet<\/em>retracted the entire paper. That same year, Wakefield was charged with deliberate falsification of research and barred from practicing medicine in the United Kingdom. Unfortunately, by then, the damage had already been done. Parents afraid that their children would develop autism had refrained from having them vaccinated. British MMR vaccination rates fell from nearly 100 per cent to 80 per cent in the years following the study. The consensus of medical experts today is that Wakefield's fraud put hundreds of thousands of children at risk because of the lower vaccination rates and also diverted research efforts and funding away from finding the true cause of autism.<\/p>\n<h2>Correlation-Causation Fallacy<\/h2>\n<p>Many statistical tests used in scientific research calculate correlations between variables. Correlation refers to how closely related two data sets are, which may be a useful starting point for further investigation. Correlation, however, is also one of the most misused types of evidence, primarily because of the logical fallacy that correlation implies causation. In reality, just because two variables are correlated does not\u00a0<em>necessarily<\/em>\u00a0mean that either variable causes the other.<\/p>\n<p>A few simple examples, illustrated by the graphs\u00a0below, can be used to demonstrate the correlation-causation fallacy. Assume a study found that both per capita consumption of mozzarella cheese and the number of Civil Engineering doctorates awarded are correlated; that is, rates of both events increase together. If correlation really did imply causation, then you could conclude from the second example that the increase in age of Miss America causes an increase in murders of a specific type or vice versa.<\/p>\n<figure id=\"attachment_126\" aria-describedby=\"caption-attachment-126\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-126\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cheese-and-Degrees-4-1.png\" alt=\"A chart showing the correlation between per capita consumption of mozzarella cheese, and the number of civil engineering doctorates awarded.\" width=\"900\" height=\"355\"><figcaption id=\"caption-attachment-126\" class=\"wp-caption-text\">Figure 1.7.4 Spurious Correlations [Causation Fallacy] - Consumption of mozzarella cheese and awarded Doctorates<\/figcaption><\/figure>\n<figure id=\"attachment_141\" aria-describedby=\"caption-attachment-141\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-141\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ms-America-and-Murder-1-1-scaled.jpg\" alt=\"A chart showing a correlation between the age of Miss America, and the number of Murders by steam, hot vapours, and hot objects.\" width=\"900\" height=\"355\"><figcaption id=\"caption-attachment-141\" class=\"wp-caption-text\">Figure 1.7.5 Spurious Correlations (Causation Fallacy)- Miss America and Murder<\/figcaption><\/figure>\n<p>An actual example of the correlation-causation fallacy occurred during the latter half of the 20th century. Numerous studies showed that women taking hormone replacement therapy (HRT) to treat menopausal symptoms also had a lower-than-average incidence of coronary heart disease (CHD). This correlation was misinterpreted as evidence that HRT protects women against CHD. Subsequent studies that controlled other factors related to CHD disproved this presumed causal connection. The studies found that women taking HRT were more likely to come from higher socio-economic groups, with better-than-average diets and exercise regimens. Rather than HRT causing lower CHD incidence, these studies concluded that HRT and lower CHD were both effects of higher socio-economic status and related lifestyle factors.<\/p>\n<p>Check out this \u201cRough Guide to Spotting Bad Science\u201d infographic from Compound Interest:<\/p>\n<figure id=\"attachment_74\" aria-describedby=\"caption-attachment-74\" style=\"width: 1754px\" class=\"wp-caption alignnone\"><img class=\"wp-image-74 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Guide-to-Spotting-Bad-Science-1.png\" alt=\"\" width=\"1754\" height=\"2480\"><figcaption id=\"caption-attachment-74\" class=\"wp-caption-text\">Figure 1.7.6 A Rough Guide to Spotting Bad Science.<\/figcaption><\/figure>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.7 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Pseudoscience is a claim, belief, or practice that is presented as scientific, but does not adhere to scientific standards and methods.<\/li>\n<li>Indicators of pseudoscience include untestable claims, lack of openness to testing by experts, absence of progress in advancing knowledge, and attacks\u00a0on the\u00a0motives and character of critics.<\/li>\n<li>Some pseudosciences, including astrology, remain popular. This suggests\u00a0that many people do not possess the scientific literacy needed to distinguish\u00a0pseudoscience\u00a0from true science, or to be convinced by scientific arguments against them.<\/li>\n<li>Belief in a pseudoscience such as astrology is unlikely to cause harm, but belief in pseudoscientific medical treatments may be harmful.<\/li>\n<li>In addition to pseudoscience, other examples of the misuse of science include scientific hoaxes (such as the Piltdown hoax), scientific frauds (such as the MMR vaccine-autism fraud), and scientific fallacies (such as the correlation-causation fallacy).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.7 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Define pseudoscience. Give three examples.<\/li>\n<li>What are some indicators that a claim, belief, or practice might be pseudoscience rather than true science?<\/li>\n<li>Astrology was once considered a science, and it was common in academic circles. Why did its status change from a science to a pseudoscience?<\/li>\n<li>What are possible reasons that some pseudosciences remain popular even after they have been shown to have no scientific validity or explanatory power?<\/li>\n<li>List three other ways besides pseudoscience that science can be misused, and identify an example of each.<\/li>\n<li>Explain how misuses of science may waste money and effort.\u00a0How can they\u00a0potentially cause harm to the public?<\/li>\n<li>Many claims made by pseudoscience cannot be tested with evidence. From a scientific perspective, why is it important that claims be testable?<\/li>\n<li>What do you think is the difference between pseudoscience and belief?<\/li>\n<li>If you see a website that claims that an herbal supplement causes weight loss and they use a lot of scientific terms to explain how it works, can you be assured that the drug is scientifically proven to work? If not, what are some steps you can take to determine whether or not the drug does in fact work?<\/li>\n<li>Why do you think it was problematic that Andrew Wakefield received funding from a group of people who were suing vaccine manufacturers?<\/li>\n<li>What do you think it says about the 1998 Wakefield paper that ten of the 12 coauthors formally retracted their conclusions?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.7 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=E91bGT9BjYk<\/p>\n<p style=\"text-align: center\">How to spot a misleading graph - Lea Gaslowitz, TED-Ed, 2017.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=sxYrzzy3cq8<\/p>\n<p style=\"text-align: center\">How statistics can be misleading - Mark Liddell, TED-Ed, 2016.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h3 id=\"x-ck12-YzhkZDM1NjIyZWY2OTkxNDVlODQ3NzVmZGJkNzkyYmY.-wlj\"><strong>Attributions<\/strong><\/h3>\n<p><strong>Figure 1.7.1<\/strong><\/p>\n<p><a href=\"https:\/\/www.maxpixel.net\/Zodiac-Gemini-Aries-Aquarius-Cancer-Signs-Leo-29420\">Zodiac Signs Cancer Aquarius Aries Gemini Leo<\/a> from <a href=\"http:\/\/maxpixel.net\">Max Pixel<\/a>, is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en\">CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).<\/p>\n<p><strong>Figure 1.7.2<\/strong><\/p>\n<p id=\"firstHeading\" class=\"firstHeading\" lang=\"en\"><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Piltdown_Man_-_McGregor_model.jpg\">Piltdown Man - McGregor model<\/a>, by <a href=\"https:\/\/archive.org\/stream\/practicaltreatis00case\/practicaltreatis00case#page\/46\/mode\/2up\">James Howard McGregor<\/a> on Wikimedia Commons is in the <a href=\"https:\/\/commons.wikimedia.org\/wiki\/Commons:Licensing#Material_in_the_public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain)<b>.<\/b><\/p>\n<p><strong>Figure 1.7.3<\/strong><\/p>\n<p id=\"firstHeading\" class=\"firstHeading\" lang=\"en\"><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sterkfontein_Piltdown_man.jpg\">Sterkfontein Piltdown man<\/a>, by <a href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Anrie\">Anrie <\/a>\u00a0on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/p>\n<p><strong>Figure 1.7.4<\/strong><\/p>\n<p><a href=\"http:\/\/www.tylervigen.com\/spurious-correlations\">Spurious Correlations (Causation Fallacy) - Consumption of mozzarella cheese and awarded Doctorates<\/a> by Tyler Vigen on <a href=\"http:\/\/tylervigen.com\">Tylervigen.com<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.<\/p>\n<p><strong>Figure 1.7.5<\/strong><\/p>\n<p><a href=\"http:\/\/www.tylervigen.com\/spurious-correlations\">Spurious Correlations (Causation Fallacy) - Miss America and Murder<\/a>, by Tyler Vigen, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.<\/p>\n<p><strong>Figure 1.7.6<\/strong><\/p>\n<p><a href=\"https:\/\/www.compoundchem.com\/2014\/04\/02\/a-rough-guide-to-spotting-bad-science\/\">A rough guide to spotting bad science,<\/a> by <a href=\"http:\/\/compoundchem.com\">Compound Interest<\/a>, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/ca\/\">CC BY-NC-ND 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/ca\/) license<\/p>\n<h3><strong>References<\/strong><\/h3>\n<p class=\"hanging-indent\">TED-Ed. (2017, July 6). How to spot a misleading graph - Lea Gaslowitz. YouTube. https:\/\/www.youtube.com\/watch?v=E91bGT9BjYk&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Wakefield, A.J., Murch, S.H., Anthony, A., Linnell, J., Casson, D.M., Malik, M., et al. (1998). Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. <em>Lancet<\/em>, 351: 637\u201341.<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, June 18). Andrew Wakefield. <i>Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Andrew_Wakefield&amp;oldid=963243135<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3975\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3975\"><div tabindex=\"-1\"><p>Created by:\u00a0CK-12\/Adapted by Christine Miller<\/p>\n<p>After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health. In Joseph\u2019s situation, chemistry is involved in the reasons why his father has diabetes, why his personal risk of getting diabetes is high, and why the dietary changes he is considering could be effective.<\/p>\n<figure id=\"attachment_1376\" aria-describedby=\"caption-attachment-1376\" style=\"width: 504px\" class=\"wp-caption alignleft\"><img class=\" wp-image-1376\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Prevalence_of_Diabetes_by_Percent_of_Country_Population_2014_Gradient_Map-1.png\" alt=\"Diagram shows a map of places in the world where diabetes is most prevalent. Northern Africa and the Middle East have high prevalence and South East Africa has low prevalence.\" width=\"504\" height=\"275\"><figcaption id=\"caption-attachment-1376\" class=\"wp-caption-text\"><em>Figure 3.13.1. Prevalence of diabetes by per cent of country population.<\/em><\/figcaption><\/figure>\n<p>Type 2 diabetes affects populations worldwide and is caused\u00a0primarily\u00a0by a lack of response in the body to the hormone insulin, which causes problems in the regulation of blood sugar, or glucose. Insulin is a peptide hormone, and as you have learned, peptides are chains of amino acids. Therefore, insulin is in the class of biochemical compounds called proteins. Joseph is at increased risk of diabetes partly because there is a genetic component to the disease. DNA, which is a type of chemical compound called a nucleic acid, is passed down from parents to their offspring, and carries the instructions for the production of proteins in units called genes. If there is a problem in a gene (or genes) that contributes to the development of a disease, such as type 2 diabetes, this can get passed down to the offspring and may raise that child\u2019s risk of getting the disease.<\/p>\n<p>But genetics is only part of the reason why Joseph is at an increased risk of diabetes. Obesity itself is a risk factor, and one that can be shared in families due to shared lifestyle factors (such as poor diet and lack of exercise), as well as\u00a0genetics. Consumption of too many refined carbohydrates (like white bread and soda) may also contribute to obesity and the development of diabetes. As you probably now know, these simple carbohydrates are more easily and quickly broken down in the digestive system into glucose than larger complex carbohydrate molecules, such as those found in vegetables and whole grains. This can lead to dramatic spikes in blood sugar levels, which is particularly problematic for people with diabetes because they have trouble maintaining their blood sugar at a safe level. You can understand why Joseph\u2019s father limits his consumption of refined carbohydrates, and in fact, some scientific studies have shown that avoiding refined carbohydrates may actually help reduce the risk of getting diabetes in the first place.<\/p>\n<figure id=\"attachment_1377\" aria-describedby=\"caption-attachment-1377\" style=\"width: 305px\" class=\"wp-caption alignright\"><img class=\" wp-image-1377\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Healthy-meal-1.jpg\" alt=\"Image shows a plate of food containing a salad, fish and broccoli.\" width=\"305\" height=\"305\"><figcaption id=\"caption-attachment-1377\" class=\"wp-caption-text\"><em>Figure 3.13.2. A diet high in vegetables and lean meats can help reduce the risk of Type 2 Diabetes.<\/em><\/figcaption><\/figure>\n<p>Joseph\u2019s friend recommended eating a low fat, high carbohydrate diet to lose weight, but you can see that\u00a0the\u00a0<em>type<\/em> of carbohydrate \u2014 simple or complex \u2014 is an important consideration. Eating a large amount of white bread and rice may not help Joseph reduce his risk of diabetes, but a healthy diet that helps him lose weight may lower his risk of diabetes, since obesity itself is a factor. Which specific diet will work best to help him lose weight probably depends on a variety of factors, including his biology, lifestyle, and food preferences. Joseph should consult with his doctor about his diet and exercise plan, so that his specific situation can be taken into account and monitored by a medical professional.<\/p>\n<p>Drinking enough water is usually good advice for everyone, especially if it replaces sugary drinks like soda. You now know that water is important for many of the chemical reactions that take place in the body. But you can have too much of a good thing \u2014 as in the case of marathon runners who can make themselves sick from drinking too much water! As you can see, proper balance, or homeostasis, is very important to the health of living organisms.<\/p>\n<p>Finally, you probably now realize that \u201cchemicals\u201d do not have to be scary, toxic substances. All matter consists of chemicals, including water, your body, and healthy fresh fruits and vegetables, like the ones pictured in Figure 3.12.2. When people advocate \u201cclean eating\u201d and avoiding \u201cchemicals\u201d in food, they are usually referring to avoiding synthetic \u2014 or man-made \u2014 chemical additives, such as preservatives. This can be a healthy way to eat because it involves eating a variety of whole, fresh, unprocessed foods. But there is no reason to be scared of chemicals in general \u2014 they are simply molecules and how they react depends on what they are, what other molecules are present, and the environmental conditions surrounding them.<\/p>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">Chapter 3 Summary<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>By now, you should have a good understanding of the basics of the chemistry of life. Specifically, you have learned:<\/p>\n<ul>\n<li>All <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3543\">matter<\/a> consists of chemical substances. A chemical substance has a definite and consistent composition and may be either an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3655\">element<\/a> or a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3641\">compound<\/a>.<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3655\">element<\/a> is a pure substance that cannot be broken down into other types of substances.\n<ul>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3619\">atom<\/a> is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3659\">electrons<\/a>, positive <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3731\">protons<\/a>, and neutral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1365\">neutrons<\/a>. The number of protons in an atom determines the element it represents.<\/li>\n<li>Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, so they have either a positive or negative charge. Atoms with the same number of protons but different numbers of neutrons are called isotopes.<\/li>\n<li>There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals, including carbon, hydrogen, and oxygen.<\/li>\n<\/ul>\n<\/li>\n<li>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3641\">compound<\/a> is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.\n<ul>\n<li>Biochemical compounds are carbon-based compounds found in living things. They make up <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3573\">cells<\/a>\u00a0and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3713\">polymers<\/a>\u00a0that consist of many repeating units of smaller molecules called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3689\">monomers<\/a>.<\/li>\n<li>There are millions of different biochemical compounds, but all of them fall into four major classes: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3563\">carbohydrates<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3559\">lipids<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">proteins<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3383\">nucleic acids<\/a>.<\/li>\n<\/ul>\n<\/li>\n<li>Carbohydrates are the most common class of biochemical compounds. They provide cells with energy, store energy, and make up organic structures, such as the cell walls of plants. The basic building block of carbohydrates is the monosaccharide.\n<ul>\n<li>Sugars are short-chain carbohydrates that supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose (or table sugar) consist of two monosaccharides and are called disaccharides.<\/li>\n<li>Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin.\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3367\">Starch<\/a> is made by plants to store energy and is readily broken down into its component sugars during digestion.<\/li>\n<li>Glycogen is made by animals and fungi to store energy and plays a critical part in the homeostasis of blood glucose levels in humans.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_333\">Cellulose<\/a> is the most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_335\">Chitin<\/a> makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<li>Lipids include fats and oils. They store energy, form cell membranes, and carry messages.\n<ul>\n<li>Lipid molecules consist mainly of repeating units called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3565\">fatty acids<\/a>. Fatty acids may be <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3601\">saturated<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3593\">unsaturated<\/a>, depending on the proportion of hydrogen atoms they contain. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.<\/li>\n<li>Types of lipids include triglycerides, phospholipids, and steroids.\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3587\">Triglycerides <\/a>contain glycerol (an alcohol) in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_368\">Phospholipids<\/a> contain phosphate and glycerol in addition to fatty acids. They are the main component of cell membranes in all living things.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_375\">Steroids<\/a> are lipids with a four-ring structure. Some steroids, such as cholesterol, are important components of cell membranes. Many other steroids are hormones.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<li>In living things, proteins include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3665\">enzymes<\/a>, antibodies, and numerous other important compounds. They\u00a0help\u00a0cells keep their shape, make up muscles, speed up chemical reactions, and carry messages and materials (among other functions).\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">Proteins<\/a> are made up of small monomer molecules called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3615\">amino acids<\/a>.<\/li>\n<li>Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Secondary structure refers to configurations such as helices and sheets within polypeptide chains. Tertiary structure is a protein's overall three-dimensional shape, which controls the molecule's basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.<\/li>\n<li>The chief characteristic that allows proteins' diverse functions is their ability to bind specifically and tightly with other molecules.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3383\">Nucleic acids<\/a> include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_519\">RNA<\/a>. They encode instructions for making proteins, helping make proteins, and passing the encoded instructions from parents to offspring.\n<ul>\n<li>Nucleic acids are built of monomers called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_518\">nucleotides<\/a>, which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.<\/li>\n<li>Each nucleotide consists of a sugar molecule, phosphate group, and nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the \"backbone\" of the polynucleotide. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.<\/li>\n<li>DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, explaining how inherited characteristics are passed from one generation to the next.<\/li>\n<\/ul>\n<\/li>\n<li>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3637\">chemical reaction<\/a> is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">reactant<\/a>, and a substance that forms in a chemical reaction is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">product<\/a>. During the chemical reaction, bonds break in reactants and new bonds form in products.<\/li>\n<li>Chemical reactions can be represented by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3415\">chemical equations<\/a>. According to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3417\">law of conservation of mass<\/a>, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.<\/li>\n<li>Many chemical reactions occur all around us each day, such as iron rusting and organic matter rotting, but not all changes are chemical processes. Some changes, such as ice melting or paper being torn into smaller pieces, are physical processes that do not involve chemical reactions and the formation of new substances.<\/li>\n<li>All chemical reactions involve energy, and\u00a0they require\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3427\">activation energy<\/a> to begin. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3421\">Exothermic reactions<\/a>\u00a0release energy. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3425\">Endothermic reactions<\/a>\u00a0absorb energy.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3581\">Biochemical reactions<\/a>\u00a0are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is\u00a0called\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3681\">metabolism<\/a>. Metabolism includes catabolic reactions (exothermic reactions) and anabolic reactions (endothermic reactions).<\/li>\n<li>Most biochemical reactions\u00a0require\u00a0a biological <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3577\">catalyst<\/a> called an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3665\">enzyme<\/a> to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme's substrate.<\/li>\n<li>Virtually all living things on Earth require liquid water. Only a tiny per cent of Earth's water is fresh liquid water. Water exists as a liquid over a wide range of temperatures, and it dissolves many substances. These properties depend on water's polarity, which causes water molecules to \"stick\" together through weak bonds called hydrogen bonds.<\/li>\n<li>The human body is about 70 per cent water (outside of fat). Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.<\/li>\n<li>A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3375\">Acidity<\/a> is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3709\">pH scale<\/a>. Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have\u00a0a lower hydronium ion concentration than pure water and a pH higher than 7.<\/li>\n<li>Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly.<\/li>\n<\/ul>\n<p>Now you understand the chemistry of the molecules that make up living things. In the next chapter, you will learn how these molecules make up the basic unit of structure and function in living organisms \u2014 cells \u2014 and you will be able to understand some of the crucial chemical reactions that occur within cells.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">Chapter 3 Review<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-54\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-54\" class=\"h5p-iframe\" data-content-id=\"54\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Cell biology True or False\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>The chemical formula for the complex carbohydrate glycogen is C<sub>24<\/sub>H<sub>42<\/sub>O<sub>21<\/sub>.\n<ol type=\"a\">\n<li>What are the elements in glycogen?<\/li>\n<li>How many atoms are in one molecule of glycogen?<\/li>\n<li>Is glycogen an ion? Why or why not?<\/li>\n<li>Is glycogen a monosaccharide or a polysaccharide? Besides memorizing this fact, how would you know this based on the information in the question?<\/li>\n<li>What is the function of glycogen in the human body?<\/li>\n<\/ol>\n<\/li>\n<li>What is the difference between an ion and a polar molecule? Give an example of each in your explanation.<\/li>\n<li>Define monomer and polymer.<\/li>\n<li>\n<div id=\"h5p-54\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-54\" class=\"h5p-iframe\" data-content-id=\"54\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Cell biology True or False\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is the difference between a protein and a polypeptide?<\/li>\n<li>\n<div id=\"h5p-56\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-56\" class=\"h5p-iframe\" data-content-id=\"56\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Linkage Map for Human X Chromosome\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>People with diabetes have trouble controlling the level of glucose in their bloodstream. Knowing this, why do you think it is often recommended that people with diabetes limit their consumption of carbohydrates?<\/li>\n<li>Identify each of the following reactions as endothermic or exothermic.\n<ol type=\"a\">\n<li>cellular respiration<\/li>\n<li>photosynthesis<\/li>\n<li>catabolic reactions<\/li>\n<li>anabolic reactions<\/li>\n<\/ol>\n<\/li>\n<li>Pepsin is an enzyme in the stomach that helps us digest protein. Answer the following questions about pepsin:\n<ol type=\"a\">\n<li>What is the substrate for pepsin?<\/li>\n<li>How does pepsin work to speed up protein digestion?<\/li>\n<li>Given what you know about the structure of proteins, what do you think are some of the products of the reaction that pepsin catalyzes?<\/li>\n<li>The stomach is normally acidic. What do you think would happen to the activity of pepsin and protein digestion if the pH is raised significantly?<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 3.13.1<\/strong><\/p>\n<section class=\"standard post-459 chapter type-chapter status-publish hentry focusable\" data-type=\"chapter\">\n<div class=\"media-atttributions\">\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Prevalence_of_Diabetes_by_Percent_of_Country_Population_(2014)_Gradient_Map.png\" rel=\"cc:attributionURL\">Prevalence_of_Diabetes_by_Percent_of_Country_Population_(2014)_Gradient_Map<\/a> by Walter Scott Wilkens [<a class=\"new\" title=\"Wwilken2 (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=Wwilken2&amp;action=edit&amp;redlink=1\">Wwilken2<\/a>], University of Illinois - Urbana Champaign Department of Geography and GIScience, on Wikimedia Commons, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 3.13.2<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/melystu\/30565936328\" rel=\"cc:attributionURL\">Healthy plate<\/a> by <a class=\"owner-name truncate\" title=\"Go to Melinda Young Stuart's photostream\" href=\"https:\/\/www.flickr.com\/photos\/melystu\/\" data-track=\"attributionNameClick\">Melinda Young Stuart<\/a> on Flickr is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/\">CC BY-NC-ND 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/) license.<\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3964\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3964\"><div tabindex=\"-1\"><p>Created by:\u00a0CK-12\/Adapted by Christine Miller<\/p>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\"><span style=\"font-size: 1.424em\">What Are You Made of?<\/span><\/h1>\n<figure id=\"attachment_250\" aria-describedby=\"caption-attachment-250\" style=\"width: 300px\" class=\"wp-caption alignleft\"><img class=\"wp-image-250 size-medium\" style=\"color: #373d3f;font-weight: bold;font-size: 1em\" title=\" Arawalk Cay, The Bahamas, by Gregory Culmer, on Unsplash, is used under the Unsplash license\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Man-sitting-1.jpg\" alt=\"\" width=\"300\" height=\"199\"><figcaption id=\"caption-attachment-250\" class=\"wp-caption-text\"><em>Figure 3.2.1 What are we?<\/em><\/figcaption><\/figure>\n<p>Your entire body is made of cells and cells are made of molecules.If you look at your hand, what do you see? Of course, you see skin, which consists of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3573\">cells<\/a><\/strong>. But what are skin cells\u00a0<em>made<\/em>\u00a0of? Like all living\u00a0cells, they are made of matter. In fact,\u00a0<em>all<\/em>\u00a0things are made of matter.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3543\">Matter<\/a><\/strong>\u00a0is anything that takes up space and has mass. Matter, in turn, is made up of chemical substances. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3639\">chemical substance<\/a><\/strong>\u00a0is matter that has a definite\u00a0composition\u00a0that is\u00a0consistent\u00a0throughout. A chemical substance may be either an\u00a0element\u00a0or a\u00a0compound.<\/p>\n<h1>Elements and Atoms<\/h1>\n<p>An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3655\">element<\/a><\/strong>\u00a0is a\u00a0pure substance. It cannot be broken down into other types of substances. Each\u00a0element\u00a0is made up of just one type of\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3619\">atom<\/a><\/strong>.<\/p>\n<h2>Structure of an\u00a0Atom<\/h2>\n<figure id=\"attachment_253\" aria-describedby=\"caption-attachment-253\" style=\"width: 226px\" class=\"wp-caption alignright\"><img class=\"wp-image-253\" title=\"Lithium Atom diagram, by AG Caesar, is used under a CC BY-SA 4.0 International license\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Atom-diagram-1.png\" alt=\"Diagram of a lithium atom. Three protons and four neutrons are in the nucleus, and three electrons are orbiting the nucleus.\" width=\"226\" height=\"226\"><figcaption id=\"caption-attachment-253\" class=\"wp-caption-text\"><em>Figure 3.2.2 An atom consists of three subatomic components: protons, neutrons and electrons.<\/em><\/figcaption><\/figure>\n<p>An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3619\">atom<\/a><\/strong> is the smallest particle of an element that still has the properties of that element. Every substance is composed of atoms. Atoms are extremely small, typically about a ten-billionth of a metre in diametre. However, atoms <em>do not<\/em> have well-defined boundaries, as suggested by the atomic model shown\u00a0below.<\/p>\n<div>\n<p><span style=\"text-align: initial;font-size: 1em\">Every <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3619\">atom<\/a><\/strong> is composed of a central area \u2014 called the\u00a0<\/span><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3625\">nucleus<\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0\u2014 and one or more subatomic particles called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3659\">electrons<\/a><\/strong>, which move around the nucleus. The nucleus\u00a0<\/span><em style=\"text-align: initial;font-size: 1em\">also<\/em><span style=\"text-align: initial;font-size: 1em\">\u00a0consists of subatomic particles. It contains one or more <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3731\">proton<\/a><\/strong>s and typically a similar number of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3703\">neutrons<\/a><\/strong>. The number of protons in the\u00a0<\/span>nucleus<span style=\"text-align: initial;font-size: 1em\">\u00a0determines the type of element an atom represents. An atom of hydrogen, for example, contains just one\u00a0<\/span>proton<span style=\"text-align: initial;font-size: 1em\">. Atoms of the same element may have different numbers of neutrons in the nucleus. Atoms of the same element with the same number of protons \u2014 but different numbers of neutrons \u2014 are called\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3675\">isotopes<\/a>.<\/strong><\/p>\n<\/div>\n<p>Protons have a positive electric charge and neutrons have no electric charge. Virtually all of an atom's mass is in the protons and neutrons in the\u00a0nucleus. Electrons surrounding the nucleus have almost no mass,\u00a0as well as\u00a0a negative electric charge. If the number of protons and electrons in an atom are equal, then an atom is electrically neutral, because the positive and negative charges cancel each other out. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3673\">ion<\/a><\/strong><strong>.<\/strong><\/p>\n<p>The negatively-charged electrons of an atom are attracted to the positively-charged protons in the nucleus by a force called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3653\">electromagnetic force<\/a>,<\/strong>\u00a0for which opposite charges attract. Electromagnetic force between protons in the nucleus causes these subatomic particles to repel each other, because they have the same charge. However, the protons and neutrons in the nucleus are attracted to each other by a different force, called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3695\">nuclear force<\/a>,<\/strong>\u00a0which is usually stronger than the electromagnetic force. Nuclear force repels the positively-charged protons from each other.<\/p>\n<h2>Periodic Table of the Elements<\/h2>\n<p>There are almost 120 known elements. As you can see in the Periodic Table of the Elements shown\u00a0below, the majority of elements are\u00a0metals. Examples of metals are iron (Fe) and copper (Cu). Metals are shiny and good conductors of electricity and\u00a0heat. Nonmetal elements are far fewer in number. They include hydrogen (H) and oxygen (O). They lack the properties of metals.<\/p>\n<div>\n<p>\u00a0The periodic table of the elements arranges elements in groups based on their properties. The element most important to life is carbon (C). Find carbon in the table. What type of element is it: metal or nonmetal?<\/p>\n<\/div>\n<div>\n<figure id=\"attachment_259\" aria-describedby=\"caption-attachment-259\" style=\"width: 573px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-259\" title=\"Periodic Table Armtuk3 by Armtuk, is used under CC BY-SA 3.0 license.\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Periodic-table-1.png\" alt=\"The Periodic Table of Elements\" width=\"573\" height=\"470\"><figcaption id=\"caption-attachment-259\" class=\"wp-caption-text\"><em>Figure 3.2.3 The Periodic Table of Elements.<\/em><\/figcaption><\/figure>\n<h2>Compounds and Molecules<\/h2>\n<\/div>\n<p>A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3641\">compound<\/a><\/strong>\u00a0is a unique substance that consists of two or more elements combined in fixed proportions. This means that the\u00a0composition\u00a0of a\u00a0compound\u00a0is always the same. The smallest particle of most compounds in living things is called a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3687\">molecule<\/a>.<\/strong><\/p>\n<figure id=\"attachment_285\" aria-describedby=\"caption-attachment-285\" style=\"width: 256px\" class=\"wp-caption alignright\"><img class=\"wp-image-285 size-full\" title=\"Water molecule, by Sakurambo, is released into the public domain.\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Water-molecule-1-1.png\" alt=\"Image shows a model of a water molecule. A large central oxygen atom is connected to two adjacent, smaller white hydrogen atoms.\" width=\"256\" height=\"184\"><figcaption id=\"caption-attachment-285\" class=\"wp-caption-text\"><em>Figure 3.2.4 A molecule of water consists of one atom of oxygen and two atoms of hydrogen connected by covalent bonds.<\/em><\/figcaption><\/figure>\n<p>Consider\u00a0water\u00a0as an example. A molecule of water always contains one atom of oxygen and two atoms of hydrogen. The\u00a0composition\u00a0of water is expressed by the\u00a0chemical formula\u00a0H<sub>2<\/sub>O. A model of a water molecule is shown in Figure 3.2.4.<\/p>\n<p>What causes the atoms of a\u00a0water\u00a0molecule to \u201cstick\u201d together? The answer is\u00a0<em>chemical bonds<\/em>. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3635\">chemical bond<\/a><\/strong>\u00a0is a force that holds together the atoms of molecules. Bonds in molecules involve the sharing of electrons among atoms. New\u00a0chemical bonds\u00a0form when substances react with one another. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3637\">chemical reaction<\/a><\/strong>\u00a0is a process that changes some chemical substances into others. A\u00a0chemical reaction\u00a0is needed to form a compound, and another chemical reaction is needed to separate the substances in that compound.<\/p>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.2 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>All <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3543\"><strong>matter<\/strong><\/a> consists of chemical substances. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3639\"><strong>chemical substance<\/strong><\/a> has a definite composition\u00a0which is consistent\u00a0throughout. A chemical substance may be either an element or a compound.<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3655\"><strong>element<\/strong><\/a> is a\u00a0pure substance\u00a0that cannot be broken down into other types of substances.<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3619\"><strong>atom<\/strong><\/a> is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3659\"><strong>electrons<\/strong><\/a>, positive <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3731\"><strong>protons<\/strong><\/a>, and neutral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3703\"><strong>neutrons<\/strong><\/a>. The number of protons in an atom determines the element it represents.<\/li>\n<li>Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons,\u00a0and as a result\u00a0have either a positive or negative charge. Atoms with the same number of protons \u2014 but different numbers of neutrons \u2014 are called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3675\"><strong>isotopes<\/strong><\/a>.<\/li>\n<li>There are almost 120 known elements. The majority of elements are\u00a0metals. A smaller number are\u00a0nonmetals. The latter include carbon, hydrogen, and oxygen.<\/li>\n<li>A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3687\"><strong>molecule<\/strong><\/a>.\u00a0Chemical bonds\u00a0hold together the atoms of molecules. Compounds can form only in\u00a0chemical reactions, and they can break down only in other chemical reactions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.2 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-10\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-10\" class=\"h5p-iframe\" data-content-id=\"10\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Definitions of Traditional Ecological Knowledge.\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is an element? Give three examples.<\/li>\n<li>Define <em>compound<\/em>. Explain how compounds form.<\/li>\n<li>Compare and contrast atoms and molecules.<\/li>\n<li>The compound called water can be broken down into its constituent elements by applying an electric current to it. What ratio of elements is produced in this process?<\/li>\n<li>Relate ions and isotopes to elements and atoms.<\/li>\n<li>What is the most important element to life?<\/li>\n<li>Iron oxide is often known as rust \u2014 the reddish substance you might find on corroded metal. The chemical formula for this type of iron oxide is Fe<sub>2<\/sub>O<sub>3<\/sub>. Answer the following questions about iron oxide and briefly explain each answer.\n<ol type=\"a\">\n<li>Is iron oxide an element or a compound?<\/li>\n<li>Would one particle of iron oxide be considered a molecule or an atom?<\/li>\n<li>Describe the relative proportion of atoms in iron oxide.<\/li>\n<li>What causes the Fe and O to stick together in iron oxide?<\/li>\n<li>Is iron oxide made of metal atoms, metalloid atoms, nonmetal atoms, or a combination of any of these?<\/li>\n<\/ol>\n<\/li>\n<li>14C is an isotope\u00a0of carbon used in the radiocarbon dating of organic material. The most common isotope of carbon is\u00a012C. Do you think\u00a014C and\u00a012C have different numbers of neutrons or protons? Explain your answer.<\/li>\n<li>Explain why ions have a positive or negative charge.<\/li>\n<li>Name the three subatomic particles described in this section.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">3.2 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=yQP4UJhNn0I&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Just how small is an atom? TED-Ed, 2012<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 3.2.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/z7uU0C-4iUo\">Man Sitting<\/a>, by <a href=\"https:\/\/unsplash.com\/@junkanoo_media\">Gregory Culmer<\/a>, on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a>, is used under the <a href=\"https:\/\/unsplash.com\/license\">Unsplash license<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 3.2.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Atom_Diagram.svg\">Lithium Atom diagram<\/a>, by <a href=\"https:\/\/commons.wikimedia.org\/wiki\/User:AG_Caesar?uselang=bn\">AG Caesar<\/a>, is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en\">CC BY-SA 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en)<\/p>\n<p><strong>Figure 3.2.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Periodic_Table_Armtuk3.svg\">Periodic Table Armtuk3<\/a>, by <a href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Armtuk&amp;action=edit&amp;redlink=1\">Armtuk<\/a>, is used under a <a href=\"http:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/<a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/\">)<\/a>\u00a0license.<\/p>\n<p><strong>Figure 3.2.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Water_molecule.svg\">Water molecule<\/a>, by <a href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Sakurambo~commonswiki\">Sakurambo<\/a>, is released into the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">TED-Ed. (2012, April 16). Just how small is an atom. YouTube. https:\/\/www.youtube.com\/watch?v=yQP4UJhNn0I&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4002\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4002\"><div tabindex=\"-1\"><p>Created by: CK-12\/Adapted by Christine Miller<\/p>\n<p>&nbsp;<\/p>\n<div id=\"h5p-75\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-75\" class=\"h5p-iframe\" data-content-id=\"75\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Mendelian Inheritance\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 5.14.1 Collage of Diverse Faces.<\/em><\/p>\n<p>This collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between.\u00a0 As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2537\">non-Mendelian inheritance<\/a>,<\/strong>\u00a0and they include inheritance of\u00a0multiple allele traits, traits with codominance or incomplete dominance, and\u00a0polygenic traits, among others. All of\u00a0these modes\u00a0are described below.<\/p>\n<div>\n<h1>Multiple Allele Traits<\/h1>\n<\/div>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2709\" aria-describedby=\"caption-attachment-2709\" style=\"width: 385px\" class=\"wp-caption alignright\"><img class=\" wp-image-2709\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Blood-types-1-1.png\" alt=\"ABO Blood types\" width=\"385\" height=\"408\"><figcaption id=\"caption-attachment-2709\" class=\"wp-caption-text\"><em>Figure 5.14.2 ABO blood types per genotype.<\/em><\/figcaption><\/figure>\n<p>The majority of human genes are thought to have more than two normal versions, or\u00a0alleles. Traits controlled by a single gene with more than two alleles are called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2540\">multiple allele traits<\/a><\/strong>. An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters A, B, and O.<\/p>\n<p>As shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both AA and AO genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both BB and BO genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the OO genotype have no antigens for ABO blood type in their blood (type O blood).<\/p>\n<div>\n<h1>Codominance<\/h1>\n<\/div>\n<p>Look at the genotype AB in the ABO blood group table. Alleles A and B for ABO blood type\u00a0are neither <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3881\">dominant<\/a> nor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2479\">recessive<\/a> to one another. Instead, they are codominant.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2544\">Codominance<\/a><\/strong>\u00a0occurs when two alleles for a gene are expressed equally in the phenotype of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2474\">heterozygotes<\/a>. In the case of ABO blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).<\/p>\n<div>\n<h1>Incomplete Dominance<\/h1>\n<\/div>\n<p>Another relationship that may occur between alleles for the same gene is\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2545\">incomplete dominance<\/a>.<\/strong>\u00a0This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce\u00a0proteins, but one\u00a0protein\u00a0is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.<\/p>\n<p>An example of incomplete dominance in humans is <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/tay-sachs-disease\/symptoms-causes\/syc-20378190\">Tay Sachs disease<\/a>. The normal allele for the gene in this case produces an\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3665\">enzyme<\/a>\u00a0that is responsible for breaking down\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3559\">lipids<\/a>. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal\u00a0development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain\u00a0starting\u00a0<em>in utero<\/em>, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_6216\" aria-describedby=\"caption-attachment-6216\" style=\"width: 551px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-6216 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Incomplete-dominance-hair-1-1.png\" alt=\"5.14.2 Incomplete dominance of hair.\" width=\"551\" height=\"348\"><figcaption id=\"caption-attachment-6216\" class=\"wp-caption-text\"><em>Figure 5.14.3 Three phenotypes of hair through the incomplete dominance model.<\/em><\/figcaption><\/figure>\n<p>Another good example of incomplete dominance in humans is hair type.\u00a0 There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.<\/p>\n<div>\n<h1>Polygenic Traits<\/h1>\n<\/div>\n<figure id=\"attachment_2549\" aria-describedby=\"caption-attachment-2549\" style=\"width: 500px\" class=\"wp-caption alignleft\"><img class=\"size-full wp-image-2549\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Average-height-1.png\" alt=\"Like many other polygenic traits, adult height has a bell-shaped distribution.\" width=\"500\" height=\"363\"><figcaption id=\"caption-attachment-2549\" class=\"wp-caption-text\"><em>Figure 5.14.4 Human Adult Height. Like many other polygenic traits, adult height has a bell-shaped distribution.<\/em><\/figcaption><\/figure>\n<p>Many human traits are controlled by more than one gene. These traits are called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2548\">polygenic traits<\/a><\/strong>. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.<\/p>\n<p>An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult\u2019s height might be 1.655 m (5.430 feet), and another adult\u2019s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in <span style=\"text-align: initial;font-size: 1em\">Figure 5.14.4.<\/span><\/p>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Environmental Effects on Phenotype<\/span><\/p>\n<\/div>\n<figure id=\"attachment_2550\" aria-describedby=\"caption-attachment-2550\" style=\"width: 236px\" class=\"wp-caption alignright\"><img class=\"wp-image-2550\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Tan-lines-1.jpg\" alt=\"Image shows a hand with a tan line where a watch had been worn.\" width=\"236\" height=\"236\"><figcaption id=\"caption-attachment-2550\" class=\"wp-caption-text\"><em>Figure 5.14.5 Due to the effects of UV radiation, the skin on the upper part of the arm is much darker in color than the\u00a0 skin that was protected by a watch strap.<\/em><\/figcaption><\/figure>\n<p>Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.<\/p>\n<div>\n<p>&nbsp;<\/p>\n<\/div>\n<div>\n<h1>Pleiotropy<\/h1>\n<\/div>\n<p>Some genes affect more than one phenotypic trait. This is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2552\">pleiotropy<\/a><\/strong>. There are numerous examples of pleiotropy in humans. They generally involve important\u00a0proteins\u00a0that are needed for the normal\u00a0development\u00a0or functioning of more than one organ system. An example of\u00a0pleiotropy in humans occurs with the gene that codes for the main\u00a0protein\u00a0in collagen, a substance that helps form\u00a0bones. This protein is also important in the ears and\u00a0eyes.\u00a0Mutations\u00a0in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.<\/p>\n<p>Another example of pleiotropy occurs with <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/sickle-cell-anemia\/symptoms-causes\/syc-20355876\">sickle cell anemia<\/a>. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">protein<\/a> called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.<\/p>\n<figure id=\"attachment_2553\" aria-describedby=\"caption-attachment-2553\" style=\"width: 385px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2553\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Sickle-cell-anemia-1.jpg\" alt=\"Image shows the difference in morphology between a sickle cell and a normal red blood cell.\" width=\"385\" height=\"434\"><figcaption id=\"caption-attachment-2553\" class=\"wp-caption-text\"><em>Figure 5.14.6 For comparison, the sickle-shaped red blood cell on the left is shown next to several normal red blood cells.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Epistasis<\/span><\/p>\n<\/div>\n<p>Some genes affect the expression of other genes. This is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3901\">epistasis<\/a><\/strong>. Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.<\/p>\n<p>Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin\u00a0color. Albinism occurs because a protein called tyrosinase, which is needed for the\u00a0production of normal skin pigment, is not produced, due to a gene\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2381\">mutation<\/a>.\u00a0If an individual has the albinism\u00a0mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.<\/p>\n<div>\n<h3>Feature: My\u00a0Human Body<\/h3>\n<\/div>\n<p>Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood\u00a0plasma\u00a0(the\u00a0liquid\u00a0part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood\u00a0cells will clump together. The agglutination reaction is serious and potentially fatal.<\/p>\n<p>Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2555\" aria-describedby=\"caption-attachment-2555\" style=\"width: 553px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2555\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/ABO_blood_type.svg_-1.png\" alt=\"Image shows a table of each blood type, which antigens and antibodies are present, and acceptable blood donor types.\" width=\"553\" height=\"356\"><figcaption id=\"caption-attachment-2555\" class=\"wp-caption-text\"><em>Figure 5.14.7 Antigens and antibodies in ABO blood types.<\/em><\/figcaption><\/figure>\n<div>\n<p>&nbsp;<\/p>\n<\/div>\n<p>You may have heard that people with blood type O are called \"universal donors,\" and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?<\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">5.14 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2537\">Non-Mendelian inheritance<\/a> refers to the inheritance of traits that have a more complex genetic basis than one <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3429\">gene<\/a> with two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3357\">alleles<\/a> and complete <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3879\">dominance<\/a>.<\/li>\n<li>Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type, for which there are three common alleles: A, B, and O.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2544\">Codominance<\/a> occurs when two alleles for a gene are expressed equally in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2477\">phenotype<\/a> of heterozygotes. A human example of codominance also occurs in the ABO blood type, in which the A and B alleles are codominant.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2545\">Incomplete dominance<\/a> is the case in which the dominant allele for a gene is not completely dominant to a recessive allele for the gene, so an intermediate phenotype occurs in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2474\">heterozygotes<\/a> who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional\u00a0enzyme\u00a0as normal homozygotes.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2548\">Polygenic traits<\/a> are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a whole continuum of phenotypes. Examples of human polygenic traits include skin color and adult\u00a0height.<\/li>\n<li>Many traits are affected by\u00a0the environment, as well as by genes. This may be especially true for polygenic traits.\u00a0Skin color, for example, may be affected by exposure to UV light, and adult stature may be affected by diet or\u00a0childhood\u00a0disease.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2552\">Pleiotropy<\/a> refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia. People who inherit two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2479\">recessive<\/a> alleles for this disorder have abnormal red blood cells and may exhibit multiple other phenotypic effects, such as stunting of physical growth, kidney failure, and strokes.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3901\">Epistasis<\/a> is the situation in which one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin color genes.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">5.14 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is non-Mendelian inheritance?<\/li>\n<li>\n<div id=\"h5p-76\">\n<div class=\"h5p-content\" data-content-id=\"76\"><\/div>\n<\/div>\n<\/li>\n<li>Explain why the human ABO blood group is an example of a multiple allele trait with codominance.<\/li>\n<li>What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans.<\/li>\n<li>Explain the genetic basis of human skin color.<\/li>\n<li>How can the human trait of adult height be influenced by the environment?<\/li>\n<li>Define pleiotropy, and give a human example.<\/li>\n<li>Compare and contrast epistasis and dominance.<\/li>\n<li>What is the difference between pleiotropy and epistasis?<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">5.14 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?time_continue=1&amp;v=YJHGfbW55l0&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!,<br \/>\nAmoeba Sisters, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=-4vsio8TZrU&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Non-Mendelian Genetics, Teacher's Pet, 2015.<\/p>\n<\/div>\n<\/div>\n<h2>Attributes<\/h2>\n<p><strong>Figure 5.14.1<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/unsplash.com\/s\/photos\/portraits\">Woman's Face from Iran<\/a> by <span class=\"_1JARO\"><a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@omidarmin\">Omid Armin<\/a><\/span> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/7ip-IOC2yvA\">Woman Wearing Black Coat<\/a> by <a href=\"https:\/\/unsplash.com\/@pinkkilla\">Anastasiya Pavlova<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><span style=\"background-color: #ffffff\"><a class=\"_2RL3a xLon9\" style=\"background-color: #ffffff\" href=\"https:\/\/unsplash.com\/photos\/jns8BPueAgU\"><span class=\"_20NLS _1ByhS\">Dark haired man, Queretaro, M\u00e9xico<\/span><\/a> by <span class=\"_1JARO\"><a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@ldhai\">Leonel Hernandez Arteaga<\/a><\/span> on <a style=\"background-color: #ffffff\" href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" style=\"background-color: #ffffff\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license). &lt;not found on Unsplash&gt;<\/span><\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/iFgRcqHznqg\">Man in White V-Neck T-Shirt (self)<\/a> by <a href=\"https:\/\/unsplash.com\/@miracletwentyone\">Joseph Gonzalez<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/u3WmDyKGsrY\">Natural Redhead in Brazil<\/a> by <a href=\"https:\/\/unsplash.com\/@eugabrielsilverio\">Gabriel Silv\u00e9rio<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/www.instagram.com\/p\/CBhxlohnSeJ\/\">Dark-Skinned Woman with Large White Rose<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@oladimeg\">Oladimeji Oduns<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<\/ul>\n<p><strong>Figure 5.14.2<\/strong><\/p>\n<p>ABO Blood Types Per Genotype by Christine Miller is released into the <a href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" rel=\"license\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 5.14.3<\/strong><\/p>\n<p>Three Phenotypes of Hair Based on Inheritance\/ <span style=\"font-size: 1em\">Incomplete Dominance<\/span><span style=\"text-align: initial;font-size: 1em\"> Hair <\/span><span style=\"text-align: initial;font-size: 1em\">by Christine Miller is released into the <\/span><a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" rel=\"license\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 5.14.4<\/strong><\/p>\n<p><a href=\"https:\/\/dr282zn36sxxg.cloudfront.net\/datastreams\/f-d%3Ad83f2c6640f7e493d532f370dbf49bb6a23ea2f578b553130dea2ad1%2BIMAGE_THUMB_POSTCARD_TINY%2BIMAGE_THUMB_POSTCARD_TINY.1\" rel=\"cc:attributionURL\">Average height \/Human Adult Height<\/a> by <a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/5.13\/\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<\/p>\n<p><img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><\/p>\n<p><strong>Figure 5.14.5<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/katiebordner\/15010445032\" rel=\"cc:attributionURL\">Tan lines<\/a> by <a class=\"owner-name truncate\" title=\"Go to katiebordner's photostream\" href=\"https:\/\/www.flickr.com\/photos\/katiebordner\/\" data-track=\"attributionNameClick\">katiebordner<\/a>\u00a0on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <span style=\"text-align: initial;font-size: 1em\">\u00a0<\/span><a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/p>\n<p><strong>Figure 5.14.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:1911_Sickle_Cells.jpg\" rel=\"cc:attributionURL\">Sickle cell anemia<\/a> by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/18-3-erythrocytes\">OpenStax College<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) \u00a9<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">Figure 5.14.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:ABO_blood_type.svg\" rel=\"cc:attributionURL\">ABO_blood_type.svg<\/a> by <a class=\"new\" title=\"User:InvictaHOG (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:InvictaHOG&amp;action=edit&amp;redlink=1\">InvictaHOG<\/a> on Wikimedia Commons is in the <a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" rel=\"license\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Amoeba Sisters. (2015, May 25). Incomplete dominance, codominance, polygenic traits, and epistasis! YouTube. https:\/\/www.youtube.com\/watch?v=YJHGfbW55l0<\/p>\n<p class=\"hanging-indent\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 18.9 Sickle cells [digital image]. In <em>Anatomy and Physiology<\/em>. OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/18-3-erythrocytes<\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). Figure 2 Human adult height [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 5.13) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/5.13\/<\/p>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Tay-Sachs disease [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tay-sachs-disease\/symptoms-causes\/syc-20378190<\/p>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/sickle-cell-anemia\/symptoms-causes\/syc-20355876<\/p>\n<p class=\"hanging-indent\">Teacher's Pet. (2015, January 25). Non-mendelian genetics. YouTube. https:\/\/www.youtube.com\/watch?v=-4vsio8TZrU<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3954\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3954\"><div tabindex=\"-1\"><div>\n<p><span style=\"text-align: initial;font-size: 1em\">Created by:\u00a0CK-12\/Adapted by: Christine Miller<\/span><\/p>\n<p><img class=\"wp-image-41 aligncenter\" title=\"CC0 1.0 Universal (CC0 1.0) from openclipart.org\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Question-Guy-2.png\" alt=\"Clip art of a person thinking with a thought bubble filled with question marks.\" width=\"166\" height=\"184\"><\/p>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\"><span style=\"text-align: initial;font-size: 1em\"><span style=\"font-size: 22.784px\">Defining Science<\/span><\/span><\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3717\">Science<\/a><\/strong>\u00a0is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question\u00a0using\u00a0evidence and logic.\u00a0It\u00a0is an exciting exploration of all the whys and hows that any curious person might\u00a0ask\u00a0about the world. You can be part of that exploration! Besides your curiosity, all you need is a basic understanding of how scientists think and how <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3717\">science<\/a> is done. In this concept, you'll learn how to think like a scientist.<\/p>\n<h2><span style=\"font-size: 1em;font-weight: bold\">Thinking Like a Scientist<\/span><\/h2>\n<p>Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:<\/p>\n<p>&nbsp;<\/p>\n<div><img class=\"aligncenter wp-image-1856\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Thinking-like-a-scientist-1.png\" alt=\"\" width=\"300\" height=\"300\"><\/div>\n<h2>Nature Is\u00a0Understandable<\/h2>\n<p>Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3561\">scientific law<\/a><\/strong>\u00a0is a statement that describes what always happens under certain conditions in nature.<\/p>\n<h2>Scientific Ideas Are Open to\u00a0Change<\/h2>\n<p>Science is\u00a0both\u00a0a process\u00a0and\u00a0body of knowledge.\u00a0Scientific knowledge is generated through systematic processes, such as\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3557\"><strong>observation<\/strong><\/a>\u00a0and experimentation.\u00a0Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often, existing ideas are simply revised. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.<\/p>\n<h2>Scientific Knowledge May Be Long Lasting<\/h2>\n<p>Many scientific ideas have\u00a0stood\u00a0the test of time. About 200 years ago, the scientist <a href=\"https:\/\/en.wikipedia.org\/wiki\/John_Dalton\">John Dalton<\/a> proposed atomic theory \u2014 the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed,\u00a0scientists have\u00a0learned a lot more about atoms and the even smaller particles\u00a0that compose them. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic\u00a0scientific\u00a0ideas that have been tested repeatedly and proven sound. You will learn about many of them as you study human biology.<\/p>\n<h2>Not All Questions Can be Answered by Science<\/h2>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3717\">Science<\/a><\/strong> rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3557\">observation<\/a><\/strong>\u00a0is anything that is detected through human\u00a0senses\u00a0or with instruments or\u00a0measuring devices that extend human\u00a0senses. Things that cannot be observed or measured by\u00a0current\u00a0means \u2014 such as supernatural beings or events \u2014 are outside the bounds of science. Consider these two questions about life on Earth:<\/p>\n<ul>\n<li>Did life on Earth evolve over time?<\/li>\n<li>How did life on Earth originate?<\/li>\n<\/ul>\n<p>The first question can be answered by science on the basis of scientific evidence (such as\u00a0fossils\u00a0and logical arguments). The second question could be a matter of belief, but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.3 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Science is a distinctive way of gaining knowledge about the natural world that tries to answer questions\u00a0using\u00a0evidence and logic.<\/li>\n<li>Scientists assume that nature can be understood\u00a0through systematic study.<\/li>\n<li>Scientific ideas are open to revision.<\/li>\n<li>Sound scientific ideas withstand the test of time.<\/li>\n<li>Science cannot provide answers to all of our questions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.3 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Define science.<\/li>\n<li>What is the general goal of science?<\/li>\n<li>\n<div id=\"h5p-46\">\n<div class=\"h5p-content\" data-content-id=\"46\"><\/div>\n<\/div>\n<\/li>\n<li>Identify four basic assumptions that scientists make when they study the natural world.<\/li>\n<li>Do observations in science have to be made by the naked eye? Can you think of a way in which scientists might be able to make observations about something they cannot directly see?<\/li>\n<li>If something cannot be observed, can it be tested scientifically? Explain your reasoning.<\/li>\n<li>Scientific knowledge builds upon itself. Give an example of a scientific idea from the reading where the initial idea\u00a0developed further\u00a0as science advanced.<\/li>\n<li>Discuss this statement: \u201cScientific ideas are always changing, so they can't be trusted.\u201d Do you think this is true?<\/li>\n<li>Why do you think that scientific knowledge expands as technology becomes more advanced?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\">1.3 Explore More<\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/3nAETHZTObk<\/p>\n<p style=\"text-align: center\">Nature of Science with the Ameoba Sisters, 2019.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<div>\n<h2><strong>References<\/strong><\/h2>\n<p class=\"hanging-indent\">Amoeba Sisters. (2019, Jun 6). Nature of science with Ameoba Sisters. YouTube. https:\/\/www.youtube.com\/watch?v=3nAETHZTObk<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, July 25). John Dalton. In\u00a0<i>Wikipedia. <\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=John_Dalton&amp;oldid=969425891<\/p>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4030\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4030\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3168\" aria-describedby=\"caption-attachment-3168\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3168\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Cappucino-Art-by-drew-coffman-tZKwLRO904E-unsplash-scaled-2.jpg\" alt=\"\" width=\"300\" height=\"169\"><figcaption id=\"caption-attachment-3168\" class=\"wp-caption-text\"><em>Figure 8.8.1 A capaccino can affect your mental state, especially when it looks like this!<\/em><\/figcaption><\/figure>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Art in a Cup<\/span><\/p>\n<p>Who knew that a cup of coffee could also be a work of art? A talented barista can make coffee look as good as it tastes. If you are a coffee drinker, you probably know that coffee can also affect your mental state. It can make you more alert, and it may improve your concentration. That\u2019s because the caffeine in coffee is a psychoactive drug. In fact, caffeine is the most widely consumed psychoactive substance in the world. In North America, for example, 90 per cent of adults consume caffeine daily.<\/p>\n<div>\n<h1>What Are Psychoactive Drugs?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3169\">Psychoactive drugs<\/a><\/strong>\u00a0are substances that change the function of the brain and result in alterations of mood, thinking, perception, and\/or behavior. Psychoactive drugs may be used for many purposes, including therapeutic, ritual, or recreational purposes. Besides <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3827\">caffeine<\/a>, other examples of psychoactive drugs include cocaine, LSD, alcohol, tobacco, codeine, and morphine. Psychoactive drugs may be legal prescription medications (codeine and morphine), legal nonprescription drugs (alcohol and tobacco), or illegal drugs (cocaine and LSD).<\/p>\n<p>Cannabis (or marijuana) is also a psychoactive drug that while illegal in many countries is legal for use in Canada by individuals over the age of 19 years. Legal prescription medications (such as opioids) are also used illegally by increasingly large numbers of people. Some legal drugs, such as alcohol and nicotine, are readily available almost everywhere, as illustrated by the images below.<\/p>\n<div id=\"h5p-129\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-129\" class=\"h5p-iframe\" data-content-id=\"129\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Scientific Method and &quot;Doing&quot; Science\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 8.8.2 These psychoactive drugs are legal and accessible almost anywhere.\u00a0\u00a0<\/em><\/p>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Classes of Psychoactive Drugs<\/span><\/p>\n<\/div>\n<p>Psychoactive drugs are divided into different classes\u00a0based on\u00a0their pharmacological effects. Several classes are listed below, along with examples of commonly used drugs in each class.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3171\">Stimulants<\/a><\/strong>\u00a0are drugs that stimulate the brain and increase alertness and wakefulness. Examples of stimulants include caffeine, nicotine, cocaine, and amphetamines (such as Adderall).<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3172\">Depressants<\/a><\/strong>\u00a0are drugs that calm the brain, reduce anxious feelings, and induce sleepiness. Examples of depressants include ethanol (in alcoholic beverages) and opioids, such as codeine and heroin.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3173\">Anxiolytics<\/a><\/strong>\u00a0are drugs that have a tranquilizing effect and inhibit anxiety. Examples of anxiolytic drugs include benzodiazepines (such as diazepam\/Valium), barbiturates (such as phenobarbital), opioids, and antidepressant drugs (such as sertraline\/Zoloft).<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3174\">Euphoriants<\/a><\/strong>\u00a0are drugs that bring about a state of euphoria, or intense feelings of well-being and happiness. Examples of euphoriants include the so-called \"club drug\" MDMA (ecstasy), amphetamines, ethanol, and opioids (such as morphine).<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3175\">Hallucinogens<\/a><\/strong>\u00a0are drugs that can cause hallucinations and other perceptual anomalies. They also cause subjective changes in thoughts, emotions, and consciousness. Examples of hallucinogens include LSD, mescaline, nitrous oxide, and psilocybin.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3176\">Empathogens<\/a><\/strong>\u00a0are drugs that produce feelings of empathy, or sympathy with other people. Examples of empathogens include amphetamines and MDMA.<\/li>\n<\/ul>\n<p>Many psychoactive drugs have multiple effects, so they may be placed in more than one class.\u00a0One\u00a0example is MDMA, pictured\u00a0below, which may act both as a euphoriant and as an empathogen. In some people, MDMA may also have stimulant or hallucinogenic effects. As of 2016, MDMA had no accepted medical uses, but it was undergoing testing for use in the treatment of post-traumatic stress disorder and certain other types of anxiety disorders.<\/p>\n<figure id=\"attachment_3177\" aria-describedby=\"caption-attachment-3177\" style=\"width: 434px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-3177\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ecstasy_monogram-1.jpg\" alt=\"\" width=\"434\" height=\"424\"><figcaption id=\"caption-attachment-3177\" class=\"wp-caption-text\"><em>Figure 8.8.3 Ecstasy (MDMA) is most commonly taken in tablet form, like the tablets shown here.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Mechanisms of Action<\/span><\/p>\n<\/div>\n<p>Psychoactive drugs generally produce their effects by affecting brain chemistry, which in turn may cause changes in a person\u2019s mood, thinking, perception, and behavior. Each drug tends to have a specific action on one or more neurotransmitters or neurotransmitter receptors in the brain. Generally, they act as either agonists or antagonists.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3178\">Agonists<\/a><\/strong>\u00a0are drugs that increase the activity of particular <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3056\">neurotransmitters<\/a>. They might act by promoting the synthesis of the neurotransmitters, reducing their reuptake from synapses, or mimicking their action by binding to receptors for the neurotransmitters.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3179\">Antagonists<\/a><\/strong>\u00a0are drugs that decrease the activity of particular neurotransmitters. They might act by interfering with the synthesis of the neurotransmitters or by blocking their receptors so the neurotransmitters cannot bind to them.<\/li>\n<\/ul>\n<p>Consider the example of the neurotransmitter <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3062\">GABA<\/a>. This is one of the most common neurotransmitters in the brain, and it normally has an inhibitory effect on\u00a0cells. GABA agonists \u2014 which increase its activity \u2014 include ethanol, barbiturates, and benzodiazepines, among other psychoactive drugs. All of these drugs work by promoting the activity of GABA receptors in the brain.<\/p>\n<div>\n<h1>Uses of Psychoactive Drugs<\/h1>\n<\/div>\n<p>You may have been prescribed psychoactive drugs by your doctor. For example, your doctor may have prescribed you\u00a0an opioid drug, such as codeine for pain (most likely in the form of Tylenol with added codeine). Chances are you also use nonprescription psychoactive drugs (like caffeine) for mental alertness. These are just two of the many possible uses of psychoactive drugs.<\/p>\n<h2>Medical Uses<\/h2>\n<figure id=\"attachment_3180\" aria-describedby=\"caption-attachment-3180\" style=\"width: 372px\" class=\"wp-caption alignright\"><img class=\"wp-image-3180\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/US_Navy_030513-N-1577S-001_Lt._Cmdr._Joe_Casey_Ships_Anesthetist_trains_on_anesthetic_procedures_with_Hospital_Corpsman_3rd_Class_Eric_Wichman_aboard_USS_Nimitz_CVN_68-1.jpg\" alt=\"Anesthetic\" width=\"372\" height=\"567\"><figcaption id=\"caption-attachment-3180\" class=\"wp-caption-text\"><em>Figure 8.8.4 This person is being prepared to receive a general anesthetic prior to surgery.<\/em><\/figcaption><\/figure>\n<p>General anesthesia is one use of psychoactive drugs in medicine. With general anesthesia, pain is blocked and unconsciousness is induced. General anesthetics are most often used during surgical procedures and may be administered in gaseous form, as in Figure 8.8.4. General anesthetics include the drugs halothane and ketamine. Other psychoactive drugs are used to manage pain without affecting consciousness. They may be prescribed either for acute pain in cases of trauma (such as broken bones) or for chronic pain caused by arthritis, cancer, or fibromyalgia. Most often, the drugs used for pain control are opioids, such as morphine and codeine.<\/p>\n<p>Many psychiatric disorders are also managed with psychoactive drugs. Antidepressants\u00a0like\u00a0sertraline, for example, are used to treat depression, anxiety, and\u00a0eating disorders. Anxiety disorders may also be treated with anxiolytics, such as buspirone and diazepam. Stimulants (such as amphetamines) are used to treat attention deficit disorder. Antipsychotics (such as clozapine and risperidone) \u2014 as well as mood stabilizers, such as lithium \u2014 are used to treat schizophrenia and bipolar disorder.<\/p>\n<h2>Ritual Uses<\/h2>\n<figure id=\"attachment_3181\" aria-describedby=\"caption-attachment-3181\" style=\"width: 278px\" class=\"wp-caption alignleft\"><img class=\" wp-image-3181\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Peyote-Lophophora_williamsii_pm-1.jpg\" alt=\"\" width=\"278\" height=\"177\"><figcaption id=\"caption-attachment-3181\" class=\"wp-caption-text\"><em>Figure 8.8.5 The peyote cactus contains a hallucinogenic drug that is still used by some Native Americans for religious rituals.<\/em><\/figcaption><\/figure>\n<p>Certain psychoactive drugs, particularly hallucinogens, have been used for ritual purposes since prehistoric times. For example, Native Americans have used the mescaline-containing peyote cactus (pictured in Figure 8.8.5) for religious ceremonies for as long as 5,700 years. In prehistoric Europe, the mushroom <em>Amanita muscaria<\/em>, which contains a hallucinogenic drug called muscimol, was used for similar purposes. Various other psychoactive drugs \u2014 including jimsonweed, psilocybin mushrooms, and cannabis \u2014 have also been used for millennia, by various peoples, for ritual purposes.<\/p>\n<p>&nbsp;<\/p>\n<div>\n<p><span style=\"font-size: 1.424em;font-weight: bold\">Recreational Uses<\/span><\/p>\n<\/div>\n<p>The recreational use of psychoactive drugs generally has the purpose of altering one\u2019s consciousness and creating a feeling of euphoria commonly called a \u201chigh.\u201d Some of the drugs used most commonly for recreational purposes are cannabis, ethanol (alcohol), opioids, and stimulants (such as nicotine). Hallucinogens are also used recreationally, primarily for the alterations they cause in thinking and perception.<\/p>\n<p>Some investigators have suggested that the urge to alter one\u2019s state of consciousness is a universal human drive, similar to the drive to satiate thirst, hunger, or sexual desire. They think that this instinct is even present in children, who may attain an altered state by repetitive motions, such as spinning or swinging. Some nonhuman animals also exhibit a drive to experience altered states. They may consume fermented berries or fruit and become intoxicated. The way cats respond to catnip (see Figure 8.8.6) is another example.<\/p>\n<figure id=\"attachment_3182\" aria-describedby=\"caption-attachment-3182\" style=\"width: 766px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3182\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cat-under-effects-of-catnip-by-katieb50-on-flickr-1.jpg\" alt=\"\" width=\"766\" height=\"574\"><figcaption id=\"caption-attachment-3182\" class=\"wp-caption-text\"><em>Figure 8.8.6 This cat is taking advantage of a catnip plant and apparently enjoying its psychoactive effects.<\/em><\/figcaption><\/figure>\n<h3>Addiction, Dependence, and Rehabilitation<\/h3>\n<p>Psychoactive substances often bring about subjective changes that the user may find pleasant (euphoria) or advantageous (increased alertness). These changes are rewarding and positively reinforcing, so they have the potential for misuse, addiction, and dependence.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3184\">Addiction<\/a><\/strong>\u00a0refers to the compulsive use of a drug, despite negative consequences that such use may entail. Sustained use of an addictive drug may produce dependence on the drug.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3461\">Dependence<\/a><\/strong>\u00a0may be physical and\/or psychological. It occurs when cessation of drug use produces withdrawal symptoms. Physical dependence produces physical withdrawal symptoms, which may include tremors, pain, seizures, or insomnia. Psychological dependence produces psychological withdrawal symptoms, such as anxiety, depression, paranoia, or hallucinations.<\/p>\n<p>Rehabilitation for drug dependence and addiction typically involves psychotherapy, which may include both individual and group therapy. Organizations such as <a href=\"https:\/\/www.aa.org\/pages\/en_US\/regional-correspondent-us-and-canada\">Alcoholics Anonymous<\/a> (AA) and <a href=\"https:\/\/www.bcrna.ca\/\">Narcotics Anonymous<\/a> (NA) may also be helpful for people trying to recover from addiction. These groups are self-described as international mutual aid fellowships,\u00a0and\u00a0their primary purpose is to help addicts achieve and maintain sobriety. In some cases, rehabilitation is aided by the temporary use of psychoactive substances that reduce cravings and withdrawal symptoms without creating addiction themselves.\u00a0The drug methadone, for example, is commonly used\u00a0to treat heroin addiction.<\/p>\n<div>\n<h1>Feature: Human Biology in the News<\/h1>\n<\/div>\n<p>In North America, a lot of media attention is currently given to a rising tide of opioid addiction and overdose deaths. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3187\">Opioids<\/a><\/strong>\u00a0are drugs derived from the opium poppy or synthetic versions of such drugs. They include the illegal drug heroin, as well as prescription painkillers such\u00a0as codeine, morphine, hydrocodone, oxycodone, and fentanyl. In 2016, fentanyl received wide media attention when it was announced that an accidental fentanyl overdose was responsible for the death of music icon Prince. Fentanyl is an extremely strong and dangerous drug, said to be 50 to 100 times stronger than morphine, making risk of overdose death from fentanyl very high.<\/p>\n<p>The dramatic increase in opioid addiction and overdose deaths has been called an opioid epidemic. It is considered to be the worst drug crisis in Canadian history. Consider the following facts:<\/p>\n<ul>\n<li>In 2016, there were almost 2,500 opioid-related deaths in Canada \u2014 almost 7 per day.<\/li>\n<li>The number of prescriptions written for opioids quadrupled between 1999 and 2010.\u00a0 If you have been prescribed codeine, fentanyl, morphine, oxycodone, hydromorphone or medical heroin, then you have been prescribed an opiate.<\/li>\n<li>There are many long-term health effects of using opioids, which include:\n<ul>\n<li>Increased tolerance to the drug.<\/li>\n<li>Liver damage.<\/li>\n<li>Substance use disorder or addiction.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p>Doctors, public health professionals, and politicians have all called for new policies, funding, programs, and laws to address the opioid epidemic. Changes that have already been made include a shift from criminalizing to medicalizing the problem,\u00a0more\u00a0treatment programs, and more widespread distribution and use of the opioid-overdose antidote naloxone (Narcan). Opioids can slow or stop a person's\u00a0breathing, which is what usually causes overdose deaths. Naloxone helps the person wake up and keeps them breathing until emergency medical treatment can be provided.<\/p>\n<p>What, if anything, will work to stop the opioid epidemic in Canada and the United States? Keep watching the news to find out.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">8.8 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3169\">Psychoactive drugs<\/a> are substances that change the function of the brain and result in alterations of mood, thinking, perception, and behavior. They include prescription medications (such as opioid painkillers), legal substances (such as nicotine and alcohol), and illegal drugs (such as LSD and heroin).<\/li>\n<li>Psychoactive drugs are divided into different classes according to their pharmacological effects. They include stimulants, depressants, anxiolytics, euphoriants, hallucinogens, and empathogens. Many psychoactive drugs have multiple effects, so they may be placed in more than one class.<\/li>\n<li>Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as agonists \u2014 which enhance the activity of particular <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3056\">neurotransmitters<\/a>\u00a0\u2014 or as antagonists, which decrease the activity of particular neurotransmitters.<\/li>\n<li>Psychoactive drugs are used for various purposes, including medical,\u00a0ritual,\u00a0and recreational purposes.<\/li>\n<li>Misuse of psychoactive drugs may lead to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3184\">addiction<\/a>, which is the compulsive use of a drug despite the negative consequences such use may entail. Sustained use of an addictive drug may produce physical or psychological <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3461\">dependence<\/a> on the drug. Rehabilitation typically involves psychotherapy, and sometimes the temporary use of other psychoactive drugs.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">8.8 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What are psychoactive drugs?<\/li>\n<li>Identify six classes of psychoactive drugs, along with an example of a drug in each class.<\/li>\n<li>Compare and contrast psychoactive drugs that are agonists and\u00a0psychoactive drugs that are antagonists.<\/li>\n<li>Describe two medical uses of psychoactive drugs.<\/li>\n<li>Give an example of a ritual use of a psychoactive drug.<\/li>\n<li>Generally speaking, why do people use psychoactive drugs recreationally?<\/li>\n<li>Define addiction.<\/li>\n<li>Identify possible withdrawal symptoms associated with physical dependence on a psychoactive drug.<\/li>\n<li>Why might a person with a heroin addiction be prescribed the psychoactive drug methadone?<\/li>\n<li>The prescription drug Prozac inhibits the reuptake of the neurotransmitter serotonin, causing more serotonin to be present in the\u00a0synapse. Prozac can elevate mood, which is why it is sometimes used to treat depression. Answer the following questions about Prozac:\n<ol type=\"a\">\n<li>Is Prozac an agonist or an antagonist for serotonin? Explain your answer.<\/li>\n<li>Is Prozac a psychoactive drug? Explain your answer.<\/li>\n<\/ol>\n<\/li>\n<li>Name three classes of psychoactive drugs that include opioids.<\/li>\n<li><em>True or False:\u00a0<\/em>All psychoactive drugs are either illegal or available by prescription only.<\/li>\n<li><em>True or False:\u00a0<\/em>Anxiolytics might be prescribed by a physician.<\/li>\n<li>Name two drugs that activate receptors for the neurotransmitter GABA.\u00a0Why do you think these drugs generally have a depressant effect?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">8.8 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=foLf5Bi9qXs<\/p>\n<p style=\"text-align: center\">How does caffeine keep us awake? - Hanan Qasim, TED-Ed, 2017.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=8qK0hxuXOC8<\/p>\n<p style=\"text-align: center\">How do drugs affect the brain? - Sara Garofalo, TED-Ed, 2017.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=Nlcr1jd_Tok<\/p>\n<p style=\"text-align: center\">Is marijuana bad for your brain? - Anees Bahji, TED-Ed, 2019.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h2>Attributions<\/h2>\n<p><strong>Figure 8.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/tZKwLRO904E\" rel=\"cc:attributionURL\">Cappucino Art by drew-coffman-tZKwLRO904E<\/a> [photo] by <a href=\"https:\/\/unsplash.com\/@drewcoffman\">Drew Coffman<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 8.8.2<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:3804,_Saint-Laurent,_Montreal_-_Cannabis_Culture_shop.jpg\">3804, Saint-Laurent, Montreal - Cannabis Culture shop<\/a> by\u00a0<a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/29442760@N00\" rel=\"nofollow\">Exile on Ontario St<\/a> (Montreal, Canada) on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/deed.en\">CC BY SA 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/deed.en) license.<\/li>\n<li><a href=\"https:\/\/www.flickr.com\/photos\/theunabonger\/478021002\">Drive Through Cigarette Store<\/a> by <a class=\"owner-name truncate no-outline\" title=\"Go to Cosmo Spacely's photostream\" href=\"https:\/\/www.flickr.com\/photos\/theunabonger\/\" data-track=\"attributionNameClick\">Cosmo Spacely<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/\">CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.<\/li>\n<li><a href=\"https:\/\/www.flickr.com\/photos\/ventriloblog\/126224469\/\">Franklin-Nicollet Liquors<\/a> by <a class=\"owner-name truncate\" title=\"Go to Max Sparber's photostream\" href=\"https:\/\/www.flickr.com\/photos\/ventriloblog\/\" data-track=\"attributionNameClick\">Max Sparber<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.<\/li>\n<\/ul>\n<p><strong>Figure 8.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Ecstasy_monogram.jpg#file\" rel=\"cc:attributionURL\">Ecstasy_monogram<\/a> by <i><a class=\"extiw\" title=\"w:Drug Enforcement Administration\" href=\"https:\/\/en.wikipedia.org\/wiki\/Drug_Enforcement_Administration\">Drug Enforcement Administration<\/a> <\/i>on Wikimedia Commons is in the <a class=\"mw-redirect\" title=\"Public domain\" href=\"https:\/\/commons.wikimedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 8.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:US_Navy_030513-N-1577S-001_Lt._Cmdr._Joe_Casey,_Ship%27s_Anesthetist,_trains_on_anesthetic_procedures_with_Hospital_Corpsman_3rd_Class_Eric_Wichman_aboard_USS_Nimitz_(CVN_68).jpg\" rel=\"cc:attributionURL\">US Navy 030513-N-1577S-001 Lt. Cmdr. Joe Casey, Ship's Anesthetist, trains on anesthetic procedures with Hospital Corpsman 3rd Class Eric Wichman aboard USS Nimitz (CVN 68)<\/a> by U.S. Navy photo by Photographer\u2019s Mate Airman Timothy F. Sosais on Wikimedia Commons is in the <a class=\"mw-redirect\" title=\"Public domain\" href=\"https:\/\/commons.wikimedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 8.8.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Lophophora_williamsii_pm.jpg\" rel=\"cc:attributionURL\">Peyote Lophophora_williamsii_pm<\/a> by <a title=\"User:PeterMansfeld\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:PeterMansfeld\">Peter A. Mansfeld<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 8.8.6<\/strong><\/p>\n<section class=\"standard post-764 chapter type-chapter status-publish hentry focusable\" data-type=\"chapter\">\n<div class=\"media-atttributions\">\n<p><a href=\"https:\/\/www.flickr.com\/photos\/10233916@N03\/2526930264\/in\/photolist-3QtaE-4Zhtt7-844WT6-5HED3R-4Ribkb-6aaVt7-ekhNvK-9QFDgP-7Khsf-21p6zx-HPAcuZ-4wKSaH-RTW4nt-8KBR3K-9K4Hg2-9K7wTL-pCriyp-C4XKJ-7Q5Q-9K4HY2-6CaNK5-dZ5sqL-8yJNjk-7feRXF-JJpCF-S1Be3J-dYYKWx-dYYKTp-auB7co-7UmiU5-adcRHZ-7fiJF9-3jnSVf-9BpzN3-7fiFjb-7fiJJ1-7fiJVA-7fiJs7-dYYL96-7feRGP-dZ5sho-6KLkkz-7feRKP-7feSbx-7feSeV-7feNyR-7feRQT-7fiJMb-f87VTi-c2TF8J\" rel=\"cc:attributionURL\">Cat under effects of catnip\/Self Indulgence<\/a> by\u00a0<a href=\"https:\/\/www.flickr.com\/photos\/10233916@N03\/\" rel=\"dc:creator\">Katieb50<\/a> on <a href=\"https:\/\/www.flickr.com\/\" rel=\"cc:attributionURL\">Flickr<\/a> is used under a\u00a0<a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.<\/p>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"glossary\"><span style=\"font-size: 1.424em;font-weight: bold;color: #333333\">References<\/span><\/div>\n<div><\/div>\n<\/section>\n<p class=\"hanging-indent\">Alcoholics Anonymous World Services, Inc. (n.d.). Regional correspondent U.S. and Canada [website]. https:\/\/www.aa.org\/pages\/en_US\/regional-correspondent-us-and-canada<\/p>\n<p class=\"hanging-indent\"><span style=\"color: #373d3f;font-size: 1em;text-align: initial;text-indent: -1em\">Belzak, L., &amp; Halverson, J. (2018). The opioid crisis in Canada: a national perspective. La crise des opio\u00efdes au Canada : une perspective nationale. <\/span><i style=\"color: #373d3f;font-size: 1em;text-align: initial;text-indent: -1em\">Health promotion and chronic disease prevention in Canada : research, policy and practice<\/i><span style=\"color: #373d3f;font-size: 1em;text-align: initial;text-indent: -1em\">,\u00a0<\/span><i style=\"color: #373d3f;font-size: 1em;text-align: initial;text-indent: -1em\">38<\/i><span style=\"color: #373d3f;font-size: 1em;text-align: initial;text-indent: -1em\">(6), 224\u2013233. https:\/\/doi.org\/10.24095\/hpcdp.38.6.02<\/span><\/p>\n<p class=\"hanging-indent\">British Columbia Regional Service Committee of Narcotics Anonymous. (n.d.). Welcome to the B.C. region of N.A. [website]. https:\/\/www.bcrna.ca\/<\/p>\n<p class=\"glossary hanging-indent\"><span style=\"text-align: initial;color: #333333;font-size: 1em\">Centers for Disease Control and Prevention (CDC). (2011 November 4). <\/span>Vital signs: overdoses of prescription opioid pain relievers\u2014United States, 1999\u20132008.<span style=\"text-align: initial;color: #333333;font-size: 1em\"><em> Morbidity and Mortality Weekly Report (MMWR),<\/em><em>60<\/em>(43):1487-1492. https:\/\/www.cdc.gov\/mmwr\/preview\/mmwrhtml\/mm6043a4.htm<\/span><\/p>\n<p class=\"hanging-indent\"><span style=\"text-align: initial;color: #333333;font-size: 1em\">TED-Ed. (2017, June 29). How do drugs affect the brain? - Sara Garofalo. YouTube. https:\/\/www.youtube.com\/watch?v=8qK0hxuXOC8&amp;feature=youtu.be<\/span><\/p>\n<p class=\"hanging-indent\">TED-Ed. (2017, July 17). How does caffeine keep us awake? - Hanan Qasim. YouTube. https:\/\/www.youtube.com\/watch?v=foLf5Bi9qXs&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2019, December 2). Is marijuana bad for your brain? - Anees Bahji. YouTube. https:\/\/www.youtube.com\/watch?v=Nlcr1jd_Tok&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4031\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4031\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3191\" aria-describedby=\"caption-attachment-3191\" style=\"width: 1023px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3191\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Alzheimers_Disease-1.jpg\" alt=\"\" width=\"1023\" height=\"791\"><figcaption id=\"caption-attachment-3191\" class=\"wp-caption-text\"><em>Figure 8.9.1 Molecular and cellular changes occur in a brain with Alzheimer\u2019s disease (AD).<\/em><\/figcaption><\/figure>\n<h1>Case Study Conclusion: Fading Memory<\/h1>\n<p>The illustration above (Figure 8.9.1) shows some of the molecular and cellular changes that occur in Alzheimer\u2019s disease (AD). Rosa was diagnosed with AD at the beginning of this chapter after experiencing memory problems and other changes in her cognitive functioning, mood, and personality. These abnormal changes in the brain include the development of amyloid plaques between brain cells and neurofibrillary tangles inside of neurons. These hallmark characteristics of AD are associated with the loss of synapses between neurons, and ultimately the death of neurons.<\/p>\n<p>After reading this chapter, you should have a good appreciation for the importance of keeping neurons alive and communicating with each other at synapses. The nervous system coordinates all of the body\u2019s voluntary and involuntary activities. It interprets information from the outside world through sensory systems, and makes appropriate responses through the motor system, through communication between the PNS and CNS. The brain directs the rest of the nervous system and controls everything from basic vital functions (such as heart rate and breathing) to high-level functions (such as problem solving and abstract thought). The nervous system\u00a0can\u00a0perform these important functions by generating action potentials in neurons in response to stimulation and sending messages between cells at synapses, typically using chemical neurotransmitter molecules. When neurons are not functioning properly, lose their synapses, or die, they cannot carry out the signaling essential for the proper functioning of the nervous system.<\/p>\n<p>AD is a progressive neurodegenerative disease, meaning that the damage to the brain becomes more extensive as time goes on. The picture in Figure 8.9.2 illustrates how the damage progresses from before AD is diagnosed (preclinical AD), to mild and moderate AD, to severe AD.<\/p>\n<figure id=\"attachment_3192\" aria-describedby=\"caption-attachment-3192\" style=\"width: 644px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3192\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Alzheimers-Disease-stagess-scaled-2.jpg\" alt=\"\" width=\"644\" height=\"1388\"><figcaption id=\"caption-attachment-3192\" class=\"wp-caption-text\"><em>Figure 8.9.2 Illustration showing the areas of the brain that become damaged as Alzheimer\u2019s disease (AD) progresses. This is a side view along the middle of the brain, with the front of the brain shown to the left. Damaged areas are shown in blue.<\/em><\/figcaption><\/figure>\n<div>\n<p>&nbsp;<\/p>\n<\/div>\n<p>You can see that the damage starts in a relatively small location toward the bottom of the brain. One of the earliest brain areas to be affected by AD is the hippocampus. As you have learned, the hippocampus is important for learning and memory, which explains why many of Rosa\u2019s symptoms of mild AD involve deficits in memory, such as trouble remembering where she placed objects, recent conversations, and appointments.<\/p>\n<p>As AD progresses, more of the brain is affected, including areas involved in emotional regulation, social behavior, planning, language, spatial navigation, and higher-level thought. Rosa is beginning to show signs of problems in these areas, including irritability, lashing out at family members, getting lost in her neighborhood, problems finding the right words, putting objects in unusual locations, and difficulty in managing her finances. You can see that as AD progresses, damage spreads further across the cerebrum, which you now know controls conscious functions\u00a0like\u00a0reasoning, language, and interpretation of sensory stimuli. You can also see how the frontal lobe \u2014 which controls executive functions such as planning, self-control, and abstract thought \u2014 becomes increasingly damaged.<\/p>\n<p>Increasing damage to the brain causes corresponding deficits in functioning. In moderate AD, patients have increased memory, language, and cognitive deficits, compared to mild AD. They may not recognize their own family members, and may wander and get lost, engage in inappropriate behaviors, become easily agitated, and have trouble carrying out daily activities such as dressing. In severe AD, much of the brain is affected. Patients usually cannot recognize family members or communicate, and they are often fully dependent on others for their care. They begin to lose the ability to control their basic functions, such as bladder control, bowel control, and proper swallowing. Eventually, AD causes death, usually as a result of this loss of basic functions.<\/p>\n<p>For now, Rosa only has mild AD and is still able to function relatively well with care from her family. The medication her doctor gave her has helped improve some of her symptoms. It is a cholinesterase inhibitor, which blocks an enzyme that normally degrades the neurotransmitter acetylcholine. With more of the neurotransmitter available, more of it can bind to neurotransmitter receptors on postsynaptic cells. Therefore, this drug acts as an agonist for acetylcholine, which enhances communication between neurons in Rosa\u2019s brain. This increase in neuronal communication can help restore some of the functions lost in early Alzheimer\u2019s disease and may slow the progression of symptoms.<\/p>\n<p>But medication such as this is only a short-term measure, and does not halt the progression of the underlying disease. Ideally, the damaged or dead neurons would be replaced by new, functioning neurons. Why does this not happen automatically in the body? As you have learned, neurogenesis is very limited in adult humans, so once neurons in the brain die, they are not normally replaced to any significant extent.\u00a0Scientists, however, are studying the ways in which neurogenesis might be increased in cases of disease or injury to the brain.\u00a0They are also investigating the possibility of using stem cell transplants to replace damaged or dead neurons with new neurons. But this research is in very early stages and is not currently a treatment for AD.<\/p>\n<p>One promising area of research is in the development of methods to allow earlier detection and treatment of AD, given that the changes in the brain may actually start ten to 20 years before diagnosis of AD.\u00a0A\u00a0radiolabeled chemical called Pittsburgh Compound B (PiB) binds to amyloid plaques in the brain, and in the future, it may be used in conjunction with brain imaging techniques to detect early signs of AD. Scientists are also looking for biomarkers in bodily fluids (such as blood and cerebrospinal fluid) that might indicate the presence of AD before symptoms appear. Finally, researchers are also investigating possible early and subtle symptoms (such as changes in how people move or a loss of smell) to see whether they can be used to identify people who will go on to develop AD. This research is in the early stages, but the hope is that patients can be identified earlier, allowing for earlier and more effective treatment, as well as more planning time for families.<\/p>\n<p>Scientists are also still trying to fully understand the causes of AD, which affects more than five million Americans. Some genetic mutations have been identified\u00a0as contributors, but environmental factors also appear to be important. With more research into the causes and mechanisms of AD, hopefully a cure can be found, and people like Rosa can live a longer and better life.<\/p>\n<div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 8 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>In this chapter, you learned about the human nervous system. Specifically, you learned that:<\/p>\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2908\">nervous system<\/a> is the organ system that coordinates all of the body\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3004\">voluntary<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3005\">involuntary<\/a> actions by transmitting signals to and from different parts of the body. It has two major divisions: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3841\">central nervous system<\/a> (CNS) and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3009\">peripheral nervous system<\/a> (PNS).\n<ul>\n<li>The CNS\u00a0includes\u00a0the brain and spinal cord.<\/li>\n<li>The PNS consists mainly of nerves that connect the CNS with the rest of the body. It has two major divisions: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3014\">somatic nervous system<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3807\">autonomic nervous system<\/a>. These divisions control different types of functions, and often interact with the CNS to carry out these functions. The somatic system controls activities that are under voluntary control. The autonomic system controls activities that are involuntary.\n<ul>\n<li>The autonomic nervous system is further divided into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3015\">sympathetic division<\/a> (which controls the fight-or-flight response), the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3016\">parasympathetic division<\/a> (which controls most routine involuntary responses), and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3895\">enteric division<\/a> (which provides local control for digestive processes).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\"><\/li>\n<li>Signals sent by the nervous system are electrical signals called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3605\">nerve impulses<\/a>. They are transmitted by special, electrically excitable cells called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2984\">neurons<\/a>, which are one of two major types of cells in the nervous system.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2985\">Neuroglia<\/a> are the other major type of nervous system cells. There are many types of glial cells, and they have many specific functions. In general, neuroglia function to support, protect, and nourish neurons.<\/li>\n<li>The main parts of a neuron include the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3839\">cell body<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3873\">dendrites<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3809\">axon<\/a>. The cell body contains the nucleus. Dendrites receive nerve impulses from other cells, and the axon transmits nerve impulses to other cells at axon terminals. A synapse is a complex membrane junction at the end of an axon terminal that transmits signals to another cell.<\/li>\n<li>Axons are often wrapped in an electrically-insulating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3028\">myelin sheath<\/a>, which is produced by oligodendrocytes or schwann cells, both of which are types of neuroglia. Electrical impulses called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3049\">action potentials<\/a>\u00a0occur at gaps in the myelin sheath, called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3041\">nodes of Ranvier<\/a>, which speeds the conduction of nerve impulses down the axon.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3025\">Neurogenesis<\/a>, or the formation of new neurons by cell division, may occur in a mature human brain \u2014 but only to a limited extent.<\/li>\n<li>The nervous tissue in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3823\">brain <\/a>and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3010\">spinal cord<\/a> consists of gray matter \u2014 which contains mainly unmyelinated cell bodies and dendrites of neurons \u2014 and white matter, which contains mainly myelinated axons of neurons. Nerves of the peripheral nervous system consist of long bundles of myelinated axons that extend throughout the body.<\/li>\n<li>There are hundreds of types of neurons in the human nervous system, but many can be classified on the basis of the direction in which they carry nerve impulses. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3030\">Sensory neurons<\/a> carry nerve impulses away from the body and toward the central nervous system, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3031\">motor neurons<\/a> carry them away from the central nervous system and toward the body, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3032\">interneurons<\/a> often carry them between sensory and motor neurons.<\/li>\n<li>A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3621\">sodium-potassium pump<\/a> maintains an electrical gradient across the plasma membrane of a neuron when it is not actively transmitting a nerve impulse. This gradient is called the resting potential of the neuron.<\/li>\n<li>An action potential is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. It begins when the neuron receives a chemical signal from another cell or some other type of stimulus. The action potential travels rapidly down the neuron\u2019s axon as an electric current.<\/li>\n<li>A nerve impulse is transmitted to another cell at either an electrical or a chemical <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3040\">synapse<\/a>. At a chemical synapse, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3056\">neurotransmitter<\/a> chemicals are released from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3052\">presynaptic cell<\/a> into the synaptic cleft between cells. The chemicals travel across the cleft to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3053\">postsynaptic cell<\/a> and bind to receptors embedded in its membrane.<\/li>\n<li>There are many different types of neurotransmitters. Their effects on the postsynaptic cell generally depend on the type of receptor they bind to. The effects may be excitatory, inhibitory, or modulatory in more complex ways. Both physical and mental disorders may occur if there are problems with neurotransmitters or their receptors.<\/li>\n<li>The CNS includes the brain and spinal cord. It is physically protected by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3821\">bones<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2929\">meninges<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3847\">cerebrospinal fluid<\/a>. It is chemically protected by the blood-brain barrier.<\/li>\n<li>The brain is the control center of the nervous system and of the entire organism. The brain uses a relatively large proportion of the body\u2019s energy, primarily in the form of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3359\">glucose<\/a>.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>The brain is divided into three major parts, each with different functions: the forebrain, the midbrain and the hindbrain.\n<ul>\n<li>The forebrain includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3849\">cerebrum<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3095\">thalamus<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2937\">hypothalamus<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3096\">hippocampus<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3791\">amygdala<\/a>. The cerebrum is further divided into left and right hemispheres. Each hemisphere has four lobes: frontal, parietal, temporal, and occipital. Each lobe is associated with specific senses or other functions.\u00a0 The cerebrum has a thin outer layer called the cerebral cortex. Its many folds give it a large surface area. This is where most information processing takes place.<\/li>\n<\/ul>\n<\/li>\n<li>The thalamus, hypothalamus, hippocampus and amygdala are all part of the limbic system which helps regulate memories, coordination and attention<\/li>\n<\/ul>\n<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3010\">spinal cord<\/a> is a tubular bundle of nervous tissues that extends from the head down the middle of the back to the pelvis. It functions mainly to connect the brain with the PNS. It also controls certain rapid responses called reflexes without input from the brain.\n<ul>\n<li>A spinal cord injury may lead to paralysis (loss of sensation and movement) of the body below the level of the injury, because nerve impulses can no longer travel up and down the spinal cord beyond that point.<\/li>\n<\/ul>\n<\/li>\n<li>The PNS consists of all the nervous tissue that lies outside of the CNS. Its main function is to connect the CNS to the rest of the organism.<\/li>\n<li>The tissues that make up the PNS are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3011\">nerves<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3935\">ganglia<\/a>. Nerves are bundles of axons and ganglia are groups of cell bodies.\u00a0Nerves are classified as sensory, motor, or\u00a0a mix of the two.\n<ul>\n<li>The PNS is not as well protected physically or chemically as the CNS, so it is more prone to injury and disease. PNS problems include injury from diabetes, shingles, and heavy metal poisoning. Two disorders of the PNS are Guillain-Barre syndrome and Charcot-Marie-Tooth disease.<\/li>\n<\/ul>\n<\/li>\n<li>The human body has two major types of senses: special senses and general senses. Special senses have specialized sense organs and include vision (eyes), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses are all associated with touch and lack special sense organs. Touch receptors are found throughout the body but particularly in the skin.<\/li>\n<li>All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3124\">mechanoreceptors<\/a>\u00a0(mechanical forces), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3125\">thermoreceptors<\/a>\u00a0(temperature), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3126\">nociceptors<\/a>\u00a0(pain), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3127\">photoreceptors<\/a>\u00a0(light), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3128\">chemoreceptors<\/a>\u00a0(chemicals).\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3129\">Touch<\/a> includes the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3164\">Vision<\/a> is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.\n<ul>\n<li>Common vision problems include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3144\">myopia<\/a> (nearsightedness), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3145\">hyperopia<\/a> (farsightedness), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3165\">presbyopia<\/a> (age-related decline in close vision).<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3151\">Hearing<\/a> is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\n<li>The ear is also the organ responsible for the sense of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3155\">balance<\/a>, which is the ability to sense and maintain an appropriate body position. The ears send impulses on head position to the brain, which sends messages to skeletal muscle via the peripheral nervous system. The muscles respond by contracting to maintain balance.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3157\">Taste<\/a> and smell are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food, and olfactory receptors in the nasal passages sense chemicals in the air. The sense of smell contributes significantly to the sense of taste.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3169\">Psychoactive drugs<\/a> are substances that change the function of the brain and result in alterations of mood, thinking, perception, and behavior. They include prescription medications (such as opioid painkillers), legal substances (such as nicotine and alcohol), and illegal drugs (such as LSD and heroin).<\/li>\n<li>Psychoactive drugs are divided into different classes according to their pharmacological effects. They include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3171\">stimulants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3172\">depressants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3173\">anxiolytics<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3174\">euphoriants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3175\">hallucinogens<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3176\">empathogens<\/a>. Many psychoactive drugs have multiple effects, so they may be placed in more than one class.<\/li>\n<li>Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3178\">agonists<\/a>, which enhance the activity of particular neurotransmitters, or as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3179\">antagonists<\/a>, which decrease the activity of particular neurotransmitters.<\/li>\n<li>Psychoactive drugs are used for\u00a0medical, ritual, and recreational\u00a0purposes.<\/li>\n<li>Misuse of psychoactive drugs may lead to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3184\">addiction<\/a>, which is the compulsive use of a drug, despite its negative consequences. Sustained use of an addictive drug may produce physical or psychological <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3461\">dependence<\/a> on the drug. Rehabilitation typically involves psychotherapy, and sometimes the temporary use of other psychoactive drugs.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\"><\/li>\n<\/ul>\n<p>In addition to the nervous system, there is another system of the body that is important for coordinating and regulating many different functions \u2013 the endocrine system. You will learn about the endocrine system in the next chapter.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 8 Review<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Imagine that you decide to make a movement. To carry out this decision, a neuron in the cerebral cortex of your brain (neuron A) fires a nerve impulse that is sent to a neuron in your spinal cord (neuron B). Neuron B then sends the signal to a muscle cell, causing it to contract, resulting in movement. Answer the following questions about this pathway.\n<ol type=\"a\">\n<li>Which part of the brain is neuron A located in \u2014 the cerebellum, cerebrum, or brain stem? Explain how you know.<\/li>\n<li>The cell body of neuron A is located in a lobe of the brain that is involved in abstract thought, problem solving, and planning. Which lobe is this?<\/li>\n<li>Part of neuron A travels all the way down to the spinal cord to meet neuron B. Which part of neuron A travels to the spinal cord?<\/li>\n<li>Neuron A forms a chemical synapse with neuron B in the spinal cord. How is the signal from neuron A transmitted to neuron B?<\/li>\n<li>Is neuron A in the central nervous system (CNS) or peripheral nervous system (PNS)?<\/li>\n<li>The axon of neuron B travels in a nerve to a skeletal muscle cell. Is the nerve part of the CNS or PNS? Is this an afferent nerve or an efferent nerve?<\/li>\n<li>What part of the PNS is involved in this pathway \u2014 the autonomic nervous system or the somatic nervous system? Explain your answer.<\/li>\n<\/ol>\n<\/li>\n<li>What are the differences between a neurotransmitter receptor and a sensory receptor?<\/li>\n<li>\n<div id=\"h5p-130\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-130\" class=\"h5p-iframe\" data-content-id=\"130\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"8.3 Review Questions Neurons and Neuroglia\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>If a person has a stroke and then has trouble using language correctly, which hemisphere of their brain was most likely damaged? Explain your answer.<\/li>\n<li>Electrical gradients are responsible for the resting potential and action potential in neurons. Answer the following questions about the electrical characteristics of neurons.\n<ol type=\"a\">\n<li>Define an electrical gradient, in the context of a cell.<\/li>\n<li>What is responsible for maintaining the electrical gradient that results in the resting potential?<\/li>\n<li>Compare and contrast the resting potential and the action potential.<\/li>\n<li>Where along a myelinated axon does the action potential occur? Why does it happen here?<\/li>\n<li>What does it mean that the action potential is \u201call-or-none?\u201d<\/li>\n<\/ol>\n<\/li>\n<li>Compare and contrast Schwann cells and oligodendrocytes.<\/li>\n<li>For the senses of smell and hearing, name their respective sensory receptor cells, what type of receptor cells they are, and what stimuli they detect.<\/li>\n<li>Nicotine is a psychoactive drug that binds to and activates a receptor for the neurotransmitter acetylcholine. Is nicotine an agonist or an antagonist for acetylcholine? Explain your answer.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 8.9.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Alzheimers_Disease.jpg\" rel=\"cc:attributionURL\">Alzheimers_Disease<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en) license.<\/p>\n<p><strong>Figure 8.9.2<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/nihgov\/24524716351\/\" rel=\"cc:attributionURL\">Alzheimer\u2019s Disease stagess<\/a> by <a class=\"owner-name truncate no-outline\" title=\"Go to NIH Image Gallery's photostream\" href=\"https:\/\/www.flickr.com\/photos\/nihgov\/\" data-track=\"attributionNameClick\">NIH Image Gallery<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is in the <a class=\"photo-license-url\" href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" target=\"_newtab\" rel=\"license cc:license noopener noreferrer\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4021\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4021\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_2947\" aria-describedby=\"caption-attachment-2947\" style=\"width: 531px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2947\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Nest_Thermostat-1.jpg\" alt=\"\" width=\"531\" height=\"528\"><figcaption id=\"caption-attachment-2947\" class=\"wp-caption-text\"><em>Figure 7.8.1\u00a0 A thermostat controls a complex system to maintain a steady temperature in our homes.\u00a0<\/em><\/figcaption><\/figure>\n<h1>Steady as She Goes<\/h1>\n<p>This device (Figure 7.8.1) looks simple, but it controls a complex system that keeps a home at a steady temperature \u2014 it's a thermostat. The device shows the current temperature in the room, and also allows the occupant to set the thermostat to the desired temperature. A thermostat is a commonly cited model of how living systems \u2014 including the human body\u2014 maintain a steady state called homeostasis.<\/p>\n<div>\n<h1>What Is Homeostasis?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">Homeostasis<\/a><\/strong>\u00a0is the condition in which a system (such as the human body) is maintained in a more or less steady state. It is the job of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3573\">cells<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2801\">tissues<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2803\">organs<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2804\">organ systems<\/a>\u00a0throughout the body to maintain many different variables within narrow ranges compatible with life. Keeping a stable internal environment requires continually monitoring the internal environment and constantly making adjustments to keep things in balance.<\/p>\n<h2>Set Point and Normal Range<\/h2>\n<p>For any given variable, such as body\u00a0temperature\u00a0or\u00a0blood\u00a0glucose level, there is a particular\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2948\">set point<\/a><\/strong>\u00a0that is the physiological optimum value.\u00a0The set point for\u00a0human body\u00a0temperature, for example, is about 37 degrees C (98.6 degrees F). As the body works to maintain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a> for temperature or any other internal variable, the value typically fluctuates around the set point. Such fluctuations are normal, as long as they do not become too extreme. The spread of values within which such fluctuations are considered insignificant is called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2949\">normal range<\/a><\/strong>. In the case of body temperature, for example, the normal range for an adult is about 36.5 to 37.5 degrees C (97.7 to 99.5 degrees F).<\/p>\n<p>A good analogy for set point, normal range, and maintenance of homeostasis is driving.\u00a0 When you are driving a vehicle on the road, you are supposed to drive in the centre of your lane \u2014 this is analogous to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2948\">set point<\/a>.\u00a0 Sometimes, you are not driving in the <em>exact<\/em> centre of the lane, but you are still within your lines, so you are in the equivalent of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2949\">normal range<\/a>.\u00a0 However, if you were to get too close to the centre line or the shoulder of the road, you would take action to correct your position.\u00a0 You'd move left if you were too close to the shoulder, or right if too close to the centre line \u2014 which is analogous to our next concept, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2955\">negative feedback<\/a> to maintain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>.<\/p>\n<h2>Maintaining Homeostasis<\/h2>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">Homeostasis<\/a> is normally maintained in the human body by an extremely complex balancing act. Regardless of the variable being kept within its normal range, maintaining homeostasis requires at least four interacting components: stimulus, sensor, control centre, and effector.<\/p>\n<ol>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2950\">stimulus<\/a><\/strong>\u00a0is provided by the variable being regulated. Generally, the stimulus indicates that the value of the variable has moved away from the set point or has left the normal range.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2951\">sensor<\/a><\/strong> monitors the values of the variable and sends data on it to the control centre.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3859\">control centre<\/a><\/strong> matches the data with normal values. If the value is not at the set point or is outside the normal range, the control centre sends a signal to the effector.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3887\">effector<\/a><\/strong> is an organ, gland, muscle, or other structure that acts on the signal from the control centre to move the variable back toward the set point.<\/li>\n<\/ol>\n<p>Each of these components is illustrated in Figure 7.8.2. The diagram on the left is a general model showing how the components interact to maintain homeostasis. The diagram on the right shows the example of body temperature. From the diagrams, you can see that maintaining homeostasis involves feedback, which is data that feeds back to control a response. Feedback may be negative (as in the example below) or positive. All the feedback mechanisms that maintain homeostasis use <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2955\">negative feedback<\/a>. Biological examples of positive feedback are much less common.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2954\" aria-describedby=\"caption-attachment-2954\" style=\"width: 777px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-2954\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Negative_Feedback_Loops-1.jpg\" alt=\"\" width=\"777\" height=\"456\"><figcaption id=\"caption-attachment-2954\" class=\"wp-caption-text\"><em>Figure 7.8.2 Maintaining homeostasis through feedback requires a stimulus, sensor, control centre, and effector.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Negative Feedback<\/span><\/p>\n<\/div>\n<p>In a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2956\">negative feedback loop<\/a><\/strong>, feedback serves to reduce an excessive response and keep a variable within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2949\">normal range<\/a>.\u00a0Two\u00a0processes controlled by negative feedback\u00a0are\u00a0body temperature regulation and control of\u00a0blood\u00a0glucose.<\/p>\n<h2>Body Temperature<\/h2>\n<p>Body temperature regulation involves <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2955\">negative feedback<\/a>, whether it lowers the temperature or raises it, as shown in Figure 7.8.3 and explained in the text that follows.<\/p>\n<figure id=\"attachment_2963\" aria-describedby=\"caption-attachment-2963\" style=\"width: 725px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-2963\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Body-Temperature-Homeostasis-1.jpg\" alt=\"Homeostasis of Body Temperature\" width=\"725\" height=\"504\"><figcaption id=\"caption-attachment-2963\" class=\"wp-caption-text\"><em>Figure 7.8.3 Homeostasis of body temperature is maintained by negative feedback loops.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.424em\">Cooling Down<\/span><\/p>\n<\/div>\n<p>The human body\u2019s temperature regulatory centre is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2937\">hypothalamus<\/a> in the brain. When the hypothalamus receives data from sensors in the skin and brain that body temperature is higher than the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2948\">set point<\/a>, it sets into motion the following responses:<\/p>\n<ul>\n<li>Blood vessels\u00a0in the skin dilate (vasodilation) to allow more\u00a0blood\u00a0from the warm body core to flow close to the surface of the body, so\u00a0heat can be radiated into\u00a0the environment.<\/li>\n<li>As blood flow to the skin increases, sweat glands in the skin are activated to increase their output of sweat (diaphoresis). When the sweat evaporates from the skin surface into the surrounding air, it takes\u00a0heat\u00a0with it.<\/li>\n<li>Breathing\u00a0becomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increases\u00a0heat\u00a0loss from the lungs.<\/li>\n<\/ul>\n<h3>Heating Up<\/h3>\n<p>When the brain\u2019s temperature regulatory centre receives data that body temperature is lower than the set point, it sets into motion the following responses:<\/p>\n<ul>\n<li>Blood vessels\u00a0in the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body, which reduces heat loss from the surface.<\/li>\n<li>As temperature falls lower, random signals to\u00a0skeletal muscles\u00a0are triggered, causing them to contract. This causes shivering, which generates a small amount of heat.<\/li>\n<li>The\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2958\">thyroid gland<\/a>\u00a0may be stimulated by the brain (via the pituitary gland) to secrete more thyroid\u00a0hormone. This hormone increases metabolic activity and heat production in\u00a0cells\u00a0throughout the body.<\/li>\n<li>The\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3777\">adrenal glands<\/a>\u00a0may also be stimulated to secrete the\u00a0hormone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3783\">adrenaline<\/a>. This hormone causes the breakdown of glycogen (the\u00a0carbohydrate\u00a0used for\u00a0energy\u00a0storage in animals) to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3359\">glucose<\/a>, which can be used as an energy source. This catabolic chemical process is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3421\">exothermic<\/a>, or heat producing.<\/li>\n<\/ul>\n<h2>Blood Glucose<\/h2>\n<p>In controlling\u00a0the blood glucose level, certain endocrine\u00a0cells\u00a0in the\u00a0pancreas\u00a0(called alpha and beta cells) detect the level of glucose in the blood. They then respond appropriately to keep the level of blood glucose within the normal range.<\/p>\n<ul>\n<li>If the blood glucose level rises above the normal range, pancreatic beta cells release the\u00a0hormone\u00a0insulin into the bloodstream. Insulin signals cells to take up the excess glucose from the blood until the level of blood glucose decreases to the normal range.<\/li>\n<li>If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormone\u00a0<strong>glucagon<\/strong>\u00a0into the bloodstream. Glucagon signals cells to break down stored glycogen to glucose and release the glucose into the blood until the level of blood glucose increases to the normal range.<\/li>\n<\/ul>\n<div>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_331\" aria-describedby=\"caption-attachment-331\" style=\"width: 632px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-331\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Homeostasis_of_blood_sugar-1.png\" alt=\"Diagram shows the way in which the liver controls homeostasis of blood sugar by either storing glucose as glycogen when blood sugar levels are too high, or releasing glucose from glycogen when blood sugar levels are too low.\" width=\"632\" height=\"474\"><figcaption id=\"caption-attachment-331\" class=\"wp-caption-text\"><em>Figure 7.8.4 Your liver plays an important role in balancing blood sugar levels. Glycogen in your liver can either collect glucose out of your blood stream to lower blood sugar, or release glucose into the bloodstream to increase blood sugar.\u00a0 This happens through a negative feedback loop.<\/em><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=Iz0Q9nTZCw4<\/p>\n<p style=\"text-align: center\">Homeostasis and Negative\/Positive Feedback, Amoeba Sisters, 2017.<\/p>\n<h1>Positive Feedback<\/h1>\n<\/div>\n<p>In a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2962\">positive feedback loop<\/a><\/strong>, feedback serves to intensify a response until an end point is reached. Examples of processes controlled by positive feedback in the human body include blood clotting and childbirth.<\/p>\n<h2>Blood Clotting<\/h2>\n<figure id=\"attachment_2967\" aria-describedby=\"caption-attachment-2967\" style=\"width: 754px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2967\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Positive_Feedback_Diagram_Blood_Clotting-1.png\" alt=\"Positive Feedback Loops Blood Clotting\" width=\"754\" height=\"306\"><figcaption id=\"caption-attachment-2967\" class=\"wp-caption-text\"><em>Figure 7.8.5 The diagram demonstrates positive feedback, using the example of blood clotting in the body. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot. Every time the blood clot builds up more, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely healed and the positive feedback loop has ended. The graph represents the number of platelets aiding in the formation of the blood clot. The exponential form of the graph represents the positive feedback mechanism.<\/em><\/figcaption><\/figure>\n<p>When a wound causes bleeding, the body responds with a positive feedback loop to clot the blood and stop blood loss. Substances released by the injured blood vessel wall begin the process of blood clotting. Platelets in the blood start to cling to the injured site and release chemicals that attract additional platelets. As the platelets continue to amass, more of the chemicals are released and more platelets are attracted to the site of the clot. The positive feedback accelerates the process of clotting until the clot is large enough to stop the bleeding.<\/p>\n<h2>Childbirth<\/h2>\n<p>Figure 7.8.6 shows the positive feedback loop that controls childbirth. The process normally begins when the head of the infant pushes against the cervix. This stimulates nerve impulses, which travel from the cervix to the hypothalamus in the brain. In response, the hypothalamus sends the hormone <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3093\">oxytocin<\/a><\/strong>\u00a0to the\u00a0pituitary gland,\u00a0which secretes it into the bloodstream so it can be carried to the uterus. Oxytocin stimulates uterine contractions, which push the baby harder against the cervix. In response, the cervix starts to dilate in preparation for the passage of the baby. This cycle of positive feedback continues, with increasing levels of oxytocin, stronger uterine contractions, and wider dilation of the cervix until the baby is pushed through the birth canal and out of the body. At that point, the cervix is no longer stimulated to send\u00a0nerve impulses\u00a0to the brain, and the entire process stops.<\/p>\n<figure id=\"attachment_2966\" aria-describedby=\"caption-attachment-2966\" style=\"width: 714px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-2966\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Pregnancy-Positive_Feedback-1.jpg\" alt=\"Positive feedback childbirth\" width=\"714\" height=\"583\"><figcaption id=\"caption-attachment-2966\" class=\"wp-caption-text\"><em>Figure 7.8.6 Normal childbirth is driven by a positive feedback loop.\u00a0<\/em><\/figcaption><\/figure>\n<div>\n<p>Normal childbirth is driven by a positive feedback loop. Positive feedback causes an increasing deviation from the normal state to a fixed end point, rather than a return to a normal set point as in homeostasis.<\/p>\n<\/div>\n<div>\n<h1>When Homeostasis Fails<\/h1>\n<\/div>\n<p>Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do,\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2968\">homeostatic imbalance<\/a><\/strong>\u00a0may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease \u2014 or even death.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2969\">Diabetes<\/a>\u00a0is an example of a disease caused by homeostatic imbalance. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention can help restore homeostasis and possibly prevent permanent damage to the organism.<\/p>\n<p>Normal aging may bring about a reduction in the\u00a0efficiency\u00a0of the body\u2019s control systems, which makes the body more susceptible to disease.\u00a0Older people, for example, may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop serious heat-induced illnesses, such as heat stroke.<\/p>\n<div>\n<h1>Feature: My Human Body<\/h1>\n<\/div>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2969\">Diabetes<\/a>\u00a0is diagnosed in people who have abnormally high levels of blood glucose after fasting for at least 12 hours. A fasting level of blood glucose below 100 is normal. A level between 100 and 125 places you in the pre-diabetes category, and a level higher than 125 results in a diagnosis of diabetes.<\/p>\n<p>Of the two types of diabetes, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2970\">type 2 diabetes<\/a> is the most common, accounting for about 90 per cent of all cases of diabetes in the United States. <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/type-2-diabetes\/symptoms-causes\/syc-20351193\">Type 2 diabetes<\/a> typically starts after the age of 40. However, because of the dramatic increase in recent decades in obesity in younger people, the age at which type 2 diabetes is diagnosed has fallen. Even children are now being diagnosed with type 2 diabetes. Today, about 3 million Canadians (8.1% of total population) are living with diabetes.<\/p>\n<p>You may at some point have your blood glucose level tested during a routine medical exam. If your blood glucose level indicates that you have diabetes, it may come as a shock to you because you may not have any symptoms of the disease. You are not alone, because as many as one in four diabetics do not know they have the disease. Once the diagnosis of diabetes sinks in, you may be devastated by the news. Diabetes can lead to heart attacks, strokes, blindness, kidney failure, nerve damage, and loss of toes or feet. The risk of death in adults with diabetes is 50 per cent greater than it is in adults without diabetes, and diabetes is the seventh leading cause of death of adults. In addition, controlling diabetes usually requires frequent blood glucose testing, watching what and when you eat, and taking medications or even insulin injections. All of this may seem overwhelming.<\/p>\n<p>The good news is that changing your lifestyle may stop the progression of type 2 diabetes or even reverse it. By adopting healthier habits, you may be able to keep your blood glucose level within the normal range without medications or insulin. Here\u2019s how:<\/p>\n<ul>\n<li><strong>Lose\u00a0weight.<\/strong> Any\u00a0weight\u00a0loss is beneficial. Losing as little as\u00a0seven\u00a0per cent of your\u00a0weight\u00a0may be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.<\/li>\n<li><strong>Exercise\u00a0regularly.<\/strong>\u00a0You should try to\u00a0exercise\u00a0for at least 30 minutes, five days a week. This will not only lower your blood sugar and help your insulin work better, but it will also lower your\u00a0blood pressure\u00a0and improve your\u00a0heart\u00a0health. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.<\/li>\n<li><strong>Adopt a healthy diet.<\/strong> Decrease your consumption of refined carbohydrates, such as sweets and sugary drinks. Increase your intake of fibre-rich foods, such as fruits, vegetables, and whole grains. About one-quarter of each meal should consist of high-protein foods, such as fish, chicken, dairy products, legumes, or nuts.<\/li>\n<li><strong>Control stress.<\/strong>\u00a0Stress can increase your blood glucose and also raise your\u00a0blood pressure\u00a0and risk of\u00a0heart\u00a0disease. When you feel stressed out, do\u00a0breathing\u00a0exercises or take a brisk walk or jog.\u00a0Try to replace stressful thoughts with more calming ones.<\/li>\n<li><strong>Establish a support system.<\/strong>\u00a0Enlist the help and support of loved ones, as well as medical professionals, such as a nutritionist and diabetes educator. Having a support system will help ensure that you are on the path to wellness, and that you can stick to your plan.<\/li>\n<\/ul>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">7.8 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">Homeostasis<\/a> is the condition in which a system (such as the human body) is maintained in a more or less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain homeostasis.<\/li>\n<li>For any given variable, such as body temperature, there is a particular <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2948\">set point<\/a> that is the physiological optimum value. The spread of values around the set point that is considered insignificant is called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2949\">normal range<\/a>.<\/li>\n<li>Homeostasis is generally maintained by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2956\">negative feedback loop<\/a> that includes a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2950\">stimulus<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2951\">sensor<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3859\">control centre<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3887\">effector<\/a>. Negative feedback serves to reduce an excessive response and to keep a variable within the normal range. Negative feedback loops control body temperature and the blood glucose level.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2962\">Positive feedback loops<\/a>\u00a0are not common in biological systems. Positive feedback serves to intensify a response until an end point is reached. Positive feedback loops control blood clotting and childbirth.<\/li>\n<li>Sometimes homeostatic mechanisms fail, resulting in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2968\">homeostatic imbalance<\/a>. Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the\u00a0efficiency\u00a0of the body\u2019s control system,\u00a0which makes\u00a0the elderly more susceptible to disease.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">7.8 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-112\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-112\" class=\"h5p-iframe\" data-content-id=\"112\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"7.7 Review Questions Interaction of Organ Systems\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>\n<div id=\"h5p-111\">\n<div class=\"h5p-content\" data-content-id=\"111\"><\/div>\n<\/div>\n<\/li>\n<li>Compare and contrast negative and positive feedback loops.<\/li>\n<li>Explain how negative feedback controls body temperature.<\/li>\n<li>Give two examples of physiological processes controlled by positive feedback loops.<\/li>\n<li>During breastfeeding, the stimulus of the baby sucking on the nipple increases the amount of milk produced by the mother. The more sucking, the more milk is usually produced. Is this an example of negative or positive feedback? Explain your answer.\u00a0What do you think might be the evolutionary benefit of the milk production regulation mechanism\u00a0you described?<\/li>\n<li>Explain why homeostasis is regulated by negative feedback loops, rather than positive feedback loops.<\/li>\n<li>The level of a sex hormone, testosterone (T), is controlled by negative feedback. Another hormone, gonadotropin-releasing hormone (GnRH), is released by the hypothalamus of the brain, which triggers the\u00a0pituitary gland to release luteinizing hormone (LH). LH stimulates the gonads to produce T. When there is too much T in the bloodstream, it feeds back on the hypothalamus, causing it to produce less GnRH. While this does not describe all the feedback loops involved in regulating T, answer the following questions about this particular feedback loop.\n<ol type=\"a\">\n<li>What is the stimulus in this system? Explain your answer.<\/li>\n<li>What is the control centre in this system? Explain your answer.<\/li>\n<li>In this system, is the pituitary considered the stimulus, sensor, control centre, or effector? Explain your answer.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">7.8 Explore More<\/span><\/h1>\n<\/header>\n<p>https:\/\/www.youtube.com\/watch?v=LSgEJSlk6W4<\/p>\n<p style=\"text-align: center\">Homeostasis - What Is Homeostasis - What Is Set Point For Homeostasis - Homeostasis In The Human Body, Whats Up Dude, 2017.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=XMsJ-3qRVJM<\/p>\n<div style=\"text-align: center\">GCSE Biology - Homeostasis #38, Cognito, 2018.<\/div>\n<div><\/div>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 7.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Nest_Thermostat.JPG\" rel=\"cc:attributionURL\">Nest_Thermostat<\/a> by <a class=\"new\" title=\"User:Amanitamano (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Amanitamano&amp;action=edit&amp;redlink=1\">Amanitamano<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en\" rel=\"license\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en) license.<\/p>\n<p><strong>Figure 7.8.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:105_Negative_Feedback_Loops.jpg\" rel=\"cc:attributionURL\">Negative_Feedback_Loops<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-5-homeostasis\">OpenStax<\/a>\u00a0 on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/deed.en\">CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/deed.en) license.<\/p>\n<p><strong>Figure 7.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/cnx.org\/contents\/GFy_h8cu@10.8:BP24ZReh@7\/Homeostasis\" rel=\"cc:attributionURL\">Body Temperature Homeostasis<\/a>\u00a0by OpenStax College, <a href=\"https:\/\/cnx.org\/contents\/GFy_h8cu@10.8:BP24ZReh@7\/Homeostasis\">Biology<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY 4.0<\/a> license.<\/p>\n<p><strong>Figure 7.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Homeostasis_of_blood_sugar.png\" rel=\"cc:attributionURL\">Homeostasis_of_blood_sugar<\/a> by <a title=\"User:Christinelmiller\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Christinelmiller\">Christinelmiller<\/a> on Wikimedia Commons is used under a \u00a0<a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en\" rel=\"license\">CC0 1.0 Universal Public Domain Dedication<\/a> (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en) license.<\/p>\n<p><strong>Figure 7.8.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Positive_Feedback_Diagram_(2).svg\" rel=\"cc:attributionURL\">Positive_Feedback_Diagram_Blood_Clotting<\/a> by <a class=\"new\" title=\"User:Elliottuttle (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Elliottuttle&amp;action=edit&amp;redlink=1\">Elliottuttle<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0 <\/a>\u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 7.8.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:106_Pregnancy-Positive_Feedback.jpg\" rel=\"cc:attributionURL\">Pregnancy-Positive_Feedback<\/a> by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-5-homeostasis\">OpenStax<\/a>\u00a0 on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/deed.en\">CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/deed.en) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Amoeba Sisters. (2017, September 7). Homeostasis and negative\/positive feedback. YouTube. https:\/\/www.youtube.com\/watch?v=Iz0Q9nTZCw4&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <span class=\"os-title-label\">Figure\u00a0<\/span><span class=\"os-number\">1.10<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"17231\" class=\"os-title\" data-type=\"title\">Negative feedback loop [digital image\/ diagram]. <\/span>\u00a0In <em>Anatomy and Physiology<\/em> (Section 1.5). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-5-homeostasis<\/p>\n<p class=\"hanging-indent\"><span class=\"os-title-label\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><span class=\"os-number\">1.11<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"83721\" class=\"os-title\" data-type=\"title\">Positive feedback loop<\/span><span class=\"os-divider\">\u00a0<\/span><span class=\"os-caption\">normal childbirth is driven by a positive feedback loop <span id=\"17231\" class=\"os-title\" data-type=\"title\">[digital image\/ diagram]. <\/span>\u00a0In <em>Anatomy and Physiology<\/em> (Section 1.5). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-5-homeostasis<\/span><\/p>\n<p class=\"hanging-indent\">Cognito. (2018, December 18). GCSE Biology - Homeostasis #38. YouTube. https:\/\/www.youtube.com\/watch?v=XMsJ-3qRVJM&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Type 2 diabetes [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/type-2-diabetes\/symptoms-causes\/syc-20351193<\/p>\n<p class=\"hanging-indent\">OpenStax CNX. (2016, March 23). Figure 4 The body is able to regulate temperature in response to signals from the nervous system [digital image]. In <em><span class=\"name\">OpenStax<\/span>, Biology <\/em>(Section 33.3). https:\/\/cnx.org\/contents\/GFy_h8cu@10.8:BP24ZReh@7\/Homeostasis<\/p>\n<p class=\"hanging-indent\">Whats Up Dude. (2017, September 20). Homeostasis - What is homeostasis - What is set point for homeostasis - Homeostasis in the human body. YouTube. https:\/\/www.youtube.com\/watch?v=LSgEJSlk6W4&amp;feature=youtu.be<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4039\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4039\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3519\" aria-describedby=\"caption-attachment-3519\" style=\"width: 320px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3519\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Maud_Stevens_Wagner-1.jpeg-1.jpeg\" alt=\"10.2.1\" width=\"320\" height=\"450\"><figcaption id=\"caption-attachment-3519\" class=\"wp-caption-text\"><em>Figure 10.2.1 The body as a canvas.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Art for All Eras<\/h1>\n<\/div>\n<p>Pictured in Figure 10.2.1, is Maud Stevens Wagner, a tattoo artist from 1907. Tattoos are not just a late 20th and early 21st century trend. They have been popular in many eras and cultures. Tattoos literally illustrate the biggest organ of the human body: the skin. The skin is very thin, but it covers a large area \u2014 about 2 m<sup>2<\/sup>\u00a0in adults. The skin is the major organ in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2905\">integumentary system<\/a>.<\/p>\n<div>\n<h1>What Is the Integumentary System?<\/h1>\n<\/div>\n<p>In addition to the skin, the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2905\">integumentary system<\/a><\/strong>\u00a0includes the\u00a0hair\u00a0and\u00a0nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense\u00a0the environment, and help the body maintain\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>.<\/p>\n<div>\n<h1>The Skin<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a><\/strong> is remarkable not only because it is the body\u2019s largest organ: the average square inch of skin has 20 blood vessels, 650 sweat glands, and more than 1,000 nerve endings. Incredibly, it also has 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick. Although the skin is thin, it consists of two distinct layers: the epidermis and dermis, as shown in the diagram (Figure 10.2.2).<\/p>\n<figure id=\"attachment_3522\" aria-describedby=\"caption-attachment-3522\" style=\"width: 587px\" class=\"wp-caption alignnone\"><img class=\"wp-image-3522\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anatomy_The_Skin_-_NCI_Visuals_Online-1.jpg\" alt=\"Skin Diagram\" width=\"587\" height=\"585\"><figcaption id=\"caption-attachment-3522\" class=\"wp-caption-text\"><em>Figure 10.2.2 The epidermis is the thinner outer layer of skin, and the dermis is the thicker inner layer of skin. The latter contains structures such as blood vessels and sweat glands.<\/em><\/figcaption><\/figure>\n<h2>Outer Layer of Skin<\/h2>\n<p>The outer layer of skin is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a><\/strong>. This layer is thinner than the inner layer (the dermis). The epidermis consists mainly of epithelial\u00a0cells, called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>,<\/strong>\u00a0which produce the tough, fibrous\u00a0protein\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>.<\/strong>\u00a0The innermost\u00a0cells\u00a0of the epidermis are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3163\">stem cells<\/a>\u00a0that divide continuously to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing more and more keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer. As the dead cells are shed from the surface of the skin, they are replaced by other cells that move up from below. The epidermis also contains <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3526\"><strong>melanocytes<\/strong><\/a>, the cells that produce the brown pigment melanin, which gives skin most of its colour. Although the epidermis contains some sensory receptor cells \u2014 called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3527\">Merkel cells<\/a> \u2014 it contains no nerves, blood vessels, or other structures.<\/p>\n<h2>Inner Layer of Skin<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a><\/strong> is the inner, thicker layer of skin. It consists mainly of tough <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2815\">connective tissue<\/a>, and is attached to the epidermis by collagen fibres. The dermis contains many structures (as shown in Figure 10.2.2), including blood vessels, sweat glands, and hair follicles, which are structures where hairs originate. In addition, the dermis contains many sensory receptors, nerves, and oil glands.<\/p>\n<h2>Functions of the Skin<\/h2>\n<p>The skin has multiple roles in the body. Many of these roles are related to\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>. The skin\u2019s main functions\u00a0are\u00a0preventing\u00a0water\u00a0loss from the body and serving as a barrier to the entry of microorganisms. Another function of the skin is synthesizing vitamin D, which occurs when the skin is exposed to ultraviolet (UV) light. Melanin in the epidermis blocks some of the UV light and protects the dermis from its damaging effects.<\/p>\n<p>Another important function of the skin is helping to regulate body temperature. When the body is too warm, for example, the skin lowers body temperature by producing sweat, which cools the body when it evaporates. The skin also increases the amount of blood flowing near the body surface through vasodilation (widening of blood vessels), bringing heat from the body core to radiate out into the environment. The sweaty hair and flushed skin of the young man pictured in Figure 10.2.3 reflect these skin responses to overheating.<\/p>\n<figure id=\"attachment_3529\" aria-describedby=\"caption-attachment-3529\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3529\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/shashank-shekhar-Db1J_qp_ctc-unsplash-scaled-2.jpg\" alt=\"Man Sweating\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-3529\" class=\"wp-caption-text\"><em>Figure 10.2.3 Both sweating and flushing of the skin are signs that the skin is working to cool the body.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Hair<\/span><\/p>\n<\/div>\n<figure id=\"attachment_3532\" aria-describedby=\"caption-attachment-3532\" style=\"width: 200px\" class=\"wp-caption alignright\"><img class=\"wp-image-3532\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Eyelashes-by-aryan-dhiman-93NBu0zG_H4-unsplash-scaled-2.jpg\" alt=\"Eyelashes\" width=\"200\" height=\"267\"><figcaption id=\"caption-attachment-3532\" class=\"wp-caption-text\"><em>Figure 10.2.4 Eyelashes protect the eyes.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3531\">Hair<\/a><\/strong> is a fibre found only in mammals. It consists mainly of keratin-producing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>. Each hair grows out of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3923\">follicle<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>. By the time the hair reaches the surface, it consists mainly of dead cells filled with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>. Hair serves several homeostatic functions. Head hair is important in preventing heat loss from the head and protecting its skin from UV radiation. Hairs in the nose trap dust particles and microorganisms in the air, and prevent them from reaching the lungs. Hair all over the body provides sensory input when objects brush against it, or when it sways in moving air. Eyelashes and eyebrows (see Figure 10.2.4) protect the eyes from water, dirt, and other irritants.<\/p>\n<div>\n<h1>Nails<\/h1>\n<\/div>\n<p>Fingernails and toenails consist of dead <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a> filled with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>. The keratin makes them hard but flexible, which is important for the functions they serve. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3403\">Nails<\/a>\u00a0prevent injury by forming protective plates over the ends of the fingers and toes. They also enhance sensation by acting as a counterforce to the sensitive fingertips when objects are handled. In addition, the fingernails can be used as tools.<\/p>\n<div>\n<h1>Interactions with Other Organ Systems<\/h1>\n<\/div>\n<p>The skin and other parts of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2905\">integumentary system<\/a> work with other organ systems to maintain\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>.<\/p>\n<ul>\n<li>The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.<\/li>\n<li>Vitamin D is needed by the\u00a0digestive system\u00a0to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.<\/li>\n<li>To control body\u00a0temperature, the skin works with the\u00a0cardiovascular system\u00a0to either lose body\u00a0heat, or to conserve it through vasodilation or vasoconstriction.<\/li>\n<li>To detect certain sensations from the outside world, the\u00a0nervous system\u00a0depends on nerve receptors in the skin.<\/li>\n<\/ul>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.2 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2905\">integumentary system<\/a> consists of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3531\">hair<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3403\">nails<\/a>. Functions of the integumentary system include providing a protective covering for the body, sensing\u00a0the environment, and helping the body maintain homeostasis.<\/li>\n<li>The skin consists of two distinct layers: a thinner outer layer called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a>, and a thicker\u00a0inner layer called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>.<\/li>\n<li>The epidermis consists mainly of epithelial cells called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>, which produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>. New keratinocytes form at the bottom of the epidermis. They become filled with keratin and die as they move upward toward the surface of the skin, where they form a protective, waterproof layer.<\/li>\n<li>The dermis consists mainly of tough <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2815\">connective tissues<\/a>\u00a0and many structures, including blood vessels, sensory receptors, nerves, hair follicles, and oil and sweat glands.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>\u2019s main functions\u00a0are\u00a0preventing\u00a0water\u00a0loss from the body, serving as a barrier to the entry of microorganisms, synthesizing vitamin D, blocking UV light, and helping to regulate body temperature.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3531\">Hair<\/a> consists mainly of dead keratinocytes and grows out of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3923\">follicles<\/a>\u00a0in the dermis. Hair helps prevent heat loss from the head, and protects its skin from UV light. Hair in the nose filters incoming air, and the eyelashes and eyebrows keep harmful substances out of the\u00a0eyes. Hair all over the body provides tactile sensory input.<\/li>\n<li>Like hair, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3403\">nails<\/a>\u00a0also consist mainly of dead keratinocytes. They help protect the ends of the fingers and toes, enhance the sense of touch in the fingertips, and may be used as tools.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.2 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Name the organs of the integumentary system.<\/li>\n<li>Compare and contrast the epidermis and dermis.<\/li>\n<li>Identify functions of the skin.<\/li>\n<li>\n<div id=\"h5p-144\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-144\" class=\"h5p-iframe\" data-content-id=\"144\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Intro to Human Biology\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is the\u00a0composition of hair?<\/li>\n<li>Describe three physiological roles played by hair.<\/li>\n<li>What do nails consist of?<\/li>\n<li>List two functions of nails.<\/li>\n<li>In terms of\u00a0composition, what do the outermost surface of the skin, the nails, and hair have in common?<\/li>\n<li>Identify two types of cells found in the epidermis of the skin. Describe their functions.<\/li>\n<li>Which structure and layer of skin does hair grow out of?<\/li>\n<li>Identify three main functions of the integumentary system. Give an example of each.<\/li>\n<li>What are two ways in which the integumentary system protects the body against UV radiation?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.2 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=OxPlCkTKhzY<\/p>\n<p style=\"text-align: center\">The science of skin - Emma Bryce, TED-Ed, 2018.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=ZSJITdsTze0&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Why do we have to wear sunscreen? - Kevin P. Boyd, TED-Ed, 2013.<\/p>\n<p>https:\/\/www.youtube.com\/watch?time_continue=1&amp;v=Lfhot7tQcWs&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Scarification | National Geographic, 2008.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<div id=\"content\" class=\"site-content\">\n<section class=\"standard post-793 chapter type-chapter status-publish hentry focusable\" data-type=\"chapter\">\n<h2>Attributions<\/h2>\n<p><strong>Figure 10.2.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Maud_Stevens_Wagner.jpeg\" rel=\"cc:attributionURL\">Maud_Stevens_Wagner <\/a>-The Plaza Gallery, Los Angeles, 1907\u00a0from the <a href=\"https:\/\/www.loc.gov\/pictures\/item\/2006687059\/\">Library of Congress<\/a> on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/p>\n<p><strong>Figure 10.2.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Anatomy_The_Skin_-_NCI_Visuals_Online.jpg\" rel=\"cc:attributionURL\">Anatomy_The_Skin_-_NCI_Visuals_Online<\/a> by Don Bliss (artist) from <a class=\"extiw\" title=\"en:National Cancer Institute\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Cancer_Institute\">National Cancer Institute<\/a>, on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/p>\n<p><strong>Figure 10.2.3<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/Db1J_qp_ctc\" rel=\"cc:attributionURL\">shashank-shekhar-Db1J_qp_ctc<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@5hashank\">Shashank Shekhar<\/a> on <a href=\"http:\/\/unsplash.com\">Unsplash<\/a> is used under the \u00a0<a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 10.2.4<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/93NBu0zG_H4\" rel=\"cc:attributionURL\">Eyelashes by aryan-dhiman-93NBu0zG_H4<\/a> [photo] by <a href=\"https:\/\/unsplash.com\/@mylifeasaryan_\">Aryan Dhiman<\/a> on <a href=\"http:\/\/unsplash.com\">Unsplash<\/a> is used under the \u00a0<a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p>&nbsp;<\/p>\n<p><span style=\"font-size: 1.424em;font-weight: bold;color: #333333\">Reference<\/span><\/p>\n<\/section>\n<\/div>\n<p class=\"hanging-indent\">National Geographic. (2008). Scarification | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=Lfhot7tQcWs&amp;t=1s<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2018, March 12). The science of skin - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=OxPlCkTKhzY&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2013, August 6). Why do we have to wear sunscreen? - Kevin P. Boyd. YouTube. https:\/\/www.youtube.com\/watch?v=ZSJITdsTze0&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4040\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4040\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3535\" aria-describedby=\"caption-attachment-3535\" style=\"width: 350px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3535\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Sunburn-1.jpg\" alt=\"10.3.1\" width=\"350\" height=\"452\"><figcaption id=\"caption-attachment-3535\" class=\"wp-caption-text\"><em>Figure 10.3.1 Feel the burn!<\/em><\/figcaption><\/figure>\n<h1>Feel the Burn<\/h1>\n<p>The person in Figure 10.3.1 is no doubt feeling the burn \u2014 sunburn, that is. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3536\">Sunburn<\/a><\/strong>\u00a0occurs when the outer layer of the skin is damaged by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a> from the\u00a0sun\u00a0or tanning lamps. Some people deliberately allow UV light to burn their skin, because after the redness subsides, they are left with a tan. A tan may look healthy, but it is actually a sign of skin damage. People who experience one or more serious sunburns are significantly more likely to develop\u00a0skin <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancer<\/a>. Natural pigment molecules in the skin help protect it from UV light damage. These pigment molecules are found in the layer of the skin called the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a>.<\/p>\n<div>\n<h1>What is the\u00a0Epidermis?<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a><\/strong>\u00a0is the outer of the two main layers of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>. The inner layer\u00a0is\u00a0the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>. It averages about 0.10 mm thick, and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with \u2014 but structurally distinct from \u2014 the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3538\">mucous membranes<\/a>\u00a0that line the mouth, anus, urethra, and vagina.<\/p>\n<div>\n<h1>Structure of the Epidermis<\/h1>\n<\/div>\n<p>There are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1655\">diffusion<\/a> of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.<\/p>\n<h2>Cells\u00a0of the Epidermis<\/h2>\n<p>There are several different types of\u00a0cells\u00a0in the epidermis. All of the cells are necessary for the important functions of the epidermis.<\/p>\n<ul>\n<li>The epidermis consists mainly of stacks of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>-producing epithelial cells called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a><\/strong>. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.<\/li>\n<li>Another eight per cent of epidermal cells are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3526\">melanocytes<\/a><\/strong>. These cells produce the pigment melanin that protects the\u00a0dermis\u00a0from UV light.<\/li>\n<li>About one per cent of epidermal cells are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3540\">Langerhans cells<\/a><\/strong>. These are immune system cells that detect and fight pathogens entering the skin.<\/li>\n<li>Less than one per cent of epidermal cells are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3527\">Merkel cells<\/a><\/strong>, which respond to light touch and connect to nerve endings in the dermis.<\/li>\n<\/ul>\n<h2>Layers of the Epidermis<\/h2>\n<p>The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 10.3.2, and described in the following text.<\/p>\n<figure id=\"attachment_3541\" aria-describedby=\"caption-attachment-3541\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-3541\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0353_Epidermis-1.png\" alt=\"Diagram of the Epidermis\" width=\"1024\" height=\"1024\"><figcaption id=\"caption-attachment-3541\" class=\"wp-caption-text\"><em>Figure 10.3.2 The epidermis has multiple layers, and structures (such as hairs from the dermis below it) pass through them. This diagram illustrates the five layers that exist on the palms and soles of the feet.<\/em><\/figcaption><\/figure>\n<h3>Stratum Basale<\/h3>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3542\">stratum basale<\/a><\/strong>\u00a0is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3815\">basement membrane<\/a><\/strong>. The stratum basale contains stem cells \u2014 called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3811\">basal cells<\/a><\/strong>\u00a0\u2014 which divide to form all the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a> of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3545\">Melanocytes<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3527\">Merkel cells<\/a>\u00a0are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.<\/p>\n<h3>Stratum Spinosum<\/h3>\n<p>Just above the stratum basale is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3546\">stratum spinosum<\/a><\/strong>. This is the\u00a0thickest\u00a0of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3540\">Langerhans cells<\/a>.<\/p>\n<h3>Stratum Granulosum<\/h3>\n<p>The next layer above the stratum spinosum is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3547\">stratum granulosum<\/a><\/strong>. In this layer, keratinocytes have become nearly filled with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>, giving their cytoplasm a granular appearance.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3559\">Lipids<\/a>\u00a0are released by keratinocytes in this layer to form a\u00a0lipid\u00a0barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from\u00a0blood vessels\u00a0in the dermis to receive\u00a0nutrients. Each dying cell digests its own\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3705\">nucleus<\/a>\u00a0and\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3465\">organelles<\/a>, leaving behind only a tough, keratin-filled shell.<\/p>\n<h3>Stratum Lucidum<\/h3>\n<p>Only on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3548\">stratum lucidum<\/a><\/strong>. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.<\/p>\n<h3>Stratum Corneum<\/h3>\n<p>The uppermost layer of the epidermis everywhere on the body is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3549\">stratum corneum<\/a><\/strong>. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.<\/p>\n<div>\n<h1>Functions of the Epidermis<\/h1>\n<\/div>\n<p>The epidermis has several crucial functions in the body. These functions include protection,\u00a0water\u00a0retention, and vitamin D synthesis.<\/p>\n<h2>Protective Functions<\/h2>\n<p>The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.<\/p>\n<h3>Protection from Physical Damage<\/h3>\n<p>Most of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.<\/p>\n<h3>Protection from Pathogens<\/h3>\n<figure id=\"attachment_3550\" aria-describedby=\"caption-attachment-3550\" style=\"width: 337px\" class=\"wp-caption alignright\"><img class=\" wp-image-3550\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Scraped-knee-by-Alpha-on-flickr-1.jpg\" alt=\"Skin: Protection from Pathogens\" width=\"337\" height=\"253\"><figcaption id=\"caption-attachment-3550\" class=\"wp-caption-text\"><em>Figure 10.3.3 This scrape on the knee provides an opportunity for bacteria to enter the body through the broken skin.<\/em><\/figcaption><\/figure>\n<p>When pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 10.3.3). That\u2019s why it is important to clean and cover even a <em>minor<\/em> wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH\u00a0of about 5.0), low amounts of\u00a0water, the presence of antimicrobial substances produced by epidermal cells, and\u00a0competition\u00a0with non-pathogenic microorganisms that normally live on the epidermis.<\/p>\n<p>&nbsp;<\/p>\n<h3>Protection from UV Light<\/h3>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a>\u00a0that penetrates the epidermis can damage epidermal cells. In particular, it can cause\u00a0mutations\u00a0in\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a>\u00a0that lead to the\u00a0development\u00a0of\u00a0skin <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancer<\/a>, in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful\u00a0reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body\u2019s vitamin B9 level by <em>50 per cent.<\/em><\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3526\">Melanocyte<\/a>s in the stratum basale of the epidermis contain small\u00a0organelles\u00a0called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3759\">melanosomes<\/a><\/strong>, which produce, store, and transport the dark brown pigment <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3753\">melanin<\/a>. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>\u00a0in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.<\/p>\n<h2>Water\u00a0Retention<\/h2>\n<p>Skin's ability\u00a0to hold water and not lose it to the surrounding environment is due mainly to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3549\">stratum corneum<\/a>.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3559\">Lipids<\/a>\u00a0arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.<\/p>\n<p>Although the skin is impermeable to water, it is not impermeable to\u00a0<em>all <\/em>substances. Instead, the skin is\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3767\"><em>selectively<\/em>\u00a0permeable<\/a>, allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.<\/p>\n<ul>\n<li>Selective permeability allows certain medications to enter the bloodstream through the capillaries in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 10.3.4) that are applied to the skin. These include steroid hormones, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a> (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).<\/li>\n<li>Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.<\/li>\n<\/ul>\n<figure id=\"attachment_3554\" aria-describedby=\"caption-attachment-3554\" style=\"width: 394px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3554\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Nicoderm-1.jpg\" alt=\"Skin - selectively permeable\" width=\"394\" height=\"262\"><figcaption id=\"caption-attachment-3554\" class=\"wp-caption-text\"><em>Figure 10.3.4 This skin patch delivers small amounts of nicotine through the skin of a person in a smoking cessation program.<\/em><\/figcaption><\/figure>\n<h3>Vitamin D Synthesis<\/h3>\n<p>Vitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.<\/p>\n<div>\n<h1>What Gives Skin Its Colour?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3753\">Melanin<\/a><\/strong> in the epidermis is the main substance that determines the colour of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin colour, however, especially in light-skinned people: carotene and hemoglobin.<\/p>\n<ul>\n<li>The pigment\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3749\">carotene<\/a><\/strong>\u00a0is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3556\">Hemoglobin<\/a><\/strong> is a red pigment found in red\u00a0blood\u00a0cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink colour is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.<\/li>\n<\/ul>\n<p>Hear what Bill Nye has to say about the subject of skin colour in the video <a href=\"https:\/\/youtu.be\/zOkj5jgC4sM\">here<\/a>.<\/p>\n<div>\n<h1>Bacteria\u00a0on Skin<\/h1>\n<\/div>\n<figure id=\"attachment_3557\" aria-describedby=\"caption-attachment-3557\" style=\"width: 251px\" class=\"wp-caption alignleft\"><img class=\" wp-image-3557\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Staphylococcus-aureus-bacteria-by-Microbe-World-on-flickr-1.jpg\" alt=\"Skin - protects against bacteria\" width=\"251\" height=\"170\"><figcaption id=\"caption-attachment-3557\" class=\"wp-caption-text\"><em>Figure 10.3.5 The bacterium Staphylococcus epidermidis is a common microorganism living on healthy human skin.<\/em><\/figcaption><\/figure>\n<p>The surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 10.3.5) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.<\/p>\n<p>In general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy.\u00a0If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain\u00a0species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as\u00a0hair\u00a0follicles) and in adjacent areas of the skin.<\/p>\n<div>\n<h1>Feature: Myth vs. Reality<\/h1>\n<\/div>\n<p>Because of the negative health effects of excessive UV light exposure, it is important to know the facts about protecting the skin from UV light.<\/p>\n<table class=\"grid\" style=\"border-collapse: collapse;width: 84.1914%;height: 84px\" border=\"0\">\n<tbody>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\">\n<h2><span style=\"color: #ff0000\">Myth<\/span><\/h2>\n<\/td>\n<td style=\"width: 55.0036%;height: 14px\">\n<h2><span style=\"color: #339966\">Reality<\/span><\/h2>\n<\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\"><em>\"Sunblock and sunscreen are just different names for the same type of product. They both work the same way and are equally effective.\"<\/em><\/td>\n<td style=\"width: 55.0036%;height: 14px\"><strong><span style=\"font-size: 16px\">Sunscreens and sunblocks are different types of products that protect the skin from UV light in different ways. They are not equally effective. Sunblocks are opaque, so they do not let light pass through. They prevent most of the rays of UV light from penetrating to the skin surface. Sunblocks are generally stronger and more effective than sunscreens. Sunblocks also do not need to be reapplied as often as sunscreens. Sunscreens, in contrast, are transparent once they are applied the skin. Although they can prevent most UV light from penetrating the skin when first applied, the active ingredients in sunscreens tend to break down when exposed to UV light. Sunscreens, therefore, must be reapplied often to remain effective.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\"><em><span style=\"font-size: 16px\">\"The skin needs to be protected from UV light only on sunny days. When the sky is cloudy, UV light cannot penetrate to the ground and harm the skin.\"<\/span><\/em><\/td>\n<td style=\"width: 55.0036%;height: 14px\"><strong><span style=\"font-size: 16px\">Even on cloudy days, a significant amount of UV radiation penetrates the atmosphere to strike Earth\u2019s surface. Therefore, using sunscreens or sunblocks to protect exposed skin is important even when there are\u00a0clouds\u00a0in the sky.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\"><em><span style=\"font-size: 16px\">\"People who have dark skin, such as African Americans, do not need to worry about skin damage from UV light.\"<\/span><\/em><\/td>\n<td style=\"width: 55.0036%;height: 14px\"><strong><span style=\"font-size: 16px\">No matter what colour skin you have, your skin can be damaged by too much exposure to UV light. Therefore, even dark-skinned people should use sunscreens or sunblocks to protect exposed skin from UV light.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\"><em><span style=\"font-size: 16px\">\"Sunscreens with an SPF (sun protection factor) of 15 are adequate to fully protect the skin from UV light.\"<\/span><\/em><\/td>\n<td style=\"width: 55.0036%;height: 14px\"><strong><span style=\"font-size: 16px\">Most dermatologists recommend using sunscreens with an SPF of at least 35 for adequate protection from UV light. They also recommend applying sunscreens at least 20 minutes before\u00a0sun\u00a0exposure and reapplying sunscreens often, especially if you are sweating or spending time in the water.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td style=\"width: 29.1878%;height: 14px\"><em><span style=\"font-size: 16px\">\"Using tanning beds is safer than tanning outside in natural sunlight.\"<\/span><\/em><\/td>\n<td style=\"width: 55.0036%;height: 14px\"><strong><span style=\"font-size: 16px\">The light in tanning beds is UV light, and it can do the same damage to the skin as the natural UV light in sunlight. This is evidenced by the fact that people who regularly use tanning beds have significantly higher rates of\u00a0skin cancer\u00a0than people who do not. It is also the reason that the use of tanning beds is prohibited in many places in people who are under the age of 18, just as youth are prohibited from using harmful substances, such as tobacco and alcohol.<\/span><\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.3 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a> is the outer of the two main layers of the skin. It is very thin, but has a complex structure.<\/li>\n<li>Cell types in the epidermis include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a> that produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a> and make up 90 per cent of epidermal cells, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3526\">melanocytes<\/a> that produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3753\">melanin<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3540\">Langerhans cells<\/a> that fight <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3707\">pathogens<\/a> in the skin, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3527\">Merkel cells<\/a> that respond to light touch.<\/li>\n<li>The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs only in the epidermis of the palms of the hands and soles of the feet.<\/li>\n<li>The innermost layer of the epidermis is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3542\">stratum basale<\/a>, which contains stem cells that divide to form new keratinocytes. The next layer is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3546\">stratum spinosum<\/a>, which is the\u00a0thickest layer and contains Langerhans cells and spiny keratinocytes. This is followed by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3547\">stratum granulosum<\/a>, in which keratinocytes are filling with keratin and starting to die. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3548\">stratum lucidum<\/a> is next, but only on the palms and soles. It consists of translucent dead keratinocytes. The outermost layer is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3549\">stratum corneum<\/a>, which consists of flat, dead, tightly packed keratinocytes that form a tough, waterproof barrier for the rest of the epidermis.<\/li>\n<li>Functions of the epidermis include protecting underlying tissues from physical damage and pathogens. Melanin in the epidermis absorbs and protects underlying tissues from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a>. The epidermis also prevents loss of water from the body and synthesizes vitamin D.<\/li>\n<li>Melanin is the main pigment that determines the colour of human skin. The pigments carotene and hemoglobin, however, also contribute to skin colour, especially in skin with low levels of melanin.<\/li>\n<li>The surface of healthy skin normally is covered by vast numbers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> representing about one thousand\u00a0species\u00a0from 19 phyla. Different areas of the body provide diverse habitats for skin microorganisms. Usually, microorganisms on the skin keep each other in check unless their balance is disturbed.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.3 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is the epidermis?<\/li>\n<li>Identify the types of cells in the epidermis.<\/li>\n<li>Describe the layers of the epidermis.<\/li>\n<li>\n<div id=\"h5p-145\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-145\" class=\"h5p-iframe\" data-content-id=\"145\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Label the atom\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>State one function of each of the four epidermal layers found all over the body.<\/li>\n<li>Explain three ways the epidermis protects the body.<\/li>\n<li>What makes the skin waterproof?<\/li>\n<li>Why is the selective permeability of the epidermis both a benefit and a risk?<\/li>\n<li>How is vitamin D synthesized in the epidermis?<\/li>\n<li>Identify three pigments that impart colour to skin.<\/li>\n<li>Describe bacteria that normally reside on the skin, and explain why they do not usually cause infections.<\/li>\n<li>Explain why the keratinocytes at the surface of the epidermis are dead, while keratinocytes located deeper in the epidermis are still alive.<\/li>\n<li>Which layer of the epidermis contains keratinocytes that have begun to die?<\/li>\n<li>\n<div id=\"h5p-146\">\n<div class=\"h5p-content\" data-content-id=\"146\"><\/div>\n<\/div>\n<\/li>\n<li>Explain why our skin is not permanently damaged if we rub off some of the surface layer by using a rough washcloth.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\" style=\"text-align: left\"><span style=\"color: #ffffff\">10.3 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=27lMmdmy-b8<\/p>\n<p style=\"text-align: center\">Jonathan Eisen: Meet your microbes, TED,\u00a02015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=9AcQXnOscQ8<\/p>\n<p style=\"text-align: center\">Why Do We Blush?, SciShow, 2014.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=_r4c2NT4naQ<\/p>\n<p style=\"text-align: center\">The science of skin colour - Angela Koine Flynn, TED-Ed, 2016.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 10.3.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sunburn.jpg\" rel=\"cc:attributionURL\">Sunburn<\/a> by <a class=\"extiw\" title=\"wikipedia:User:QuinnHK\" href=\"https:\/\/en.wikipedia.org\/wiki\/User:QuinnHK\">QuinnHK<\/a>\u00a0at\u00a0<a class=\"extiw\" title=\"wikipedia:\" href=\"https:\/\/en.wikipedia.org\/wiki\/\">English Wikipedia<\/a> on Wikimedia Commons is released into the <a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 10.3.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0353_Epidermis.png\" rel=\"cc:attributionURL\">Blausen_0353_Epidermis<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 10.3.3<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/avlxyz\/10975825785\" rel=\"cc:attributionURL\">Isaac's scraped knee close-up<\/a> by <a class=\"owner-name truncate\" title=\"Go to Alpha's photostream\" href=\"https:\/\/www.flickr.com\/photos\/avlxyz\/\" data-track=\"attributionNameClick\">Alpha<\/a> on <a href=\"http:\/\/flickr.com\" rel=\"cc:attributionURL\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/\" rel=\"license\">CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.<\/p>\n<p><strong>Figure 10.3.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Nicoderm.JPG\" rel=\"cc:attributionURL\">Nicoderm<\/a> by <a title=\"User:RegBarc\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:RegBarc\">RegBarc<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en\">CC BY-SA 3.0<\/a> (http:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/) license. (No machine-readable author provided for original.)<\/p>\n<p><strong>Figure 10.3.5<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/microbeworld\/5619255092\">Staphylococcus aureus bacteria, MRSA<\/a> by <a class=\"owner-name truncate\" title=\"Go to Microbe World's photostream\" href=\"https:\/\/www.flickr.com\/photos\/microbeworld\/\" data-track=\"attributionNameClick\">Microbe World<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/\" rel=\"license\">CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\n<p class=\"hanging-indent\">Jeff Bone 'n' Pookie. (2020, July 19). Bill Nye the science guy explains we have different skin color. Youtube. https:\/\/www.youtube.com\/watch?v=zOkj5jgC4sM&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">SciShow. (2014, July 15). Why do we blush? YouTube. https:\/\/www.youtube.com\/watch?v=9AcQXnOscQ8<\/p>\n<p class=\"hanging-indent\">TED. (2015, July 17). Jonathan Eisen: Meet your microbes. YouTube. https:\/\/www.youtube.com\/watch?v=27lMmdmy-b8<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2016, February 16). The science of skin color - Angela Koine Flynn. YouTube. https:\/\/youtu.be\/_r4c2NT4naQ<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4041\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4041\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<div>\n<figure id=\"attachment_3560\" aria-describedby=\"caption-attachment-3560\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3560\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Goose_bumps-1.jpg\" alt=\"10.4.1\" width=\"400\" height=\"209\"><figcaption id=\"caption-attachment-3560\" class=\"wp-caption-text\"><em>Figure 10.4.1 Goose bumps!<\/em><\/figcaption><\/figure>\n<h1>Goose Bumps<\/h1>\n<\/div>\n<p>No doubt you\u2019ve experienced the tiny, hair-raising skin bumps called goose bumps, like those you see in Figure 10.4.1. They happen when you feel chilly. Do you know what causes goose bumps, or why they pop up when you are cold? The answers to these questions involve the layer of skin known as the dermis.<\/p>\n<div>\n<h1>What is the Dermis?<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a><\/strong>\u00a0is the inner of the two major layers that make up the skin, the outer layer being the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3899\">epidermis<\/a><\/strong>. The dermis consists mainly of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2815\">connective tissues<\/a>. It also contains most skin structures, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2936\">glands<\/a>\u00a0and\u00a0blood vessels. The dermis is anchored to the tissues below it by flexible collagen bundles that permit most areas of the skin to move freely over subcutaneous (\u201cbelow the skin\u201d) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body\u00a0temperature, sensing\u00a0the environment, and excreting wastes.<\/p>\n<div>\n<h1>Anatomy of the Dermis<\/h1>\n<\/div>\n<p>The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibres \u2014 which provide toughness \u2014 and elastin fibres, which provide elasticity. Surrounding these fibres, the matrix also includes a gel-like substance made of proteins. The tissues of the matrix give the dermis both strength and flexibility.<\/p>\n<p>The dermis is divided into two layers: the papillary layer and the reticular layer. Both layers are shown in Figure 10.4.2 below and described in the text that follows.<\/p>\n<figure id=\"attachment_3561\" aria-describedby=\"caption-attachment-3561\" style=\"width: 376px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3561\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Layers_of_the_Dermis-1.jpg\" alt=\"Layers of the Dermis\" width=\"376\" height=\"485\"><figcaption id=\"caption-attachment-3561\" class=\"wp-caption-text\"><em>Figure 10.4.2 This photomicrograph shows a cross-section of the papillary and reticular layers of the dermis.<\/em><\/figcaption><\/figure>\n<h2>Papillary Layer<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3751\">papillary layer<\/a><\/strong> is the upper layer of the dermis, just below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3815\">basement membrane<\/a> that connects the dermis to the epidermis above it. The papillary layer is the thinner of the two dermal layers. It is composed mainly of loosely arranged collagen fibres. The papillary layer is named for its fingerlike projections \u2014 or papillae \u2014 that extend upward into the epidermis. The papillae contain capillaries and sensory touch receptors.<\/p>\n<figure id=\"attachment_3563\" aria-describedby=\"caption-attachment-3563\" style=\"width: 390px\" class=\"wp-caption alignleft\"><img class=\" wp-image-3563\" src=\"http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/Fingerprint_detail_on_male_finger_in_T\u0159eb\u00ed\u010d_T\u0159eb\u00ed\u010d_District.jpg#fixme\" alt=\"10.3 Fingerprints\" width=\"390\" height=\"260\"><figcaption id=\"caption-attachment-3563\" class=\"wp-caption-text\"><em>Figure 10.4.3 This photo is an enlarged image of epidermal ridges on a finger.<\/em><\/figcaption><\/figure>\n<p>The papillae give the dermis a bumpy surface that interlocks with the epidermis above it, strengthening the connection between the two layers of skin. On the palms and soles, the papillae create epidermal ridges. Epidermal ridges on the fingers are commonly called fingerprints (see Figure 10.4.3). Fingerprints are genetically determined, so no two people (other than identical twins) have exactly the same fingerprint pattern. Therefore, fingerprints can be used as a means of identification, for example, at crime scenes. Fingerprints were much more commonly used forensically before DNA analysis was introduced for this purpose.<\/p>\n<h2>Reticular Layer<\/h2>\n<p>The\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3755\"><strong>reticular layer<\/strong> <\/a>is the lower layer of the dermis, located below the papillary layer. It is the thicker of the two dermal layers. It is composed of densely woven collagen and elastin fibres. These protein fibres give the dermis its properties of strength and elasticity. This layer of the dermis cushions subcutaneous tissues of the body from stress and strain. The reticular layer of the dermis also contains most of the structures in the dermis, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2936\">glands<\/a> and hair <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3923\">follicle<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">.<\/span><\/p>\n<div>\n<h1>Structures in the Dermis<\/h1>\n<\/div>\n<p>Both papillary and reticular layers of the dermis contain numerous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3006\">sensory receptors<\/a>, which make the skin the body\u2019s primary sensory organ for the sense of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3129\">touch<\/a>. Both dermal layers also contain blood vessels. They provide nutrients to remove wastes from dermal cells, as well as cells in the lowest layer of the epidermis, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3542\">stratum basale<\/a>. The circulatory components of the dermis are shown in Figure 10.4.4 below.<\/p>\n<figure id=\"attachment_3565\" aria-describedby=\"caption-attachment-3565\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-3565\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0802_Skin_DermalCirculation-1.png\" alt=\"10.3 Dermal Circulation\" width=\"1024\" height=\"819\"><figcaption id=\"caption-attachment-3565\" class=\"wp-caption-text\"><em>Figure 10.4.4 Both the papillary layer and the reticular layer of the dermis contain blood vessels, as shown in this diagram.<\/em><\/figcaption><\/figure>\n<h2>Glands<\/h2>\n<p>Glands in the reticular layer of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram in Figure 10.4.5 shows these glands, as well as several other structures in the dermis.<\/p>\n<figure id=\"attachment_3567\" aria-describedby=\"caption-attachment-3567\" style=\"width: 702px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3567 \" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anatomy_The_Skin_-_NCI_Visuals_Online-1-e1591224136617-1.jpg\" alt=\"10.4 Skin Glands\" width=\"702\" height=\"580\"><figcaption id=\"caption-attachment-3567\" class=\"wp-caption-text\"><em>Figure 10.4.5 The dermis contains sweat and oil (sebaceous) glands, as well as hair follicles and blood vessels.<\/em><\/figcaption><\/figure>\n<h3>Sweat Glands<\/h3>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3566\">Sweat glands<\/a><\/strong>\u00a0produce the fluid called sweat, which contains mainly\u00a0water\u00a0and salts. The glands have ducts that carry the sweat to\u00a0hair\u00a0follicles, or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3568\">Eccrine sweat gland<strong>s<\/strong><\/a><\/strong>\u00a0occur in skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in\u00a0temperature\u00a0regulation.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3569\">Apocrine sweat gland<strong>s<\/strong><\/a><\/strong>\u00a0are larger than eccrine glands, and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until\u00a0puberty, at which point they start producing an oily sweat that is consumed by\u00a0bacteria\u00a0living on the skin. The digestion of apocrine sweat by bacteria\u00a0causes\u00a0body odor.<\/li>\n<\/ul>\n<h3>Sebaceous Glands<\/h3>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3570\">Sebaceous gland<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0are exocrine glands that produce a thick, fatty substance called sebum.\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3571\">Sebum<\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0is secreted into hair follicles and makes its way to the skin surface along hairs. It waterproofs the hair and skin, and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin \u2014 except for the palms of the hands and soles of the feet, where hair does not grow.<\/span><\/p>\n<h2>Hair Follicles<\/h2>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3572\">Hair follicle<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0are the structures where hairs originate (see the diagram\u00a0above). Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin. Associated with each hair follicle is a sebaceous gland, which secretes sebum that coats and waterproofs the hair. Each follicle also has a bed of capillaries, a nerve ending, and a tiny muscle called an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3747\">arrector pili<\/a>.<\/span><\/p>\n<div>\n<h1>Functions of the Dermis<\/h1>\n<\/div>\n<p>The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.<\/p>\n<h2>Temperature Regulation<\/h2>\n<p>Several structures in the reticular layer of the dermis are involved in regulating body temperature. For example, when body temperature rises, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2937\">hypothalamus<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3823\">brain<\/a> sends nerve signals to sweat glands, causing them to release sweat. An adult can sweat up to four litres an hour. As the sweat evaporates from the surface of the body, it uses energy in the form of body heat, thus cooling the body. The hypothalamus also causes dilation of blood vessels in the dermis when body temperature rises. This allows more blood to flow through the skin, bringing body heat to the surface, where it can radiate into the environment.<\/p>\n<p>When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body\u00a0heat. The arrector pili\u00a0muscles\u00a0also contract, moving hair follicles and lifting hair shafts. This results in more air being trapped under the hairs to insulate the surface of the skin. These contractions of arrector pili muscles are the cause of goose bumps.<\/p>\n<h2>Sensing\u00a0the Environment<\/h2>\n<p>Sensory receptors in the dermis are mainly responsible for the body\u2019s tactile\u00a0senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send\u00a0nerve impulses\u00a0to the brain, which interprets and responds to the sensory information. Sensory receptors in the dermis can be classified on the basis of the type of touch stimulus they sense.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3124\">Mechanoreceptor<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0sense mechanical forces such as pressure, roughness, vibration, and stretching.\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3125\">Thermoreceptor<\/strong><strong style=\"font-size: 1em;text-align: initial\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0sense variations in temperature that are above or below body temperature.\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3126\">Nociceptor<\/strong><strong style=\"font-size: 1em;text-align: initial\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\"> sense painful stimuli. Figure 10.4.6 shows several specific kinds of tactile receptors in the dermis. Each kind of receptor senses one or more types of touch stimuli.<\/span><\/p>\n<ul>\n<li>Free nerve endings sense pain and temperature variations.<\/li>\n<li>Merkel\u00a0cells\u00a0sense light touch, shapes, and textures.<\/li>\n<li>Meissner\u2019s corpuscles sense light touch.<\/li>\n<li>Pacinian corpuscles sense pressure and vibration.<\/li>\n<li>Ruffini corpuscles sense stretching and sustained pressure.<\/li>\n<\/ul>\n<figure id=\"attachment_3574\" aria-describedby=\"caption-attachment-3574\" style=\"width: 836px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3574 \" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0809_Skin_TactileReceptors-e1591224860583-1.png\" alt=\"10.4 Tactile Receptors\" width=\"836\" height=\"669\"><figcaption id=\"caption-attachment-3574\" class=\"wp-caption-text\"><em>Figure 10.4.6 A variety of types of tactile receptors are located in the dermis of the skin.<\/em><\/figcaption><\/figure>\n<h2>Excreting Wastes<\/h2>\n<p>The sweat released by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3568\">eccrine sweat gland<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0is one way the body excretes waste products. Sweat contains excess\u00a0water, salts (electrolytes), and other waste products that the body must get rid of to maintain\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>. The most common electrolytes in sweat are sodium and chloride. Potassium, calcium, and magnesium electrolytes may be excreted in sweat, as well. When these electrolytes reach high levels in the\u00a0blood, more are excreted in sweat. This helps to bring their blood levels back into balance. Besides electrolytes, sweat contains small amounts of waste products from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3681\">metabolism<\/a>, including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.<\/span><\/p>\n<div>\n<h1>Feature: My\u00a0Human Body<\/h1>\n<\/div>\n<figure id=\"attachment_3575\" aria-describedby=\"caption-attachment-3575\" style=\"width: 343px\" class=\"wp-caption alignright\"><img class=\"wp-image-3575\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Akne-jugend-1.jpg\" alt=\"10.4.7\" width=\"343\" height=\"234\"><figcaption id=\"caption-attachment-3575\" class=\"wp-caption-text\"><em>Figure 10.4.7 Acne can be embarrassing, but most people will experience it at one point in their lives.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3576\">Acne<\/a><\/strong> is the most common skin disorder in the Canada. At least 20% of Canadians have acne at any given time and it affects approximately 90% of adolescents (as in Figure 10.4.7). Although acne occurs most commonly in teens and young adults, but it can occur at any age. Even newborn babies can get acne.<\/p>\n<p>The main sign of acne is the appearance of pimples (pustules) on the skin, like those in the photo above. Other signs of acne may include whiteheads, blackheads, nodules, and other lesions. Besides the face, acne can appear on the back, chest, neck, shoulders, upper arms, and buttocks. Acne can permanently scar the skin, especially if it isn\u2019t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.<\/p>\n<p>Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of\u00a0bacteria. The bacteria cause infection, and the immune system responds with inflammation. Inflammation, in turn, causes swelling and redness, and may be associated with the formation of pus. If the inflammation goes deep into the skin, it may form an acne nodule.<\/p>\n<p>Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic\u00a0acid. Treatment with these products may take a month or two to clear up the acne. Once the skin clears, treatment generally needs to continue for some time to prevent future breakouts.<\/p>\n<p>If acne fails to respond to OTC products, nodules develop, or acne is affecting self-esteem, a visit to a dermatologist is in order. A dermatologist can determine which treatment is best for a given patient. A dermatologist can also prescribe prescription medications (which are likely to be more effective than OTC products) and provide other medical treatments, such as laser light therapies or chemical peels.<\/p>\n<p>What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:<\/p>\n<ul>\n<li>Wash affected or acne-prone skin (such as the face) twice a day, and after sweating.<\/li>\n<li>Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.<\/li>\n<li>Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.<\/li>\n<li>Rinse with lukewarm\u00a0water, and avoid using very hot or cold water.<\/li>\n<li>Shampoo your hair regularly.<\/li>\n<li>Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.<\/li>\n<li>Keep your hands off your face. Avoid touching your skin throughout the day.<\/li>\n<li>Stay out of the\u00a0sun\u00a0and tanning beds. Some acne medications make your skin very sensitive to UV light.<\/li>\n<\/ul>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.4 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a> is the inner and thicker of the two major layers that make up the skin. It consists mainly of a matrix of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2815\">connective tissue<\/a>s that provide strength and stretch. It also contains almost all skin structures, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3006\">sensory receptors<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3743\">blood vessel<\/a>s.<\/li>\n<li>The dermis has two layers. The upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3751\">papillary layer<\/a> has papillae extending upward into the epidermis and loose connective tissues. The lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3755\">reticular layer<\/a> has denser connective tissues and structures, such as glands and hair follicles. Glands in the dermis include eccrine and apocrine sweat glands and sebaceous glands. Hair <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3923\">follicles<\/a>\u00a0are structures where hairs originate.<\/li>\n<li>Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes. The dense connective tissues of the dermis provide cushioning. The dermis regulates body temperature mainly by sweating and by vasodilation or vasoconstriction. The many tactile sensory receptors in the dermis make it the main organ for the sense of touch. Wastes excreted in sweat include excess water, electrolytes, and certain metabolic wastes.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.4 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is the dermis?<\/li>\n<li>Describe the basic anatomy of the dermis.<\/li>\n<li>Compare and contrast the papillary and reticular layers of the dermis.<\/li>\n<li>What causes epidermal ridges, and why can they be used to identify individuals?<\/li>\n<li>Name the two types of sweat glands in the dermis, and explain\u00a0how they differ.<\/li>\n<li>What is the function of sebaceous glands?<\/li>\n<li>Describe the structures associated with hair follicles.<\/li>\n<li>Explain how the dermis helps regulate body temperature.<\/li>\n<li>Identify three specific kinds of tactile receptors in the dermis, along with the type of stimuli they sense.<\/li>\n<li>How does the dermis excrete wastes? What waste products does it excrete?<\/li>\n<li>What are subcutaneous tissues?\u00a0Which layer of the dermis provides cushioning for subcutaneous tissues? Why does this layer provide most of the cushioning, instead of the other layer?<\/li>\n<li>For each of the functions listed below, describe which structure within the dermis carries it out.\n<ol type=\"a\">\n<li>Brings nutrients to and removes wastes from dermal and lower epidermal cells<\/li>\n<li>Causes hairs to move<\/li>\n<li>Detects painful stimuli on the skin<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.4 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=FX-FwK0IIrE<\/p>\n<p style=\"text-align: center\">How do you get rid of acne? SciShow, 2016.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=VcHQWMAClhQ&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">When You Can't Scratch Away An Itch, Seeker, 2013.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 10.4.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Goose_bumps.jpg\" rel=\"cc:attributionURL\">Goose_bumps<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/photos\/61532128@N00\" rel=\"nofollow\">EverJean<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en\" rel=\"license\">CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<p><strong>Figure 10.4.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:506_Layers_of_the_Dermis.jpg\" rel=\"cc:attributionURL\">Layers_of_the_Dermis<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/5-1-layers-of-the-skin\">OpenStax College<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 10.4.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Fingerprint_detail_on_male_finger_in_T%C5%99eb%C3%AD%C4%8D,_T%C5%99eb%C3%AD%C4%8D_District.jpg\" rel=\"cc:attributionURL\">Fingerprint_detail_on_male_finger_in_T\u0159eb\u00ed\u010d,_T\u0159eb\u00ed\u010d_District<\/a> by <a title=\"User:Frettie\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Frettie\">Frettie<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">Figure 10.4.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0802_Skin_DermalCirculation.png\" rel=\"cc:attributionURL\">Blausen_0802_Skin_Dermal Circulation<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia commons is used under a <span style=\"text-align: initial;font-size: 1em\">\u00a0<a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span><\/p>\n<p><strong>Figure 10.4.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Anatomy_The_Skin_-_NCI_Visuals_Online.jpg\" rel=\"cc:attributionURL\">Anatomy_The_Skin_-_NCI_Visuals_Online<\/a>\u00a0by Don Bliss (artist) \/ \u00a0<a class=\"extiw\" title=\"en:National Cancer Institute\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Cancer_Institute\">National Cancer Institute<\/a> (<a class=\"extiw\" title=\"en:National Institutes of Health\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Institutes_of_Health\">National Institutes of Health<\/a>, with the ID\u00a0<a class=\"external text\" href=\"https:\/\/visualsonline.cancer.gov\/details.cfm?imageid=4604\" rel=\"nofollow\">4604<\/a>) is in the<span style=\"font-size: 1em\"> <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0<\/span><\/p>\n<p><strong>Figure 10.4.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0809_Skin_TactileReceptors.png\" rel=\"cc:attributionURL\">Blausen_0809_Skin_TactileReceptors<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia commons is used under a <span style=\"text-align: initial;font-size: 1em\">\u00a0<a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span><\/p>\n<p><strong>Figure 10.4.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Akne-jugend.jpg\" rel=\"cc:attributionURL\">Akne-jugend<\/a> by <a title=\"User:Ellywa\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Ellywa\">Ellywa<\/a> on Wikimedia Commons is released into the <a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> <span style=\"font-size: 1em\">(https:\/\/en.wikipedia.org\/wiki\/public_domain)<\/span>. (No machine-readable author provided. <a title=\"User:Ellywa\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Ellywa\">Ellywa<\/a>\u00a0assumed, based on copyright claims).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 5.7 Layers of the dermis [digital image]. In <em>Anatomy and Physiology<\/em> (Section 5.1 Layers of the skin). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/5-1-layers-of-the-skin<\/p>\n<p class=\"hanging-indent\">Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\n<p class=\"hanging-indent\">SciShow. (2016, October 26). How do you get rid of acne? YouTube. https:\/\/www.youtube.com\/watch?v=FX-FwK0IIrE<\/p>\n<p class=\"hanging-indent\">Seeker. (2013, October 26). When you can't scratch away an itch. YouTube. https:\/\/www.youtube.com\/watch?v=VcHQWMAClhQ&amp;amp;feature=emb_logo<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4042\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4042\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3579\" aria-describedby=\"caption-attachment-3579\" style=\"width: 350px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3579\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Hair-by-jessica-dabrowski-TETR8YLSqt4-unsplash-scaled-e1591285640537-1.jpg\" alt=\"10.5.1 Fancy hair\" width=\"350\" height=\"396\"><figcaption id=\"caption-attachment-3579\" class=\"wp-caption-text\"><em>Figure 10.5.1 Hair envy.<\/em><\/figcaption><\/figure>\n<h1>Fashion Statement<\/h1>\n<p>This colourful hairstyle makes quite a fashion statement. Many people spend a lot of time and money on their hair, even if they don\u2019t have an exceptional hairstyle like this one. Besides its display value, hair actually has important physiological functions.<\/p>\n<h1>What is Hair?<\/h1>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3531\">Hair<\/a><\/strong>\u00a0is a filament that grows from a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3572\">hair follicle<\/a><\/strong>\u00a0in the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>\u00a0of the skin. It consists mainly of tightly packed, keratin-filled\u00a0cells\u00a0called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a><\/strong>. The\u00a0human body\u00a0is covered with hair follicles, with the exception of a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.<\/p>\n<h1>Structure of Hair<\/h1>\n<p>The part of the hair located within the follicle is called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3745\">hair root<\/a>.<\/strong>\u00a0The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.<\/p>\n<h2>Follicle and Root<\/h2>\n<p>Hair growth begins inside a follicle (see Figure 10.5.2 below). Each hair follicle contains stem cells that can keep dividing, which allows hair to grow. The stem cells can also regrow a new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum. The sebum lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves, and the hair in the follicle stands up.<\/p>\n<figure id=\"attachment_3581\" aria-describedby=\"caption-attachment-3581\" style=\"width: 731px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3581\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0438_HairFollicleAnatomy_02-1.png\" alt=\"10.5 Hair Follicle\" width=\"731\" height=\"731\"><figcaption id=\"caption-attachment-3581\" class=\"wp-caption-text\">Figure 10.5.2 A hair follicle has a sebaceous gland and an arrector pili muscle.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h2>Shaft<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3741\">hair shaft<\/a><\/strong>\u00a0is a hard filament that may grow very long. Hair normally grows in length by about half an inch a month. In cross-section, a hair shaft can be divided into three zones, called the cuticle, cortex, and medulla.<\/p>\n<ul>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3739\">cuticle<\/a><\/strong>\u00a0(or outer coat) is the outermost zone of the hair shaft. It consists of several layers of flat, thin keratinocytes that overlap one another like shingles on a roof. This arrangement helps the cuticle repel\u00a0water. The cuticle is also covered with a layer of\u00a0lipids, just one molecule thick, which increases its ability to repel water. This is the zone of the hair shaft that is visible to the eye.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3737\">cortex<\/a><\/strong> is the middle zone of the hair shaft, and it is also the widest part. The cortex is highly structured and organized, consisting of keratin bundles in rod-like structures. These structures give hair its mechanical strength. The cortex also contains melanin, which gives hair its\u00a0colour.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3735\">medulla<\/a><\/strong>\u00a0is the innermost zone of the hair shaft. This is a small, disorganized, and more open area at the center of the hair shaft. The medulla is not always present. When it is present, it contains highly pigmented\u00a0cells\u00a0full of keratin.<\/li>\n<\/ul>\n<h1>Characteristics of Hair<\/h1>\n<p>Two visible characteristics of hair are its colour and texture. In adult males, the extent of balding is another visible characteristic. All three characteristics are genetically controlled.<\/p>\n<h2>Hair Colour<\/h2>\n<p>All natural hair colours are the result of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3753\">melanin<\/a><\/strong>, which is produced in hair follicles and packed into granules in the hair. Two forms of melanin are found in human hair: eumelanin and pheomelanin.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3894\">Eumelanin<\/a><\/strong>\u00a0is the dominant pigment in brown hair and black hair, and\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3895\">pheomelanin<\/a><\/strong> is the dominant pigment in red hair. Blond hair results when you have\u00a0only a small amount of melanin in the hair. Gray and white hair occur when melanin production slows down, and eventually stops.<\/p>\n<div id=\"h5p-147\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-147\" class=\"h5p-iframe\" data-content-id=\"147\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"8.7 Sensory Receptors\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 10.5.3 Variation in hair colouration. Which types of melanin are present for each hair colour shown?<\/em><\/p>\n<h2>Hair Texture<\/h2>\n<p>Hair exists in a variety of textures. The main aspects of hair texture are the curl pattern, thickness, and consistency.<\/p>\n<ul>\n<li>The shape of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3572\">hair follicle<\/a> determines the shape of the hair shaft. The shape of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3741\">hair shaft<\/a>, in turn, determines the curl pattern of the hair. Round hair shafts produce straight hair. Hair shafts that are oval or have other shapes produce wavy or curly hair .<\/li>\n<li>The size of the hair follicle determines the thickness of hair. Thicker hair has greater volume than thinner hair.<\/li>\n<li>The consistency of hair is determined by the hair follicle volume and the condition of the hair shaft. The consistency of hair is generally classified as fine, medium, or coarse. Fine hair has the smallest circumference, and coarse hair has the largest circumference. Medium hair falls in between these two extremes. Coarse hair also has a more open cuticle than thin or medium hair does,\u00a0which causes\u00a0it to be more porous.<br \/>\n<figure id=\"attachment_3896\" aria-describedby=\"caption-attachment-3896\" style=\"width: 346px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3896\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Curly-hair-by-chris-benson-clvEami9RN4-unsplash-scaled-2.jpg\" alt=\"10.5 Hair Texture\" width=\"346\" height=\"231\"><figcaption id=\"caption-attachment-3896\" class=\"wp-caption-text\">Figure 10.5.4 Curly hair has a differently shaped shaft than straight hair.<\/figcaption><\/figure>\n<\/li>\n<\/ul>\n<h1>Functions of Hair<\/h1>\n<p>In humans, one function of head hair is to provide insulation and help the head retain\u00a0heat. Head hair also protects the skin on the head from damage by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a>.<\/p>\n<p>The function of hair in\u00a0<em>other<\/em> locations on the body is debated. One idea is that body hair helps keep us warm in cold weather. When the body is too cold, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3747\">arrector pili<\/a> muscles contract and cause hairs to stand up (shown in Figure 10.5.5), trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.<\/p>\n<figure id=\"attachment_3897\" aria-describedby=\"caption-attachment-3897\" style=\"width: 423px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-3897\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-PilioerectionAnimation-1.gif\" alt=\"10.5 Arrector Pili\" width=\"423\" height=\"423\"><figcaption id=\"caption-attachment-3897\" class=\"wp-caption-text\"><em>Figure 10.5.5 Arrector pili muscles will make hairs stand erect, more commonly recognized as goose bumps.\u00a0 (1) Epidermis (2) Arrector pili muscle (3) Hair follicle (4) Dermis<\/em><\/figcaption><\/figure>\n<p>Human hair has an important sensory function, as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze, or because of the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin.<\/p>\n<figure id=\"attachment_3903\" aria-describedby=\"caption-attachment-3903\" style=\"width: 147px\" class=\"wp-caption alignright\"><img class=\" wp-image-3903\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Pout-by-alexander-dummer-Em8I8Z_DwA4-unsplash-scaled-e1591298337111-1.jpg\" alt=\"10.5 Eyebrows and Emotions\" width=\"147\" height=\"158\"><figcaption id=\"caption-attachment-3903\" class=\"wp-caption-text\"><em>Figure 10.5.6 This young child is using her eyebrows to good effect to convey her displeasure.<\/em><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Some hairs, such as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3900\">eyelashes<\/a>, are especially sensitive to the presence of potentially harmful matter. The eyelashes grow at the edge of the eyelid and can sense when dirt, dust, or another potentially harmful object is too close to the eye. The eye reflexively closes as a result of this sensation. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3901\">eyebrows<\/a> also provide some protection to the eyes. They protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (see Figure 10.5.6). They help express emotions such as sadness, anger, surprise, and excitement.<\/p>\n<h1>Hair in Human Evolution<\/h1>\n<p>Among mammals, humans are nearly unique in having undergone significant loss of body hair during their evolution. Humans are also unlike most other mammals in having curly hair as one variation in hair texture. Even non-human primates (see Figure 10.5.7) all have straight hair. This suggests that curly hair evolved at some point during human evolution.<\/p>\n<figure id=\"attachment_3905\" aria-describedby=\"caption-attachment-3905\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3905\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cotton_top_tamarin_monkey._12046035746-1.jpg\" alt=\"10.5 Straight hair in non-human primates\" width=\"400\" height=\"293\"><figcaption id=\"caption-attachment-3905\" class=\"wp-caption-text\"><em>Figure 10.5.7 Like other non-human primates, this tamarin monkey has straight hair.<\/em><\/figcaption><\/figure>\n<h2>Loss of Body Hair<\/h2>\n<p>One\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">hypothesis<\/a>\u00a0for the loss of body hair in the human lineage is that it would have facilitated cooling of the body by the\u00a0evaporation\u00a0of sweat. Humans also evolved far more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3566\">sweat glands<\/a>\u00a0than other mammals, which is consistent with this hypothesis, because sweat evaporates more quickly from less hairy skin. Another hypothesis for human hair loss is that it would have led to fewer parasites on the skin. This might have been especially important when humans started living together in larger, more crowded social groups.<\/p>\n<p>These hypotheses may explain why we lost body hair, but they can\u2019t explain why we didn\u2019t also lose head hair and hair in the pubic region and armpits. It is possible that head hair was retained because it protected the scalp from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a>. As our bipedal ancestors walked on the open savannas of equatorial Africa, the skin on the head would have been an area exposed to the most direct rays of sunlight in an upright hominid. Pubic and armpit hair may have been retained because they served as signs of sexual maturity, which would have been important for successful mating and\u00a0reproduction.<\/p>\n<h2>Evolution of Curly Hair<\/h2>\n<p>Greater protection from UV light has also been posited as a possible selective agent favoring the evolution of curly hair. Researchers have found that straight hair allows more light to pass into the body through the hair shaft via the follicle than does curly hair. In this way, human hair is like a fibre optic cable. It allows light to pass through easily when it is straight, but it impedes the passage of light when it is kinked or coiled. This is indirect evidence that UV light may have been a selective agent leading to the evolution of curly hair.<\/p>\n<h1>Social and Cultural Significance of Hair<\/h1>\n<p>Hair has great social significance for human beings. Body hair is an indicator of biological sex, because hair distribution is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3906\">sexually dimorphic<\/a>. Adult males are generally hairier than adult females, and facial hair in particular is a notable secondary male sex characteristic. Hair may also be an indicator of age. White hair is a sign of older age in both males and females, and male pattern baldness is a sign of older age in males. In addition, hair colour and texture can be a sign of ethnic ancestry.<\/p>\n<p>Hair also has great cultural significance. Hairstyle and colour may be an indicator of social group membership and for better or worse can be associated with specific stereotypes. Head shaving has been used in many times and places as a punishment, especially for women. On the other hand, in some cultures, cutting off one\u2019s hair symbolizes liberation from one\u2019s past. In other cultures, it is a sign of mourning. There are also many religious-based practices involving hair. For example, the majority of Muslim women hide their hair with a headscarf. Sikh men grow their hair long and cover it with a turban. Amish men (like the one pictured in Figure 10.5.8) grow facial hair only after they marry \u2014 but just a beard, and not a mustache.<\/p>\n<figure id=\"attachment_3907\" aria-describedby=\"caption-attachment-3907\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3907\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Amish-hairstyle-1.jpg\" alt=\"10.5\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-3907\" class=\"wp-caption-text\"><em>Figure 10.5.8 This style of facial hair is adopted by most Amish men after they marry.<\/em><\/figcaption><\/figure>\n<p>Unfortunately, sometimes hairstyle, colour and characteristics are used to apply stereotypes, particularly with respect to women.\u00a0 \"Blonde jokes\" are a good example of how negative stereotypes are maintained despite having no actual truth behind them.\u00a0 Many stereotypes related to hair are hidden, even from persons perpetrating the stereotype.\u00a0 Often a hairstyle is judged by another as having ties to gender, sexuality, worldview and\/or socioeconomic status; even when these inferences are woefully inaccurate.\u00a0 It is important to be aware of our own biases and determine if these biases are appropriate - take a look at the collage in Figure 10.5.9.\u00a0 What are your initial reactions?\u00a0 Are these reactions founded in fact?\u00a0 Do you harbor an unfair bias?<\/p>\n<div id=\"h5p-148\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-148\" class=\"h5p-iframe\" data-content-id=\"148\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"8.7 Eye Diagram\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 10.5.9 What are your biases?\u00a0 Are they fair?<\/em><\/p>\n<p>&nbsp;<\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.5 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Hair is a filament that grows from a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3572\">hair follicle<\/a> in the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3875\">dermis<\/a>\u00a0of the skin. It consists mainly of tightly packed, keratin-filled cells called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>. The\u00a0human body\u00a0is almost completely covered with hair follicles.<\/li>\n<li>The part of a hair that is within the follicle is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3745\">hair root<\/a>. This is the only living part of a hair. The part of a hair that is visible above the skin surface is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3741\">hair shaft<\/a>. It consists of dead cells.<\/li>\n<li>Hair growth begins inside a follicle when stem cells within the follicle divide to produce new keratinocytes. An individual hair may grow to be very long.<\/li>\n<li>A hair shaft has three zones: the outermost zone called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3739\">cuticle<\/a>; the middle zone called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3737\">cortex<\/a>; and the innermost zone called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3735\">medulla<\/a>.<\/li>\n<li>Genetically controlled, visible characteristics of hair include hair colour, hair texture, and the extent of balding in adult males. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3753\">Melanin<\/a> (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3894\">eumelanin<\/a> and\/or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3895\">pheomelanin<\/a>) is the pigment that gives hair its colour. Aspects of hair texture include curl pattern, thickness, and consistency.<\/li>\n<li>Functions of head hair include providing insulation and protecting skin on the head from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3537\">UV light<\/a>. Hair everywhere on the body has an important sensory function. Hair in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3900\">eyelashes<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3901\">eyebrows<\/a>\u00a0protects the eyes from dust, dirt, sweat, and other potentially harmful substances. The eyebrows also play a role in nonverbal communication.<\/li>\n<li>Among mammals, humans are nearly unique in having undergone significant loss of body hair during their evolution, probably because sweat evaporates more quickly from less hairy skin. Curly hair also is thought to have evolved at some point during human evolution, perhaps because it provided better protection from UV light.<\/li>\n<li>Hair has social significance for human beings,\u00a0because it is\u00a0an indicator of biological sex, age, and ethnic ancestry. Human hair also has cultural significance.\u00a0Hairstyle may be an indicator of social group membership, for example.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.5 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li style=\"list-style-type: none\">\n<ol>\n<li>Compare and contrast the hair root and hair shaft.<\/li>\n<li>Describe hair follicles.<\/li>\n<li>\n<div id=\"h5p-149\">\n<div class=\"h5p-content\" data-content-id=\"149\"><\/div>\n<\/div>\n<\/li>\n<li>\n<div id=\"h5p-150\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-150\" class=\"h5p-iframe\" data-content-id=\"150\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 8 Review Questions\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Explain variation in human hair colour.<\/li>\n<li>What factors determine the texture of hair?<\/li>\n<li>Describe two functions of human hair.<\/li>\n<li>What hypotheses have been proposed for the loss of body hair during human evolution?<\/li>\n<li>Discuss the social and cultural significance of human hair.<\/li>\n<li>Describe one way in which hair can be used as a method of communication in humans.<\/li>\n<li>Explain why waxing or tweezing body hair, which typically removes hair down to the root, generally keeps the skin hair-free for a longer period of time than shaving, which cuts hair off at the surface of the skin.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.5 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=8diYLhl8bWU<\/p>\n<p style=\"text-align: center\">Why do some people go bald? - Sarthak Sinha, TED-Ed, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=kNw8V_Fkw28<\/p>\n<p style=\"text-align: center\">Hair Love | Oscar\u00ae-Winning Short Film (Full) | Sony Pictures Animation, 2019.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=hDW5e3NR1Cw<\/p>\n<p style=\"text-align: center\">Why do we care about hair | Naomi Abigail | TEDxBaDinh, TEDx Talks, 2015.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 10.5.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/TETR8YLSqt4\" rel=\"cc:attributionURL\">Hair by jessica-dabrowski-TETR8YLSqt4<\/a>\u00a0[photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@jessiedee\">Jessica Dabrowski<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 10.5.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0438_HairFollicleAnatomy_02.png\" rel=\"cc:attributionURL\">Blausen_0438_HairFollicleAnatomy_02<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0 <\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 10.5.3<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/unsplash.com\/photos\/uX0yUOEYFFU\">Standing tall<\/a> by <a href=\"https:\/\/unsplash.com\/@caticlicks\">Ilaya Raja<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/3C7Qri2QQx8\">Blond-haired woman smiling<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@realbench\">Carlos Lindner<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/nRDopRnkEfI\">Smith Mountain Lake redhead<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@chrisrossharris\">Chris Ross Harris<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/fH55IFx6kXE\">Through the look of experience<\/a> by <a href=\"https:\/\/unsplash.com\/@lauramc\">Laura Margarita Cede\u00f1o Peralta<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<\/ul>\n<p><strong style=\"text-align: initial;font-size: 1em\">Figure 10.5.4<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/clvEami9RN4\" rel=\"cc:attributionURL\">Curly hair by chris-benson-clvEami9RN4<\/a>\u00a0[photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@lordmaui\">Chris Benson<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 10.5.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:PilioerectionAnimation.gif\" rel=\"cc:attributionURL\">1024px-PilioerectionAnimation<\/a> by <a class=\"new\" title=\"User:AnthonyCaccese (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:AnthonyCaccese&amp;action=edit&amp;redlink=1\">AnthonyCaccese<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 10.5.6<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/Em8I8Z_DwA4\" rel=\"cc:attributionURL\">Pout by alexander-dummer-Em8I8Z_DwA4<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@4dgraphic\">Alexander Dummer<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 10.5.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Cotton_top_tamarin_monkey._(12046035746).jpg\" rel=\"cc:attributionURL\">Cotton_top_tamarin_monkey._(12046035746)<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/88123769@N02\" rel=\"nofollow\">Bernard Spragg. NZ<\/a>, from Christchurch, New Zealand on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en\">CC0 1.0<\/a> Universal<br \/>\nPublic Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).<\/p>\n<p><strong>Figure 10.5.8<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/12.5\/\" rel=\"cc:attributionURL\">Amish hairstyle<\/a>\u00a0by <a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/12.5\/\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\">CC BY-NC 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<br \/>\n<img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><br \/>\n<strong>Figure 10.5.9<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/unsplash.com\/photos\/XuHbLw1sUyk\">Rainbow Hair Bubble Man<\/a><sup>\u00a0<\/sup>by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@behrouzix\">Behrouz Jafarnezhad<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/KIg7V5AbTpw\">Pink hair in <span class=\"_20NLS _1ByhS\">Atlanta, United States<\/span><\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@tammeallen\">Tammie Allen<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/vx-PQnfLDDk\">Magdalena 2<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@elashv\">Valerie Elash<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/g_2fwjrDj0M\">Perfect Style<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@darias_big_world\">Daria Volkova<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license)<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/sMR__Unqh5A\">Stay Classy<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@fayizdasma\">Fayiz Musthafa<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license)<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/2OycBCpnUVI\">Take your time<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@craft_ear\">Jan Tinneberg<\/a>\u00a0on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license)<\/li>\n<\/ul>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). Figure 7 This style of facial hair is adopted by most Amish men after they marry [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 12<span style=\"font-size: 1em\">.5<\/span><span style=\"text-align: initial;font-size: 1em\">) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/12.5\/<\/span><\/p>\n<p class=\"hanging-indent\">Sony Pictures Animation. (2019, December 5). Hair love | Oscar\u00ae-winning short film (Full) | Sony Pictures Animation. YouTube. https:\/\/www.youtube.com\/watch?v=kNw8V_Fkw28<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, August 25). Why do some people go bald? \u2013 Sarthak Sinha. YouTube. https:\/\/www.youtube.com\/watch?v=8diYLhl8bWU<\/p>\n<p class=\"hanging-indent\">TEDx Talks. (2015, February 4). Why do we care about hair | Naomi Abigail | TEDxBaDinh. YouTube. https:\/\/www.youtube.com\/watch?v=hDW5e3NR1Cw<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4043\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4043\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_3911\" aria-describedby=\"caption-attachment-3911\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3911\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Nails-by-allison-christine-vPrqHSLdF28-unsplash-scaled-e1591304396521-1.jpg\" alt=\"10.6.1\" width=\"400\" height=\"400\"><figcaption id=\"caption-attachment-3911\" class=\"wp-caption-text\"><em>Figure 10.6.1 Neat nails!<\/em><\/figcaption><\/figure>\n<div>\n<h1>Nail Art<\/h1>\n<\/div>\n<p>Painting nails with coloured polish for aesthetic reasons is nothing new. In fact, there is evidence of this practice dating back to at least 3000 BCE. Today, painting and otherwise decorating the nails is big business, with annual revenues in the billions of dollars in North America alone! With all the attention (and money) given to nails as decorative objects, it\u2019s easy to forget that they also have important biological functions.<\/p>\n<div>\n<h1>What Are Nails?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3403\">Nail<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0are accessory organs of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>. They are made of sheets of dead <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a> and are found on the far (or distal) ends of the fingers and toes. The keratin in nails makes them hard, but flexible. Nails serve a number of purposes, including protecting the digits, enhancing sensations, and acting as tools.<\/span><\/p>\n<div>\n<h1>Nail Anatomy<\/h1>\n<\/div>\n<figure id=\"attachment_3918\" aria-describedby=\"caption-attachment-3918\" style=\"width: 300px\" class=\"wp-caption alignright\"><img class=\" wp-image-3918\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0406_FingerNailAnatomy-1.png\" alt=\"10.5 Nail Anatomy\" width=\"300\" height=\"600\"><figcaption id=\"caption-attachment-3918\" class=\"wp-caption-text\"><em>Figure 10.6.2 The top diagram in this diagram shows the external, visible part of the nail and the cuticle. The bottom diagram shows internal structures in a cross-section of the nail and nail bed.<\/em><\/figcaption><\/figure>\n<p>A nail has three main parts: the root, plate, and free margin. Other structures around or under the nail include the nail bed, cuticle, and nail fold. These structures are shown in Figure 10.6.2.<\/p>\n<ul>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3912\">nail root<\/a><\/strong>\u00a0is the portion of the nail found under the surface of the skin at the near (or proximal) end of the nail. It is where the nail begins.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">nail plate<\/a><\/strong>\u00a0(or body) is the portion of the nail that is external to the skin. It is the visible part of the nail.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3916\">free margin<\/a><\/strong>\u00a0is the portion of the nail that protrudes beyond the distal end of the finger or toe. This is the part that is cut or filed to keep the nail trimmed.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3914\">nail bed<\/a><\/strong> is the area of skin under the nail plate. It is pink in\u00a0colour, due to the presence of capillaries in the\u00a0dermis.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3917\">cuticle<\/a><\/strong>\u00a0is a layer of dead epithelial\u00a0cells\u00a0that overlaps and covers the edge of the nail plate. It helps to seal the edges of the nail to prevent infection of the underlying tissues.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3915\">nail fold<\/a><\/strong>\u00a0is a groove in the skin in which the side edges of the nail plate are embedded.<\/li>\n<\/ul>\n<div>\n<p>&nbsp;<\/p>\n<\/div>\n<div>\n<h1>Nail Growth<\/h1>\n<\/div>\n<p>Nails grow from a deep layer of living epidermal tissue, known as the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3920\">nail matrix<\/a>,<\/strong> at the proximal end of the nail (see the bottom of the diagram in Figure 10.6.2). The nail matrix surrounds the nail root. It contains stem cells that divide to form keratinocytes, which are cells that produce keratin and make up the nail.<\/p>\n<h2>Formation of the Nail Root and Nail Plate<\/h2>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a> produced by the nail matrix accumulate to form tough, hard, translucent sheets of dead\u00a0cells\u00a0filled with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3525\">keratin<\/a>. The sheets make up the nail root, which slowly grows out of the skin and becomes the nail plate when it reaches the skin surface. As the nail grows longer, the cells of the nail root and nail plate are pushed toward the distal end of the finger or toe by new cells being formed in the nail matrix. The upper epidermal cells of the nail bed also move along with the nail plate as it grows toward the tip of the digit.<\/p>\n<p>The proximal end of the nail plate near the root has a whitish crescent shape called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3921\">lunula<\/a><\/strong>. This is where a small amount of the nail matrix is visible through the nail plate. The lunula is most pronounced in the nails of the thumbs, and may not be visible in the nails of the little fingers.<\/p>\n<h2>Rate of Nail Growth<\/h2>\n<p>Nails grow at an average rate of 3 mm a month. Fingernails, however, grow up to four times as fast as toenails. If a fingernail is lost, it takes between three and six months to regrow completely, whereas a toenail takes between 12 and 18 months to regrow. The actual rate of growth of an individual\u2019s nails depends on many factors, including age, sex, season, diet, exercise level, and genes. If protected from breaking, nails can sometimes grow to be very long. The Chinese doctor in the photo below (Figure 10.6.3) has very long nails on two fingers of his left hand. This picture was taken in 1920 in China, where having long nails was a sign of aristocracy since it implied that one was wealthy enough to not have to do physical labour.<\/p>\n<figure id=\"attachment_3922\" aria-describedby=\"caption-attachment-3922\" style=\"width: 313px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-3922\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Chinese_doctor_with_long_finger_nails_an_aristocrat_ca.1920_CHS-249-scaled-2.jpg\" alt=\"10.6 Fingernails and aristocracy\" width=\"313\" height=\"500\"><figcaption id=\"caption-attachment-3922\" class=\"wp-caption-text\"><em>Figure 10.6.3 Nails, like hair, can have sociocultural relevance, as the man in this photo illustrates. His long nails indicate his aristocratic heritage.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Functions of Nails<\/h1>\n<\/div>\n<p>Both fingernails and toenails protect the soft tissues of the fingers and toes from injury. Fingernails also serve to enhance sensation and precise movements of the fingertips through the counter-pressure exerted on the pulp of the fingers by the nails. In addition, fingernails can function as several different types of tools. For example, they enable a fine precision grip like tweezers, and can also be used for cutting and scraping.<\/p>\n<div>\n<h1>Nails and Health<\/h1>\n<\/div>\n<p>Healthcare providers, particularly EMTs, often examine the fingernail beds as a quick and easy indicator of oxygen saturation of the blood, or the amount of blood reaching the extremities. If the nail beds are bluish or purple, it is generally a sign of low oxygen saturation. To see if blood flow to the extremities is adequate, a blanch test may be done. In this test, a fingernail is briefly depressed to turn the nail bed white by forcing the blood out of its capillaries. When the pressure is released, the pink colour of the nail bed should return within a second or two if there is normal blood flow. If the return to a pink colour is delayed, then it can be an indicator of low blood volume, due to dehydration or shock.<\/p>\n<figure id=\"attachment_3923\" aria-describedby=\"caption-attachment-3923\" style=\"width: 386px\" class=\"wp-caption alignleft\"><img class=\" wp-image-3923\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Toenail-fungus-Nagelpilz-3-1.jpg\" alt=\"10.6 Toenail fungus\" width=\"386\" height=\"290\"><figcaption id=\"caption-attachment-3923\" class=\"wp-caption-text\"><em>Figure 10.6.4 Fungus infections of the toenails are common. They often look worse than they are. Generally, they are more unsightly than painful or dangerous.<\/em><\/figcaption><\/figure>\n<p>How the visible portion of the nails appears can be used as an indicator of recent health status. In fact, nails have been used as diagnostic tools for hundreds \u2014 if not thousands \u2014 of years. Nail abnormalities, such as deep grooves, brittleness, discolouration, or unusually thin or thick nails, may indicate various illnesses, nutrient deficiencies, drug reactions, or other health problems.<\/p>\n<p>Nails \u2014 especially toenails \u2014 are common sites of fungal infections (shown in Figure 10.6.4), causing nails to become thickened and yellowish in colour. Toenails are more often infected than fingernails because they are often confined in shoes, which creates a dark, warm, moist environment where fungi can thrive. Toes also tend to have less blood flow than fingers, making it harder for the immune system to detect and stop infections in toenails.<\/p>\n<p>Although nails are harder and tougher than skin, they are also more permeable. Harmful substances may be absorbed through the nails and cause health problems. Some of the substances that can pass through the nails include the herbicide Paraquat, fungicidal agents such as miconazole (e.g., Monistat), and sodium hypochlorite, which is an ingredient in common household bleach. Care should be taken to protect the nails from such substances when handling or immersing the hands in them by wearing latex or rubber gloves.<\/p>\n<div>\n<h1>Feature: Reliable Sources<\/h1>\n<figure id=\"attachment_3924\" aria-describedby=\"caption-attachment-3924\" style=\"width: 400px\" class=\"wp-caption alignright\"><img class=\"wp-image-3924 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Pedicure-in-progress-from-wikimedia-commons-1.jpg\" alt=\"10.6.5 Nail salon\" width=\"400\" height=\"300\"><figcaption id=\"caption-attachment-3924\" class=\"wp-caption-text\"><em>Figure 10.6.5 Nail salons must follow very strict cleanliness guidelines in order to reduce the chances of transmitting pathogens from one customer to the next.<\/em><\/figcaption><\/figure>\n<\/div>\n<p>Do you get regular manicures or pedicures from a nail technician? If so, there is a chance that you are putting your health at risk. Nail tools that are not properly disinfected between clients may transmit infections from one person to another. Cutting the cuticles with scissors may create breaks in the skin that let infective agents enter the body. Products such as acrylics, adhesives, and UV gels that are applied to the nails may be harmful, especially if they penetrate the nails and enter the skin.<\/p>\n<p>Use the Internet to find several reliable sources that address the health risks of professional manicures or pedicures. Try to find answers to the following questions:<\/p>\n<ol>\n<li>What training and certification are required for professional nail technicians?<\/li>\n<li>What licenses and inspections are required for nail salons?<\/li>\n<li>What hygienic practices should be followed in nail salons to reduce the risk of infections being transmitted to clients?<\/li>\n<li>Which professional nail products are potentially harmful to the\u00a0human body\u00a0and which are safer?<\/li>\n<li>How likely is it to have an adverse health consequence when you get a professional manicure or pedicure?<\/li>\n<li>What steps can you take to ensure that a professional manicure or pedicure is safe?<\/li>\n<\/ol>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.6 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3403\">Nails<\/a>\u00a0are accessory organs of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>, consisting of sheets of dead, keratin-filled <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3524\">keratinocytes<\/a>. The keratin in nails makes them hard, but flexible.<\/li>\n<li>A nail has three main parts: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3912\">nail root<\/a> (which is under the epidermis), the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">nail plate<\/a> (which is the visible part of the nail), and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3916\">free margin<\/a> (which is the distal edge of the nail). Other structures under or around a nail include the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3914\">nail bed<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3917\">cuticle<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3915\">nail fold<\/a>.<\/li>\n<li>A nail grows from a deep layer of living epidermal tissues \u2014 called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3920\">nail matrix<\/a> \u2014 at the proximal end of the nail. Stem cells in the nail matrix keep dividing to allow nail growth, forming first the nail root and then the nail plate as the nail continues to grow longer and emerges from the\u00a0epidermis.<\/li>\n<li>Fingernails grow faster than toenails. Actual rates of growth depend on many factors, such as age, sex, and season.<\/li>\n<li>Functions of nails include protecting the digits, enhancing sensations and precise movements of the fingertips, and acting as tools.<\/li>\n<li>The colour of the nail bed can be used to quickly assess oxygen and blood flow in a patient. How the nail plate grows out can reflect recent health problems, such as illness or nutrient deficiency.<\/li>\n<li>Nails \u2014 and especially toenails \u2014 are prone to fungus infections. Nails are more permeable than skin and can absorb several harmful substances, such as herbicides.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.6 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What are nails?<\/li>\n<li>\n<div id=\"h5p-151\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-151\" class=\"h5p-iframe\" data-content-id=\"151\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9.2 Review Questions\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Explain why most of the nail plate looks pink.<\/li>\n<li>Describe a lunula.<\/li>\n<li>Explain how a nail grows.<\/li>\n<li>Identify three functions of nails.<\/li>\n<li>Give several examples of how nails are related to health.<\/li>\n<li>What is the cuticle of the nail composed of?\u00a0What is the function of the cuticle?\u00a0Why is it a bad idea to cut the cuticle during a manicure?<\/li>\n<li>Is the nail plate composed of living or dead cells?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">10.6 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=G35kPhbUZdg<\/p>\n<p style=\"text-align: center\">Longest Fingernails - Guinness World Records 60th Anniversary,<br \/>\nGuinness World Records, 2014.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=aTSVHwzkYI4&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">5 Things Your Nails Can Say About Your Health, SciShow, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=7w2gCBL1MCg<\/p>\n<p style=\"text-align: center\">Claws vs. Nails - Matthew Borths, TED-Ed, 2019.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 10.6.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/vPrqHSLdF28\" rel=\"cc:attributionURL\">Nails by allison-christine-vPrqHSLdF28<\/a>\u00a0[photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@happpyal\">allison christine<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 10.6.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0406_FingerNailAnatomy.png\" rel=\"cc:attributionURL\">Blausen_0406_FingerNailAnatomy<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a>\u00a0 on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 10.6.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Chinese_doctor_with_long_finger_nails_(an_aristocrat),_ca.1920_(CHS-249).jpg\" rel=\"cc:attributionURL\">Chinese_doctor_with_long_finger_nails_(an_aristocrat),_ca.1920_(CHS-249)<\/a> by Pierce, C.C. (Charles C.), 1861-1946 from the <a href=\"http:\/\/digitallibrary.usc.edu\/cdm\/ref\/collection\/p15799coll65\/id\/8203\">USC Digital Library<\/a> on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/p>\n<p><strong>Figure 10.6.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Nagelpilz-3.jpg\" rel=\"cc:attributionURL\">Toenail fungus Nagelpilz-3<\/a> by <a class=\"new mw-userlink\" title=\"User:Pepsyrock (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Pepsyrock&amp;action=edit&amp;redlink=1\">Pepsyrock<\/a> on Wikimedia Commons is released into the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/p>\n<p><strong>Figure 10.6.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Pedicure_1.jpg\" rel=\"cc:attributionURL\">OLYMPUS DIGITAL CAMERA<\/a> by <a class=\"extiw\" title=\"en:User:Stoive\" href=\"https:\/\/en.wikipedia.org\/wiki\/User:Stoive\">Stoive<\/a> at the English language Wikipedia, on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en\" rel=\"license\">CC BY-SA 3.0 <\/a>(http:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/) license.<\/p>\n<p><span style=\"font-size: 1.424em;font-weight: bold\"><br \/>\nReferences<\/span><\/p>\n<p class=\"hanging-indent\">Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\n<p class=\"hanging-indent\">Guiness World Records. (2014, December 8). Longest fingernails - Guinness World Records 60th Anniversary. YouTube. https:\/\/www.youtube.com\/watch?v=G35kPhbUZdg<\/p>\n<p class=\"hanging-indent\">SciShow. (2015, September 14). 5 things your nails can say about your health. YouTube. https:\/\/www.youtube.com\/watch?v=aTSVHwzkYI4<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2019, October 29). Claws vs. nails - Matthew Borths. YouTube. https:\/\/www.youtube.com\/watch?v=7w2gCBL1MCg<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4049\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4049\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4020\" aria-describedby=\"caption-attachment-4020\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4020\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Bone_marrow_grilled_on_the_barbecue_sliced_young_raw_garlic_salted_leek_flowers_from_last_year_lovage_and_kale_19098148350-1.jpg\" alt=\"11.4.1 Roasted Bone Marrow\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-4020\" class=\"wp-caption-text\"><em>Figure 11.4.1 Roasted bone marrow.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Roasted Bone Marrow<\/h1>\n<\/div>\n<p>Do you recognize the food item in the top left of Figure 11.4.1? It\u2019s roasted <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">bone marrow<\/a>, still inside the bones, and it is considered a delicacy in some cuisines. Marrow is a type of tissue found inside many animal bones, including our own. It\u2019s a soft tissue that, in adults, may be mostly fat. You\u2019ll learn more about bone marrow and other tissues that make up bones when you read this section.<\/p>\n<div>\n<h1>What Are Bones?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3821\">Bones<\/a><\/strong>\u00a0are organs that consist primarily of bone tissue, also called osseous tissue.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4022\">Bone tissue<\/a><\/strong>\u00a0is a type of connective tissue consisting mainly of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3970\">collagen<\/a> matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and hard mineral crystals makes bone tissue hard, without making it brittle.<\/p>\n<div>\n<h1>Bone Anatomy<\/h1>\n<\/div>\n<p>There are several different types of tissues in bones, including two types of osseous tissues. Osseous tissues, in turn, consist of several different types of bone\u00a0cells.<\/p>\n<h2>Types of Osseous Tissue<\/h2>\n<p>The two different types of osseous tissue are compact bone tissue (also called hard or cortical bone) and spongy bone tissue (also called cancellous or trabecular bone). Both are shown in the diagrams of a typical bone in Figures 11.4.2 and 11.4.3.<\/p>\n<p>Flat bones are typically enveloped by compact bone, with a center of spongy bone.<\/p>\n<table class=\"no-lines aligncenter\" style=\"border-collapse: collapse;width: 48.1291%;height: 620px\" border=\"0\">\n<tbody>\n<tr style=\"height: 305px\">\n<td style=\"width: 50%;height: 305px\">\n<figure id=\"attachment_4027\" aria-describedby=\"caption-attachment-4027\" style=\"width: 588px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4027\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cross-section.svg_-1.png\" alt=\"11.4.2 Bone Cross Section\" width=\"588\" height=\"294\"><figcaption id=\"caption-attachment-4027\" class=\"wp-caption-text\"><em>Figure 11.4.2 Bones are more complex on the inside than you would expect from their outer appearance.\u00a0 This long bone has many different structural regions performing unique functions.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 50%\">\n<div class=\"mceTemp\"><\/div>\n<figure id=\"attachment_4034\" aria-describedby=\"caption-attachment-4034\" style=\"width: 678px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4034\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anatomy_of_a_Flat_Bone-1.jpg\" alt=\"11.4.3 Anatomy of a flat bone\" width=\"678\" height=\"343\"><figcaption id=\"caption-attachment-4034\" class=\"wp-caption-text\"><em>Figure 11.4.3 Flat bones are typically enveloped by compact bone, with a center of spongy bone.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div style=\"text-align: center\">\n<p>&nbsp;<\/p>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4030\">Compact bone tissue<\/a><\/strong> forms the extremely hard outside layer of bones. Compact bone tissue gives bone its smooth, dense, solid appearance. It accounts for about 80% of the total bone mass of the adult skeleton. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4031\">Spongy bone tissue<\/a><\/strong> fills part or all of the interior of many bones. As its name suggests, spongy bone is porous like a sponge, containing an irregular network of spaces, as shown in Figures 11.4.4 and 11.4.5. This makes spongy bone much less dense than compact bone. Spongy bone has a greater surface area than compact bone, but makes up only 20% of bone mass.<\/p>\n<table class=\"aligncenter\" style=\"border-collapse: collapse;width: 100%;height: 322px\" border=\"0\">\n<tbody>\n<tr>\n<td style=\"width: 50%\">\n<figure id=\"attachment_4028\" aria-describedby=\"caption-attachment-4028\" style=\"width: 414px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4028\" src=\"http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/the-detail-of-the-bones-the-structure-of-the-bones-spongy-bone-tram\u010dina-close-up-structure.jpg#fixme\" alt=\"11.4.4 Spongy Bone\" width=\"414\" height=\"233\"><figcaption id=\"caption-attachment-4028\" class=\"wp-caption-text\"><em>Figure 11.4.4 Spongy bone has a lattice-like appearance. The empty spaces you can see here would be filled with bone marrow in a living person.<\/em><\/figcaption><\/figure>\n<\/td>\n<td style=\"width: 50%\">\n<figure id=\"attachment_4033\" aria-describedby=\"caption-attachment-4033\" style=\"width: 253px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4033\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Spongy_bone_-_Trabecular_bone_2_-_Smart-Servier-1.png\" alt=\"11.4.5 Spongy Bone Diagram\" width=\"253\" height=\"257\"><figcaption id=\"caption-attachment-4033\" class=\"wp-caption-text\"><em>Figure 11.4.5 Spongy bone is made up of a lattice-like network of tissue and is found at the ends of long bones and in the center of many flat bones.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p style=\"text-align: left\">Both compact and spongy bone tissues have the same types of cells, but they differ in how the cells are arranged. The cells in compact bone are arranged in multiple microscopic columns, whereas the cells in spongy bone are arranged in a looser, more open network. These cellular differences explain why compact and spongy bone tissues have such different structures.<\/p>\n<h2>Other Tissues in Bones<\/h2>\n<p>Besides compact and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">Bone marrow<\/a><\/strong> is a soft connective tissue found inside a cavity, called the marrow cavity. There are two types of marrow in adults \u2014 yellow bone marrow (which consists mostly of fat) and red bone marrow. All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones. Only red bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3961\">hematopoiesis<\/a>.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4039\">Periosteum<\/a><\/strong> is a tough, fibrous membrane that covers the outer surface of bones. It provides a protective covering for compact bone tissue. It is also the source of new bone cells.<\/li>\n<\/ul>\n<h2>Bone Cells<\/h2>\n<p>As shown in Figure 11.4.6, bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">Osteoblasts<\/a><\/strong>\u00a0are bone cells with a single\u00a0nucleus\u00a0that make and mineralize bone matrix. They make a\u00a0protein\u00a0mixture\u00a0that is composed primarily of collagen and creates the organic part of the matrix. They also release calcium and phosphate ions that form mineral crystals within the matrix. In addition, they produce hormones that play a role in the mineralization of the matrix.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4041\">Osteocytes<\/a><\/strong>\u00a0are mainly inactive bone cells that form from osteoblasts that have become entrapped within their own bone matrix. Osteocytes help regulate the formation and breakdown of bone tissue. They have multiple cell projections that are thought to be involved in\u00a0<a href=\"https:\/\/www.ck12.org\/c\/life-science\/communication?referrer=crossref\">communication<\/a>\u00a0with other bone cells.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">Osteoclasts<\/a><\/strong>\u00a0are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the\u00a0minerals\u00a0in bone and release them into the\u00a0blood.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4043\">Osteogenic cells<\/a><\/strong>\u00a0are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.<\/li>\n<\/ul>\n<figure id=\"attachment_4048\" aria-describedby=\"caption-attachment-4048\" style=\"width: 807px\" class=\"wp-caption alignnone\"><img class=\"wp-image-4048 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cells-1.jpg\" alt=\"11.4.6 Bone Cells\" width=\"807\" height=\"567\"><figcaption id=\"caption-attachment-4048\" class=\"wp-caption-text\"><em>Figure 11.4.6 Different types of bones cells have different functions.<\/em><\/figcaption><\/figure>\n<p>Bone is very active tissue. It is constantly remodeled by the work of osteoblasts and osteoclasts. Osteoblasts continuously make new bone, and osteoclasts keep breaking down bone. This allows for minor repair of bones, as well as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a> of mineral ions in the blood.<\/p>\n<div>\n<h1>Types of Bones<\/h1>\n<\/div>\n<p>There are six types of bones in the human body, categorized based on their shape or location: long, short, flat, sesamoid, sutural, and irregular bones. You can see an example of each type of bone in Figure 11.4.7.<\/p>\n<ul>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4049\">Long bones<\/a><\/strong> are characterized by a shaft that is much longer than it is wide, and by a rounded head at each end of the shaft. Long bones are made mostly of compact bone, with lesser amounts of spongy bone and marrow. Most bones of the limbs, including those of the fingers and toes, are long bones.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4050\">Short bones<\/a><\/strong> are roughly cube-shaped and have only a thin layer of compact bone surrounding a spongy bone interior. The bones of the wrists and ankles are short bones.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4052\">Flat bones<\/a><\/strong>\u00a0are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum (breast bone).<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4053\">Sesamoid bones<\/a><\/strong>\u00a0are embedded in tendons, the connective tissues that bind\u00a0muscles\u00a0to bones. Sesamoid bones hold tendons farther away from\u00a0joints, so the angle of the tendons is increased, thus increasing the leverage of muscles. The patella (knee cap) is an example of a sesamoid bone.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4054\">Sutural bones<\/a><\/strong>\u00a0are very small bones located between the major bones of the skull, within the\u00a0joints\u00a0(sutures) between the larger bones. They are not always present.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4055\">Irregular bones<\/a><\/strong> are those that do not fit into any of the above categories. They generally consist of thin layers of compact bone surrounding a spongy bone interior. Their shapes are irregular and complicated. Examples of irregular bones include the vertebrae and the bones of the pelvis.<\/li>\n<\/ul>\n<figure id=\"attachment_4056\" aria-describedby=\"caption-attachment-4056\" style=\"width: 2048px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4056 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0229_ClassificationofBones-1.png\" alt=\"11.4.7 Bone Types of the Body\" width=\"2048\" height=\"2048\"><figcaption id=\"caption-attachment-4056\" class=\"wp-caption-text\"><em>Figure 11.4.7 This diagram shows an example of each of six types of bones classified by shape or location.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Feature: Reliable Sources<\/h1>\n<\/div>\n<p>Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often\u00a0<em>cure<\/em>\u00a0many life-threatening conditions, including leukemia, lymphoma, sickle cell anemia, and thalassemia. If a bone marrow transplant is successful, the new bone marrow will start making healthy blood cells and improve the patient\u2019s condition.<\/p>\n<p>Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:<\/p>\n<ol>\n<li>How does one become a potential bone marrow donor?<\/li>\n<li>Who can and who cannot donate bone marrow?<\/li>\n<li>How is a bone marrow donation made?<\/li>\n<li>What risks are there in donating bone marrow?<\/li>\n<\/ol>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.4 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3821\">Bones<\/a>\u00a0are organs that consist mainly of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4022\">bone tissue<\/a> (or osseous tissue). Osseous tissue is a type of connective tissue consisting of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3970\">collagen<\/a> matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and\u00a0minerals\u00a0makes bone hard, without making it brittle.<\/li>\n<li>There are two types of osseous tissues: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4030\">compact bone<\/a> tissue and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4031\">spongy bone<\/a> tissue. Compact bone tissue is smooth and dense. It forms the outer layer of bones. Spongy bone tissue is porous and light, and it is found inside many bones.<\/li>\n<li>Besides osseous tissues, bones also contain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3011\">nerves<\/a>,\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3743\">blood vessels<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4039\">periosteum<\/a>.<\/li>\n<li>Bone tissue is composed of four different types of bone cells: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4043\">osteogenic cells<\/a>. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Bone is a very active tissue, constantly being remodeled by the work of osteoblasts and osteoclasts.<\/li>\n<li>There are six types of bones in the\u00a0human body: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4055\">irregular bones<\/a> (such as the vertebrae).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.4 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Describe osseous tissue.<\/li>\n<li>Why are bones hard, but not brittle?<\/li>\n<li>Compare and contrast the compact and spongy bone.<\/li>\n<li>What non-osseous tissues are found in bones?<\/li>\n<li>List four types of bone cells and their functions.<\/li>\n<li>Identify six types of bones. Give an example of each type.<\/li>\n<li>\n<div id=\"h5p-158\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-158\" class=\"h5p-iframe\" data-content-id=\"158\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9.4 Review Questions\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Compare and contrast yellow bone marrow and red bone marrow.<\/li>\n<li>Which type of bone cell divides to produce new bone cells?\u00a0Where is this cell type located?<\/li>\n<li>Where do osteoblasts and osteocytes come from? How are they related to each other?<\/li>\n<li>Which type of bone is embedded in tendons?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.4 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=rDGqkMHPDqE&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">The Skeletal System: Crash Course A&amp;P #19, CrashCourse, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=0dV1Bwe2v6c&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Bone Remodeling and Modeling, Amgen, 2012.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=1Qfmkd6C8u8<\/p>\n<p style=\"text-align: center\">How bones make blood - Melody Smith, TED-Ed, 2020.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 11.4.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bone_marrow_grilled_on_the_barbecue,_sliced_young_raw_garlic,_salted_leek_flowers_from_last_year,_lovage,_and_kale_(19098148350).jpg\" rel=\"cc:attributionURL\">Bone_marrow_grilled_on_the_barbecue,_sliced_young_raw_garlic,_salted_leek_flowers_from_last_year,_lovage,_and_kale_(19098148350)<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/89060048@N03\" rel=\"nofollow\">City Foodsters<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<p><strong>Figure 11.4.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bone_cross-section.svg\" rel=\"cc:attributionURL\">Bone_cross-section.svg<\/a> by <a title=\"User:Pbroks13\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Pbroks13\">Pbroks13<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 11.4.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:621_Anatomy_of_a_Flat_Bone.jpg\" rel=\"cc:attributionURL\">Anatomy_of_a_Flat_Bone<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-3-bone-structure\">OpenStax College<\/a>\u00a0 on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 11.4.4<\/strong><\/p>\n<p><a href=\"https:\/\/www.pxfuel.com\/en\/free-photo-xihdl\" rel=\"cc:attributionURL\">the-detail-of-the-bones-the-structure-of-the-bones-spongy-bone-tram\u010dina-close-up-structure<\/a> on <a href=\"http:\/\/pxfuel.com\">pxfuel<\/a> are used according to the <a href=\"https:\/\/www.pxfuel.com\/terms-of-use\">pxfuel Terms of Use<\/a>.<\/p>\n<p><strong>Figure 11.4.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Spongy_bone_-_Trabecular_bone_2_--_Smart-Servier.png\" rel=\"cc:attributionURL\">Spongy_bone_-_Trabecular_bone_2_\u2013_Smart-Servier<\/a> by <a href=\"https:\/\/smart.servier.com\/smart_image\/bone-structure-8\/\">Laboratoires Servier\/ <i>Smart Servier website<\/i><\/a>\u00a0on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\" rel=\"license\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/p>\n<p><strong>Figure 11.4.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:604_Bone_cells.jpg\" rel=\"cc:attributionURL\">Bone_cells<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-3-bone-structure\">OpenStax College<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 11.4.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0229_ClassificationofBones.png\" rel=\"cc:attributionURL\">Blausen_0229_ClassificationofBones<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en) license.<\/p>\n<p><span style=\"font-size: 1.424em;font-weight: bold\"><br \/>\nReferences<\/span><\/p>\n<p class=\"hanging-indent\">Amgen. (2012, January 19). Bone remodeling and modeling. YouTube. https:\/\/www.youtube.com\/watch?v=0dV1Bwe2v6c<\/p>\n<p class=\"hanging-indent\"><span class=\"os-title-label\"><span class=\"search-highlight text focus\" data-timestamp=\"1596466959729\" data-highlight-id=\"dcef32c5-03c0-4ce7-bc53-4a009f49f357\" data-highlighted=\"true\"><span class=\"search-highlight text\" data-timestamp=\"1596466959742\" data-highlight-id=\"c22d9e48-4a84-467e-ad93-a456dd50461e\" data-highlighted=\"true\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure <\/span><\/span><\/span><span class=\"os-number\"><span class=\"search-highlight text focus\" data-timestamp=\"1596466959729\" data-highlight-id=\"dcef32c5-03c0-4ce7-bc53-4a009f49f357\" data-highlighted=\"true\"><span class=\"search-highlight text\" data-timestamp=\"1596466959742\" data-highlight-id=\"c22d9e48-4a84-467e-ad93-a456dd50461e\" data-highlighted=\"true\">6.9<\/span><\/span><\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"51327\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight text focus\" data-timestamp=\"1596466959729\" data-highlight-id=\"dcef32c5-03c0-4ce7-bc53-4a009f49f357\" data-highlighted=\"true\"><span class=\"search-highlight text\" data-timestamp=\"1596466959742\" data-highlight-id=\"c22d9e48-4a84-467e-ad93-a456dd50461e\" data-highlighted=\"true\">Anatomy of a flat bone <\/span><\/span><\/span>[digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 6.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-3-bone-structure<\/p>\n<p class=\"hanging-indent\"><span class=\"os-title-label\"><span class=\"search-highlight text focus\" data-timestamp=\"1596466959729\" data-highlight-id=\"dcef32c5-03c0-4ce7-bc53-4a009f49f357\" data-highlighted=\"true\"><span class=\"search-highlight text\" data-timestamp=\"1596466959742\" data-highlight-id=\"c22d9e48-4a84-467e-ad93-a456dd50461e\" data-highlighted=\"true\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). <\/span><\/span>Figure\u00a0<\/span><span class=\"os-number\">6.11<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"3633\" class=\"os-title\" data-type=\"title\">Bone cells [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 6.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-3-bone-structure<\/span><\/p>\n<p class=\"hanging-indent\">CK-12 Foundation. (n.d.). Communication: Identifies means of communication between animals. ck12.org. https:\/\/www.ck12.org\/c\/life-science\/communication\/<\/p>\n<p class=\"hanging-indent\">CrashCourse. (2015, May 18). The skeletal system: Crash Course A&amp;P #19. YouTube. https:\/\/www.youtube.com\/watch?v=rDGqkMHPDqE<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2020, January 27). How bones make blood - Melody Smith. YouTube. https:\/\/www.youtube.com\/watch?v=1Qfmkd6C8u8<\/p>\n<\/div>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4050\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4050\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4076\" aria-describedby=\"caption-attachment-4076\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4076\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/First_plaster_long_leg_cast...._-_9383569051-1.jpg\" alt=\"11.5.1 Intro\" width=\"400\" height=\"226\"><figcaption id=\"caption-attachment-4076\" class=\"wp-caption-text\"><em>Figure 11.5.1 Water and casts don't mix.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Break A Leg<\/h1>\n<\/div>\n<p>Did you ever break a leg or other bone, like the man looking longingly at the water in this swimming pool (Figure 11.5.1)? Having a broken bone can really restrict your activity. Bones are very hard, but they will break (or fracture) if enough force is applied to them. Fortunately, bones are highly active organs that can repair themselves if they break. Bones can also remodel themselves and grow. You\u2019ll learn how bones can do all of these things in this section.<\/p>\n<div>\n<h1>Bone Growth<\/h1>\n<\/div>\n<p>Early in the\u00a0development\u00a0of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4077\">Ossification<\/a><\/strong> is a process in which bone tissue is created from cartilage. The steps in which bones of the skeleton form from cartilage are illustrated in 11.5.2. The steps are as follows:<\/p>\n<ol>\n<li>Cartilage \u201cmodel\u201d of bone forms. This model continues to grow as ossification takes place.<\/li>\n<li>Ossification begins at a primary ossification center in the middle of bone.<\/li>\n<li>Ossification then starts to occur at secondary ossification centers at the ends of bone.<\/li>\n<li>The medullary cavity forms. This cavity will contain red bone marrow.<\/li>\n<li>Areas of ossification meet at epiphyseal plates, and articular cartilage forms. Bone growth ends.<\/li>\n<\/ol>\n<figure id=\"attachment_4078\" aria-describedby=\"caption-attachment-4078\" style=\"width: 520px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4078 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_growth-1.png\" alt=\"11.5.2 Bone Growth\" width=\"520\" height=\"310\"><figcaption id=\"caption-attachment-4078\" class=\"wp-caption-text\"><em>Figure 11.5.2 The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.<\/em><\/figcaption><\/figure>\n<div>\n<p>The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.<\/p>\n<\/div>\n<h2>Primary and Secondary Ossification Centers<\/h2>\n<p>When bone forms from cartilage, ossification begins with a point in the cartilage called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4079\">primary ossification center<\/a><\/strong>. This generally appears during fetal\u00a0development, although a few short\u00a0bones\u00a0begin their primary ossification after birth. Ossification happens\u00a0toward both ends of the bone from the primary ossification center, and \u2014 in the case of long bones \u2014 it eventually forms the shaft of the bone.<\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4080\"><strong>Secondary ossification centers<\/strong><\/a> form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.<\/p>\n<h2>Skeletal Maturity<\/h2>\n<p>Throughout childhood, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3951\">cartilage<\/a> remaining in the skeleton keeps growing, and allows for bones to grow in size. Once all of the cartilage has been replaced by bone, and fusion has taken place at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4108\">epiphyseal plate<\/a>s, bones can no longer keep growing in length. At this point, skeletal maturity has been reached. It generally takes place by age 18 to 25.<\/p>\n<p>The use of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3438\">anabolic steroids<\/a>\u00a0by teens can\u00a0speed\u00a0up the process of skeletal maturity, resulting in a shorter period of cartilage growth before fusion takes place. This means that teens who use steroids are likely to end up shorter as adults than they would otherwise have been.<\/p>\n<div>\n<h1>Bone Remodeling<\/h1>\n<\/div>\n<p>Even after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process\u00a0called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4109\">bone remodeling<\/a><\/strong>. In this lifelong process, mature bone tissue is continually turned over, with about ten per cent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">osteoclasts<\/a><\/strong>\u00a0\u2014<strong>\u00a0<\/strong>which are bone\u00a0cells\u00a0that resorb bone and dissolve its\u00a0minerals\u00a0\u2014 and\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">osteoblasts<\/a><\/strong>, which are bone\u00a0cells\u00a0that make new bone matrix.<\/p>\n<p>Bone remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>, because it either releases mineral from bones into the blood or absorbs mineral from the blood into bones. Figure 11.5.3 shows how osteoclasts in bones are involved in calcium regulation.<\/p>\n<figure id=\"attachment_4110\" aria-describedby=\"caption-attachment-4110\" style=\"width: 825px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4110 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Calcium_Homeostasis-1.jpg\" alt=\"11.5.3 Calcium Homeostasis\" width=\"825\" height=\"737\"><figcaption id=\"caption-attachment-4110\" class=\"wp-caption-text\"><em>Figure 11.5.3 Keeping the calcium level in homeostasis involves the work of osteoclasts, the bone cells that resorb bone and release calcium into the blood.<\/em><\/figcaption><\/figure>\n<p>The action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3665\">enzymes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3569\">hormones<\/a>, and other substances that either promote or inhibit the activity of the cells. In this way, these substances control the rate at which bone is made, destroyed, and changed in shape. For example, the rate at which osteoclasts resorb bone and release calcium into the blood is promoted by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3467\">parathyroid hormone<\/a> (PTH) and inhibited by calcitonin, which is produced by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2958\">thyroid gland<\/a> (see the diagram in Figure 11.5.3). The rate at which osteoblasts create new bone is stimulated by growth hormone, which is produced by the anterior lobe of the pituitary gland. Thyroid hormone and sex hormones (estrogens and androgens) also stimulate osteoblasts to create new bone.<\/p>\n<div>\n<h1>Bone Repair<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4111\">Bone repair<\/a><\/strong> (or healing) is the process in which a bone repairs itself following a bone fracture. You can see an X-ray of a bone fracture in Figure 11.5.4. In this fracture, the humerus in the upper arm has been completely broken through its shaft. Before this fracture heals, a physician must push the displaced bone parts back into their correct positions. Then, the bone must be stabilized \u2014 with a cast and\/or pins surgically inserted into the bone, for example (as shown in Figure 11.5.5) \u2014 until the bone\u2019s natural healing process is complete. This process may take several weeks.<\/p>\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\">\n<tbody>\n<tr>\n<td style=\"width: 55.0786%\">\n<figure id=\"attachment_4112\" aria-describedby=\"caption-attachment-4112\" style=\"width: 390px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4112\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Broken-Arm-by-Ashley-Chung-scaled-e1591658172394-1.jpg\" alt=\"11.5.4 Broken Arm\" width=\"390\" height=\"257\"><figcaption id=\"caption-attachment-4112\" class=\"wp-caption-text\"><em>Figure 11.5.4 A bone fracture does not always involve a complete break in the bone, as in this X-ray. Sometimes, a fracture is just a crack in the bone. In other cases, the bone not only breaks all the way through, but also breaks through the soft tissues around it so it protrudes from the skin. This is called an open fracture.<\/em><\/figcaption><\/figure>\n<\/td>\n<td style=\"width: 44.9214%\">\n<figure id=\"attachment_4113\" aria-describedby=\"caption-attachment-4113\" style=\"width: 241px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4113\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Broken-Arm-with-plate-and-pins-by-Ashley-Chung-scaled-2.jpg\" alt=\"11.5.5 Broken Arm with plate and pins\" width=\"241\" height=\"321\"><figcaption id=\"caption-attachment-4113\" class=\"wp-caption-text\"><em>Figure 11.5.5 While some bones can heal by wearing a cast, others may require more invasive treatments, such as bone fracture repair. Bone fracture repair is a surgery to fix a broken bone using metal screws, pins, rods, or plates to hold the bone in place. It's also known as open reduction and internal fixation (ORIF) surgery.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Although bone repair is a natural physiological process, it may be promoted or inhibited by several factors.\u00a0Fracture repair is more likely to be successful with adequate nutrient intake. Age, bone type, drug therapy, and pre-existing bone disease are additional factors that may affect healing. Bones that are weakened by disease (such as osteoporosis or bone cancer) are not only likely to heal more slowly, but are also more likely to fracture in the first place.<\/p>\n<div>\n<h1>Feature: Myth vs. Reality<\/h1>\n<\/div>\n<p>Bone fractures are fairly common, and there are many myths about them. Knowing the facts is important, because fractures generally require emergency medical treatment.<\/p>\n<table class=\"grid\" style=\"border-collapse: collapse;width: 100%;height: 299px\" border=\"0\">\n<tbody>\n<tr style=\"height: 98px\">\n<th scope=\"col\"><span style=\"color: #ff0000\">Myth<\/span><\/th>\n<th scope=\"col\"><span style=\"color: #008000\">Reality<\/span><\/th>\n<\/tr>\n<tr style=\"height: 31px\">\n<td><em>\"A bone fracture is a milder injury than a broken bone.\"<\/em><\/td>\n<td>A bone fracture is the same thing as a broken bone.<\/td>\n<\/tr>\n<tr style=\"height: 127px\">\n<td><em>\"If you still have full range of motion in a limb, then it must not be fractured.\"<\/em><\/td>\n<td>Even if a bone is fractured, the\u00a0muscles\u00a0and tendons attached to it may still be able to move the bone normally. This is especially likely if the bone is cracked \u2014 but not broken \u2014 into two pieces. Even if a bone is broken all the way through, range of motion may not be affected if the bones on either side of the fracture remain properly aligned.<\/td>\n<\/tr>\n<tr style=\"height: 15px\">\n<td><em>\"A fracture always produces a bruise.\"<\/em><\/td>\n<td>Many \u2014 but not all \u2014 fractures produce a bruise. If a fracture does produce a bruise, it may take several hours (or even a day or more!) for the bruise to appear.<\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td><em>\"Fractures are so painful that you will immediately know if you break a bone.\"<\/em><\/td>\n<td>Ligament sprains and muscle strains are also very painful, sometimes more painful than fractures. Additionally, every person has a different pain tolerance. People with a high pain tolerance may continue using a broken bone in spite of the pain.<\/td>\n<\/tr>\n<tr style=\"height: 14px\">\n<td><em>\"You can tell when a bone is fractured because there will be very localized pain over the break.\"<\/em><\/td>\n<td>A broken bone is often accompanied by injuries to surrounding\u00a0muscles\u00a0or ligaments. As a result, the pain may extend far beyond the\u00a0location\u00a0of the fracture. The pain may be greater directly over the fracture, but the intensity of the pain may make it difficult to pinpoint exactly where the pain originates.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.5 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3821\">Bone<\/a> is very active tissue. Its cells are constantly forming and resorbing bone matrix.<\/li>\n<li>Early in the\u00a0development\u00a0of a human fetus, the skeleton is made almost entirely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3951\">cartilage<\/a>. The relatively soft cartilage gradually turns into hard bone. This is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4077\">ossification<\/a>. It begins at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4079\">primary ossification center<\/a> in the middle of bone, and later also occurs at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4080\">secondary ossification centers<\/a>\u00a0in the ends of bone. The bone can no longer grow in length after the areas of ossification meet and fuse at the time of skeletal maturity.<\/li>\n<li>Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">osteoclasts<\/a>\u00a0resorb bone, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">osteoblasts<\/a>\u00a0make new bone to replace it. Bone remodeling shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>\u00a0in the\u00a0blood.<\/li>\n<li>Bone repair is the natural process in which a bone repairs itself following a bone fracture. This process may take several weeks. In the process, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4039\">periosteum<\/a> produces cells that develop into osteoblasts, and the osteoblasts form new bone matrix to heal the fracture. Bone repair may be affected by diet, age, pre-existing bone disease, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.5 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Outline how bone develops starting\u00a0early in the\u00a0fetal stage, and through the age of skeletal maturity.<\/li>\n<li>Describe the process of bone remodeling. When does it occur?<\/li>\n<li>What purposes does bone remodeling serve?<\/li>\n<li>Define bone repair. How long does this process take?<\/li>\n<li>Explain how bone repair occurs.<\/li>\n<li>Identify factors that may affect bone repair.<\/li>\n<li>\n<div id=\"h5p-160\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-160\" class=\"h5p-iframe\" data-content-id=\"160\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Size of Proteins\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>If there is a large region between the primary and secondary ossification centers in a bone, is the person young or old? Explain your answer.<\/li>\n<li>If bones can repair themselves, why are casts and pins sometimes necessary in the process?<\/li>\n<li>When calcium levels are low, which type of bone cell causes the release of calcium to the bloodstream?<\/li>\n<li>Which tissue and bone cell type are primarily\u00a0involved in bone repair after a fracture?<\/li>\n<li>Describe one way in which hormones are involved in bone remodeling.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.5 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=yJoQj5-TIvE<\/p>\n<p style=\"text-align: center\">How to grow a bone - Nina Tandon, TED-Ed, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=-P6LsendHxU&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Healing Process of Bone Fracture, Aldo Fransiskus Marsetio, 2015.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=RC2w_9DcY38<\/p>\n<p style=\"text-align: center\">The Skeleton From Fetal to Adult, Samantha Espolt, 2012.<\/p>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h2>Attributions<\/h2>\n<p><strong>Figure 11.5.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:First_plaster_long_leg_cast...._-_9383569051.jpg\" rel=\"cc:attributionURL\">First_plaster_long_leg_cast\u2026._-_9383569051<\/a> by <a class=\"owner-name truncate\" title=\"Go to 4x4king10's photostream\" href=\"https:\/\/www.flickr.com\/photos\/rockincast\/\" data-track=\"attributionNameClick\">4x4king10<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<p><strong>Figure 11.5.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bone_growth.png\" rel=\"cc:attributionURL\">Bone_growth<\/a> by <a title=\"User:Chaldor\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Chaldor\">Chaldor<\/a> (derivative work) on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:en:Public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:Public_domain\">public domain<\/a>\u00a0(https:\/\/en.wikipedia.org\/wiki\/en:Public_domain). (Original, <a title=\"File:Illu bone growth.jpg\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Illu_bone_growth.jpg\">Illu_bone_growth.jpg<\/a> is by <a class=\"mw-userlink\" title=\"User:Fuelbottle~commonswiki\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Fuelbottle~commonswiki\">Fuelbottle<\/a>)<\/p>\n<p><strong>Figure 11.5.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:625_Calcium_Homeostasis.jpg\" rel=\"cc:attributionURL\">Calcium_Homeostasis<\/a> by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-7-calcium-homeostasis-interactions-of-the-skeletal-system-and-other-organ-systems\">OpenStax<\/a> on Wikimedia commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0\/deed.en) license.<\/p>\n<p><strong>Figure 11.5.4<\/strong><\/p>\n<p>Broken Arm by Ashley Chung is used with permission.<\/p>\n<p><strong>Figure 11.5.5<\/strong><\/p>\n<p>Broken Arm with plate and pins by Ashley Chung is used with permission.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Aldo Fransiskus Marsetio. (2015, ). Healing process of bone fracture. YouTube. https:\/\/www.youtube.com\/watch?v=-P6LsendHxU<\/p>\n<p class=\"hanging-indent\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 6.24 Pathways in calcium homeostasis [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 6.7). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-7-calcium-homeostasis-interactions-of-the-skeletal-system-and-other-organ-systems<\/p>\n<p class=\"hanging-indent\">Samantha Espolt. (2012, ). The skeleton from fetal to adult. YouTube. https:\/\/www.youtube.com\/watch?v=RC2w_9DcY38&amp;t=3s<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, ). How to grow a bone - Nina Tandon. YouTube. https:\/\/www.youtube.com\/watch?v=yJoQj5-TIvE<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4053\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4053\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<div>\n<figure id=\"attachment_4138\" aria-describedby=\"caption-attachment-4138\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4138\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Running-Shoes-by-bruno-nascimento-PHIgYUGQPvU-unsplash-scaled-2.jpg\" alt=\"11.8.1 Running Shoes\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-4138\" class=\"wp-caption-text\"><em>Figure 11.8.1 Wear supportive shoes \u2014 your feet will thank you!<\/em><\/figcaption><\/figure>\n<h1>Case Study Conclusion: A Pain in the Foot<\/h1>\n<\/div>\n<p>As Sophia discovered in the beginning of the chapter, wearing high heels can result in a condition called metatarsalgia. Metatarsalgia is named for the metatarsal bones, which are the five bones that run through the ball of the foot\u00a0 just behind the toes (highlighted in Figures 11.8.2 and 11.8.3). Wearing high heels causes excessive pressure on the ball of the foot, as described in the beginning of this chapter. Additionally, the toes are forced to pull upward in high heels, which moves the fleshy padding away from the ball of the foot and adds to the overall pressure placed on this region. Over time, this can cause inflammation and direct stress on the bones, resulting in the pain in the ball of the foot known as metatarsalgia. The pain occurs especially in weight-bearing positions, such as standing, walking, or running \u2014 which is what Sophia was experiencing. There may also be pain, numbness, or tingling in the toes associated with metatarsalgia.<\/p>\n<table class=\"no-lines\" style=\"border-collapse: collapse;width: 100%;height: 79px\" border=\"0\">\n<tbody>\n<tr>\n<td style=\"width: 50%\">\n<figure id=\"attachment_4140\" aria-describedby=\"caption-attachment-4140\" style=\"width: 367px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4140\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Metatarsalgia-by-Esther-Max-on-flickr-1.jpg\" alt=\"11.7.2 Metatarsalgia pain\" width=\"367\" height=\"203\"><figcaption id=\"caption-attachment-4140\" class=\"wp-caption-text\"><em>Figure 11.8.2 Metatarsalgia is a painful and even debilitating condition characterized by pain in the ball of your foot with worsens when you stand, run, walk or flex your foot.<\/em><\/figcaption><\/figure>\n<\/td>\n<td style=\"width: 50%\">\n<figure id=\"attachment_4141\" aria-describedby=\"caption-attachment-4141\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4141 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gray290_-_Mratatarsus-1-1.png\" alt=\"11.7.3 Metatarsals\" width=\"500\" height=\"189\"><figcaption id=\"caption-attachment-4141\" class=\"wp-caption-text\"><em>Figure 11.8.3 Illustration of the bones of the foot, with the metatarsal bones highlighted in pink.<\/em><\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Wearing high heels can also cause stress fractures in the feet, which are tiny breaks in bone that occur due to repeated mechanical stress. This is caused by the excessive pressure that high heels put on some of the bones of the feet. These fractures are somewhat similar to what occurs in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4122\">osteoporosis <\/a>when the bone mass decreases to the point where bones can fracture easily as a person goes about their daily activities. In both cases, a major noticeable injury is not necessary to create the tiny fractures. As you have learned, tiny fractures that accrue over time are the cause of dowager\u2019s hump (or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4143\">kyphosis<\/a>), which is often seen in women with osteoporosis.<\/p>\n<p>Don\u2019t think you are immune to stress fractures just because you don\u2019t wear high heels! This injury also commonly occurs in people who participate in sports involving repetitive striking of the foot on the ground, such as running, tennis, basketball, or gymnastics. They may be avoided by taking preventative measures. You should ramp up any increase in activity slowly, cross-train by engaging in a variety of different sports or activities, rest if you experience pain, and wear well-cushioned and supportive running shoes.\u00a0 It is important to know that your cardiovascular and muscular systems adapt to an increase in physical activity much more quickly than the skeletal system.<\/p>\n<figure id=\"attachment_4144\" aria-describedby=\"caption-attachment-4144\" style=\"width: 411px\" class=\"wp-caption alignright\"><img class=\"wp-image-4144\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Heels-by-gavin-allanwood-ndpX28miBtE-unsplash-scaled-2.jpg\" alt=\"11.8.4 Heels\" width=\"411\" height=\"411\"><figcaption id=\"caption-attachment-4144\" class=\"wp-caption-text\"><em>Figure 11.8.4 High heels with a narrow, pointed toe box and thin stiletto heels<\/em><\/figcaption><\/figure>\n<p>Sophia learned through her online research that wearing high heels can also lead to foot deformities, such as bunions and <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839\">hammertoes<\/a>. As you learned in an earlier chapter, a bunion is a protrusion on the side of the foot, most often at the base of the big toe. It can be caused by wearing shoes with a narrow, pointed toe box \u2014 a common shape for high heels (see Figure 11.8.4). The pressure of the shoes on the side of the foot causes an enlargement of bone or inflammation of other tissues in the region, which pushes the big toe toward the other toes.<\/p>\n<p><span style=\"font-size: 1em;text-align: initial\">Hammertoes are an abnormal bend in the middle joint of the second, third, or fourth toe (with the big toe being the first toe), causing the toe to be shaped similarly to a hammer. The narrow, pointed toe box of many high heels, combined with the way the toes are squished into the front of the shoe as a result of the height of the heel, can cause the toes to become deformed this way. Treatments for bunions and hammertoe include wearing shoes with a roomy toe box, padding or taping the toes, and toe exercises and stretches. If the bunion or hammertoe does not respond to these treatments, surgery may be necessary to correct the deformity.<\/span><\/p>\n<p>Because the bones of the skeleton are connected and work together with other systems to support the body, wearing high heels can also cause physical problems in areas other than the feet. Wearing high heels shifts a person\u2019s posture and alignment, and can put strain on tendons, muscles, and other joints in the body. Research published in 2014 from a team at Stanford University suggests that wearing high heels, particularly if the person is overweight or the heels are very high, may increase the risk of osteoarthritis (OA) in the knee, due to added stress on the knee joint as the person walks. As you have learned, OA results from the breakdown of cartilage and bone at the joint. Because it can only be treated to minimize symptoms \u2014 and not for a cure \u2014 OA could be an unfortunate long-term consequence of wearing high heels.<\/p>\n<p>Sophia has decided that wearing high heels regularly is not worth the pain and potential long-term damage to her body. After consulting with her doctor, who confirmed she had metatarsalgia, she was able to successfully treat it with ice, rest, and wearing comfortable, supportive shoes instead of heels.<\/p>\n<p>High heels are not the only kind of shoes that can cause problems. Flip-flops, worn-out sneakers, and shoes that are too tight can all cause foot issues. To prevent future problems from her shoe choices, Sophia is following guidelines recommended by medical experts. The guidelines include:<\/p>\n<ul>\n<li>Wearing shoes that fit well, have plenty of room in the toes, are supportive, and are comfortable right away. There should be no \u201cbreak-in\u201d period needed for shoes.<\/li>\n<li>Avoiding high heels, especially those with heels over two inches high, or those that have narrow, pointed toe boxes or very thin heels. The heels pictured in Figure 11.8.4 are an example of a type of shoe that should be avoided.<\/li>\n<li>If high heels\u00a0<em>must<\/em>\u00a0be worn, it\u00a0should only be for a limited period of time.<\/li>\n<\/ul>\n<p>As you have learned in this chapter, your skeletal system carries out a variety of important functions in your body, including physical support. But even though it is strong, your skeletal system can become damaged and deformed \u2014 even through such a seemingly innocuous act as wearing a certain type of shoe. Taking good care of your skeletal system is necessary to help it continue to take good care of the rest of you.<\/p>\n<div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 11 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>In this chapter, you learned about the skeletal system. Specifically, you learned that:<\/p>\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2906\">skeletal system<\/a> is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3821\">Bones<\/a> are organs made of supportive connective tissues, mainly the tough protein <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3970\">collagen<\/a>. Bones also contain blood vessels, nerves, and other tissues. Bones are hard and rigid, due to deposits of calcium and other mineral salts within their living tissues. Besides bones, the skeletal system includes cartilage and ligaments.<\/li>\n<li>The skeletal system has many different functions, including supporting the body and giving it shape, protecting internal organs, providing attachment surfaces for skeletal muscles, allowing body movements, producing blood cells, storing minerals, helping to maintain mineral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3669\">homeostasis<\/a>, and producing endocrine hormones.<\/li>\n<li>There is relatively little sexual dimorphism in the human skeleton, although the female skeleton tends to be smaller and less robust than the male skeleton. The greatest sex difference is in the pelvis, which is adapted for childbirth in females.<\/li>\n<li>The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3953\">axial skeleton<\/a> consists of a total of 80 bones. It includes the skull, vertebral column, and rib cage. It also includes the three tiny ossicles in the middle ear and the hyoid bone in the throat.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3957\">skull<\/a> provides a bony framework for the head. It consists of 22 different bones: eight in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3975\">cranium<\/a>, which encloses the brain, and 14 in the face, which includes the upper and lower jaw.<\/li>\n<li>The vertebral column is a flexible, S-shaped column of 33 <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3964\">vertebrae<\/a> that connects the trunk with the skull and encloses the spinal cord. The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal regions. The S shape of the vertebral column allows it to absorb shocks and distribute the weight of the body.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4002\">rib cage<\/a> holds and protects the organs of the upper part of the trunk, including the heart and lungs. It includes the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs.<\/li>\n<\/ul>\n<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3954\">appendicular skeleton<\/a> consists of a total of 126 bones. It includes the bones of the four limbs, shoulder girdle, and pelvic girdle. The girdles attach the appendages to the axial skeleton.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Each upper limb consists of 30 bones. There is one bone (called the humerus) in the upper arm, and two bones (called the ulna and radius) in the lower arm. The wrist contains eight\u00a0carpal bones, the hand contains\u00a0five\u00a0metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.<\/li>\n<li>Each lower limb also consists of 30 bones. There is one bone (called the femur) in the upper leg, and two bones (called the tibia and fibula) in the lower leg. The patella covers the knee joint. The ankle contains\u00a0seven\u00a0tarsal bones, and the foot contains five\u00a0metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.<\/li>\n<li>The shoulder girdle attaches the upper limbs to the trunk of the body. It is connected to the axial skeleton only by muscles, allowing mobility of the upper limbs. Bones of the shoulder girdle include a right and left clavicle, and a right and left scapula.<\/li>\n<li>The pelvic girdle attaches the legs to the trunk of the body and supports the organs of the abdomen. It is connected to the axial skeleton by ligaments. The pelvic girdle consists of two halves that are fused together in adults. Each half consists of three bones: the ilium, pubis, and ischium.<\/li>\n<\/ul>\n<\/li>\n<li>Bones are organs that consist mainly of bone (or osseous) tissue. Osseous tissue is a type of supportive connective tissue consisting of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard, without making it brittle.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>There are two types of osseous tissues: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4030\">compact bone tissue<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4031\">spongy bone tissue<\/a>. Compact bone tissue is smooth and dense. It forms the outer layer of bones. Spongy bone tissue is porous and light, and it is found inside many bones.<\/li>\n<\/ul>\n<\/li>\n<li>Besides osseous tissues, bones also contain nerves, blood vessels, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4039\">periosteum<\/a>.<\/li>\n<li>Bone tissue is composed of four different types of bone cells: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4043\">osteogenic cells<\/a>. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Bone is a very active tissue, constantly being remodeled by the work of osteoblasts and osteoclasts.<\/li>\n<li>There are six types of bones in the human body: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4055\">irregular bones<\/a> (such as the vertebrae).<\/li>\n<li>Early in the development of a human fetus, the skeleton is made almost entirely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3951\">cartilage<\/a>. The relatively soft cartilage gradually turns into hard bone \u2014 a process that is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4077\">ossification<\/a>. It begins at a primary ossification center in the middle of bone, and later also occurs at secondary ossification centers in the ends of bone. The bone can no longer grow in length after the areas of ossification meet and fuse at the time of skeletal maturity.<\/li>\n<li>Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, osteoclasts resorb bone and osteoblasts make new bone to replace it. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4109\">Bone remodeling<\/a> shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral homeostasis in the blood.<\/li>\n<li>Bone repair is the natural process in which a bone repairs itself following a bone fracture. This process may take several weeks. In the process, the periosteum produces cells that develop into osteoblasts, and the osteoblasts form new bone matrix to heal the fracture. Bone repair may be affected by diet, age, pre-existing bone disease, or other factors.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3949\">Joints<\/a>\u00a0are locations at which bones of the skeleton connect with one another.<\/li>\n<li>Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.<\/li>\n<li>The structural classification of joints depends on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.<\/li>\n<li>The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4094\">pivot<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4096\">hinge<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4098\">saddle<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4100\">plane<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3391\">condyloid<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4093\">ball-and-socket<\/a> joints.<\/li>\n<\/ul>\n<\/li>\n<li>A number of disorders affect the skeletal system, including bone fractures and bone cancers. The two most common disorders of the skeletal system are osteoporosis and osteoarthritis.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4122\">Osteoporosis<\/a> is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation and bone resorption in bone remodeling. Osteoporosis may also occur as a side effect of other disorders or certain medications.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Osteoporosis is diagnosed by measuring a patient\u2019s bone density and comparing it with the normal level of peak bone density. Fractures are the most dangerous aspect of osteoporosis. Osteoporosis is rarely fatal, but complications of fractures often are.<\/li>\n<li>Risk factors for osteoporosis include older age, female sex, European or Asian ancestry, family history of osteoporosis, short stature and small bones, smoking, alcohol consumption, lack of exercise, vitamin D deficiency, poor nutrition, and consumption of soft drinks.<\/li>\n<li>Osteoporosis is often treated with medications (such as bisphosphonates) that may slow or even reverse bone loss. Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior, such as undertaking weight-bearing exercise.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4102\">Osteoarthritis<\/a> (OA) is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage, coupled with low-grade inflammation of the joints.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Diagnosis of OA is typically made on the basis of signs and symptoms, such as joint deformities, pain, and stiffness. X-rays or other tests are sometimes used to either support the diagnosis or rule out other disorders. Age is the chief risk factor for OA. Other risk factors include joint injury, excess body weight, and a family history of OA.<\/li>\n<li>OA cannot be cured, but the symptoms can often be treated successfully. Treatments may include exercise, efforts to decrease stress on joints, pain medications, and surgery to replace affected hip or knee joints.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p>As you have learned in this chapter, one of the important functions of the skeletal system is to allow movement of the body. But it doesn\u2019t do it alone. Movement is caused by the contraction of muscles, which pull on the bones, causing them to move. Read the next chapter to learn about this and other important functions of the muscular system.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 11 Review<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li style=\"list-style-type: none\">\n<ol>\n<li>\n<div id=\"h5p-162\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-162\" class=\"h5p-iframe\" data-content-id=\"162\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"3.7 True\/False\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Why does the rib cage need to be flexible? Why can it\u00a0be flexible?<\/li>\n<li>In general, what do \u201cgirdles\u201d in the skeletal system do?<\/li>\n<li>Would swimming be more effective as an exercise for preventing osteoporosis or as a treatment for osteoarthritis? Explain your answer.<\/li>\n<li>Explain why some of the vertebrae become misshapen in the condition called dowager\u2019s hump (or kyphosis).<\/li>\n<li>Explain why osteoarthritis often involves inflammation in the joints.<\/li>\n<li>Osteoporosis can involve excess bone resorption, as well as insufficient production of new bone tissue. What are the two main bone cell types that carry out these processes, respectively?<\/li>\n<li>Describe two roles that calcium in bones play in the body.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 11.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/PHIgYUGQPvU\" rel=\"cc:attributionURL\">Running Shoes by bruno-nascimento-PHIgYUGQPvU<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@bruno_nascimento\">Bruno Nascimento<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<p><strong>Figure 11.8.2<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/26430974194\">Metatarsalgia\/ Best Shoes for Metatarsalgia<\/a> by <a href=\"https:\/\/www.flickr.com\/photos\/esthermax\/\" rel=\"dc:creator\">Esther Max<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/p>\n<\/div>\n<p><strong>Figure 11.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Gray290_-_Mratatarsus.png\" rel=\"cc:attributionURL\">Gray290_-_Mratatarsus (1)<\/a> by\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Henry_Vandyke_Carter\">Henry Vandyke Carter<\/a> (1831-1897) (Revised by Warren H. Lewis, coloured by <a class=\"mw-userlink\" title=\"User:Was a bee\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Was_a_bee\">Was a bee<\/a>) on Wikimedia Commons is in the <a class=\"extiw\" title=\"en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain. (<a class=\"extiw\" title=\"en:Bartleby.com\" href=\"https:\/\/en.wikipedia.org\/wiki\/Bartleby.com\">Bartleby.com<\/a>:\u00a0<a class=\"external text\" href=\"http:\/\/www.bartleby.com\/107\/\" rel=\"nofollow\">Gray\u2019s Anatomy<\/a>,\u00a0<a class=\"external text\" href=\"https:\/\/www.bartleby.com\/107\/illus290.html\" rel=\"nofollow\">Plate 290<\/a>)<\/p>\n<p><strong>Figure 11.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/ndpX28miBtE\" rel=\"cc:attributionURL\">Heels by gavin-allanwood-ndpX28miBtE-unsplash<\/a> by Photo by <a href=\"https:\/\/unsplash.com\/@gavla\">Gavin Allanwood<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Hammertoe and mallet toe [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/hammertoe-and-mallet-toe\/symptoms-causes\/syc-20350839<\/p>\n<p class=\"hanging-indent\">VanDyke Carter, H. (1858). Illustration plate 290. In H. Gray,\u00a0 <i>Anatomy of the Human Body.<\/i> Lea &amp; Febiger. Bartleby.com, 2000. www.bartleby.com\/107\/.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4054\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4054\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4147\" aria-describedby=\"caption-attachment-4147\" style=\"width: 333px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4147\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Whiskeys-2nd-Birthday-by-Kelly-Hunter-on-Flickr-1.jpg\" alt=\"12.1.1 Doggy body language\" width=\"333\" height=\"500\"><figcaption id=\"caption-attachment-4147\" class=\"wp-caption-text\"><em>Figure 12.1.1 Who's a good boy?<\/em><\/figcaption><\/figure>\n<div>\n<h1>Case Study: Needing to Relax<\/h1>\n<\/div>\n<p>This dog (Figure 12.1.1) is expressing his interest in something \u2014 perhaps a piece of food \u2014 by using the neck muscles to tilt its head in an adorable fashion. Humans also sometimes tilt their heads to express interest. But imagine how disturbing and painful it would be if your neck tilted involuntarily, without you being able to control it! Forty-three year old Edward unfortunately knows just how debilitating this can be.<\/p>\n<p>Edward has a rare condition called <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/cervical-dystonia\/symptoms-causes\/syc-20354123\">cervical dystonia<\/a>, which is also called spasmodic torticollis. In this condition, the muscles in the neck contract involuntarily, often causing the person\u2019s head to twist to one side. Figure 12.1.2 shows one type of abnormal head positioning that can be caused by cervical dystonia. The muscles may contract in a sustained fashion, holding the head and neck in one position, or they may spasm repeatedly, causing jerky movements of the head and neck.<\/p>\n<figure id=\"attachment_4148\" aria-describedby=\"caption-attachment-4148\" style=\"width: 467px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4148\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Dystonia2010-1.jpg\" alt=\"12.1 Dystonia\" width=\"467\" height=\"480\"><figcaption id=\"caption-attachment-4148\" class=\"wp-caption-text\"><em>Figure 12.1.2 Dystonia is a movement disorder in which a person's muscles contract uncontrollably. The contraction causes the affected body part to twist involuntarily, resulting in repetitive movements or abnormal postures. Dystonia can affect one muscle, a muscle group, or the entire body.<\/em><\/figcaption><\/figure>\n<p>Cervical dystonia is painful and can significantly interfere with a person\u2019s ability to carry out their usual daily activities. In Edward\u2019s case, he can no longer drive a car, because his uncontrollable head and neck movements and abnormal head positioning prevent him from navigating the road safely. He also has severe neck and shoulder pain much of the time.<\/p>\n<p>Although it can be caused by an injury, there is no known cause of <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/cervical-dystonia\/symptoms-causes\/syc-20354123\">cervical dystonia<\/a> \u2014 and there is also no cure. Fortunately for Edward, and others who suffer from cervical dystonia,\u00a0 there is a treatment that can significantly reduce symptoms in many people. You may be surprised to learn that this treatment is the same substance which, when injected into the face, is used for cosmetic purposes to reduce wrinkles!<\/p>\n<p>The substance is botulinum toxin, one preparation of which may be familiar to you by its brand name \u2014 <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4150\">Botox<\/a>. It is a neurotoxin produced by the bacterium\u00a0<em>Clostridium botulinum<\/em>, and can cause a life-threatening illness called <a href=\"https:\/\/www.who.int\/news-room\/fact-sheets\/detail\/botulism#:~:text=Clostridium%20botulinum%20is%20a%20bacterium,to%20respiratory%20and%20muscular%20paralysis.\">botulism<\/a>. However, when injected in very small amounts by a skilled medical professional, botulinum toxins have some safe and effective uses. In addition to cervical dystonia, botulinum toxins can be used to treat other disorders involving the muscular system, such as strabismus (misalignment of the eyes); eye twitches; excessive muscle contraction due to neurological conditions\u00a0like\u00a0cerebral palsy; and even overactive bladder.<\/p>\n<p>Botulinum toxin has its effect on the muscular system by inhibiting muscle contractions. When used to treat wrinkles, it relaxes the muscles of the face, lessening the appearance of wrinkles. When used to treat cervical dystonia and other disorders involving excessive muscle contraction, it reduces the abnormal contractions.<\/p>\n<p>In this chapter, you will learn about the muscles of the body, how they contract to produce movements and carry out their functions, and some disorders that affect the muscular system. At the end of the chapter, you will find out if botulinum toxin helped relieve Edward\u2019s cervical dystonia, and how this toxin works to inhibit muscle contraction.<\/p>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter Overview: Muscular System<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>In this chapter, you will learn about the muscular system, which carries out both voluntary body movements and involuntary contractions of internal organs and structures. Specifically, you will learn about:<\/p>\n<ul>\n<li>The different types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2817\">muscle tissue<\/a> \u2014 skeletal, cardiac, and smooth muscle \u2014 and their different characteristics and functions.<\/li>\n<li>How muscle cells are specialized to contract and cause <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3004\">voluntary<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3005\">involuntary<\/a> movements.<\/li>\n<li>The ways in which muscle contraction is controlled.<\/li>\n<li>How <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2981\">skeletal muscles<\/a> can grow or shrink, causing changes in strength.<\/li>\n<li>The structure and organization of skeletal muscles, including the different types of muscle fibres, and how actin and myosin filaments move across each other \u2014 according to the sliding filament theory \u2014 to cause muscle contraction.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3833\">Cardiac muscle<\/a> tissue in the heart that contracts to pump blood through the body.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2982\">Smooth muscle<\/a> tissue that makes up internal organs and structures, such as the digestive system, blood vessels, and uterus.<\/li>\n<li>The physical and mental health benefits of aerobic and anaerobic exercise, such as running and weight lifting.<\/li>\n<li>How individuals vary in their response to exercise.<\/li>\n<li>Disorders of the muscular system, including musculoskeletal disorders (such as strains and carpal tunnel syndrome) and neuromuscular disorders (such as muscular dystrophy, myasthenia gravis, and Parkinson\u2019s disease).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox shaded\">\n<p>As you read the chapter, think about the following questions:<\/p>\n<ol>\n<li>How is the contraction of skeletal muscles controlled?<\/li>\n<li>Botulinum toxin works on the cellular and molecular level to inhibit muscle contraction. Based on what you learn about how muscle contraction works, can you think of some ways it could potentially be inhibited?<\/li>\n<li>What is one disorder involving a lack of sufficient muscle contraction? Why does it occur?<\/li>\n<\/ol>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 12.1.1<\/strong><\/p>\n<p><a href=\"https:\/\/www.flickr.com\/photos\/inspirekelly\/7828690186\" rel=\"cc:attributionURL\">Whiskey\u2019s 2nd Birthday<\/a> by <a class=\"owner-name truncate\" title=\"Go to Kelly Hunter's photostream\" href=\"https:\/\/www.flickr.com\/photos\/inspirekelly\/\" data-track=\"attributionNameClick\">Kelly Hunter<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/p>\n<p><strong>Figure 12.1.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Dystonia2010.JPG\" rel=\"cc:attributionURL\">1024px-Dystonia2010<\/a> by <a class=\"mw-redirect\" title=\"User:Jmh649\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Jmh649\">James Heilman, MD<\/a> on Wikimedia Commons is used under a\u00a0 <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\" rel=\"license\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Botulism [online article]. (2018, January 10). World Health Organization (WHO). https:\/\/www.who.int\/news-room\/fact-sheets\/detail\/botulism<\/p>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.) Cervical dystonia [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/cervical-dystonia\/symptoms-causes\/syc-20354123<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4055\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4055\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4154\" aria-describedby=\"caption-attachment-4154\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4154\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Natalia_Zabolotnaya_2012b-1.jpg\" alt=\"12.2 Natalia Zabolotnaya\" width=\"400\" height=\"460\"><figcaption id=\"caption-attachment-4154\" class=\"wp-caption-text\"><em>Figure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Marvelous Muscles<\/h1>\n<\/div>\n<p>Does the word\u00a0<em>muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is <a href=\"https:\/\/en.wikipedia.org\/wiki\/Natalya_Zabolotnaya\">Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n<div>\n<h1>What Is the Muscular System?<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2907\">muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2981\">skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3833\">cardiac muscle<\/a>, which makes up the walls of the heart, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2982\">smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\n<figure id=\"attachment_4155\" aria-describedby=\"caption-attachment-4155\" style=\"width: 304px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4155\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bougle_whole2_retouched-1.png\" alt=\"11.2.2 Muscular System\" width=\"304\" height=\"900\"><figcaption id=\"caption-attachment-4155\" class=\"wp-caption-text\"><em>Figure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Muscle Structure and Function<\/span><\/p>\n<\/div>\n<p>Muscles are organs composed mainly of muscle cells, which are also called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4156\">muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4157\">myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3457\">ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0<em>all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\n<p>Skeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n<p>Smooth muscles in the walls of blood vessels contract to cause <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3942\">vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2756\">vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\n<p>Cardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n<div>\n<h1>Muscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n<p>Muscles can grow larger, or\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4159\">hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\n<p>Muscles can also grow smaller, or\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4160\">atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\n<figure id=\"attachment_4161\" aria-describedby=\"caption-attachment-4161\" style=\"width: 479px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-4161\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Daniel_Tani_iss016e027910-1.jpg\" alt=\"12.1\" width=\"479\" height=\"467\"><figcaption id=\"caption-attachment-4161\" class=\"wp-caption-text\"><em>Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\n<p>Many diseases, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancer<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3523\">AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4163\">sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\n<p>In both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n<div>\n<h1>Interactions with Other Body Systems<\/h1>\n<\/div>\n<p>Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4164\">neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\n<p>Involuntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3807\">autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3569\">Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\n<p>Muscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4165\">musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4166\">tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3949\">joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">12.2 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2907\">muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2981\">skeletal muscle<\/a> (which is attached to bones and enables <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3004\">voluntary<\/a> body movements), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3833\">cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2982\">smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n<li>Muscles are organs composed mainly of muscle cells, which may also be called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4156\">muscle fibres<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4157\">myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3457\">ATP<\/a>.<\/li>\n<li>Muscles can grow larger, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4159\">hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4160\">atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n<li>Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3005\">Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3807\">autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">12.2 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is the muscular system?<\/li>\n<li>Describe muscle cells and their function.<\/li>\n<li>Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n<li>Define muscle hypertrophy and muscle atrophy.<\/li>\n<li>What are some possible causes of muscle hypertrophy?<\/li>\n<li>Give three reasons that muscle atrophy may occur.<\/li>\n<li>How do muscles change when they increase or decrease in size?<\/li>\n<li>How do changes in muscle size affect strength?<\/li>\n<li>Explain why astronauts can easily lose muscle mass in space.<\/li>\n<li>Describe how the terms\u00a0<em>muscle cells<\/em>,\u00a0<em>muscle fibres<\/em>, and\u00a0<em>myocytes<\/em>\u00a0relate to each other.<\/li>\n<li>\n<div id=\"h5p-163\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-163\" class=\"h5p-iframe\" data-content-id=\"163\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"DNA Complementary Base Pairing Drag and Drop\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\n<li>Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">12.2 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4<\/p>\n<p style=\"text-align: center\">How your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 12.2.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Natalia_Zabolotnaya_2012b.jpg\" rel=\"cc:attributionURL\">Natalia_Zabolotnaya_2012b<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/10287726@N02\" rel=\"nofollow\">Simon Q<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 12.2.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bougle_whole2_retouched.png\" rel=\"cc:attributionURL\">Bougle_whole2_retouched<\/a> by <a href=\"https:\/\/locatorplus.gov\/cgi-bin\/Pwebrecon.cgi?SC=Author&amp;SA=Bougle%CC%81%2C%20Julien%2E&amp;PID=gqesdtV7B_MJOCmbavf7c_g&amp;BROWSE=1&amp;HC=4&amp;SID=2\">Bougl\u00e9, Julien<\/a> from the <a href=\"https:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html\">National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the <a class=\"mw-redirect\" title=\"Public domain\" href=\"https:\/\/commons.wikimedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 12.2.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Daniel_Tani_iss016e027910.jpg\" rel=\"cc:attributionURL\">Daniel_Tani_iss016e027910<\/a> by <a href=\"https:\/\/spaceflight.nasa.gov\/gallery\/images\/station\/crew-16\/html\/iss016e027910.html\">NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the <a class=\"mw-redirect\" title=\"Public domain\" href=\"https:\/\/commons.wikimedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In <em>Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0<i>Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&amp;oldid=962630409<\/p>\n<\/div>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3951\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3951\"><div tabindex=\"-1\"><p>A peptide hormone, produced by alpha cells of the pancreas. It works to raise the concentration of glucose and fatty acids in the bloodstream, and is considered to be the main catabolic hormone of the body. It is also used as a medication to treat a number of health conditions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4077\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4077\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4630\" aria-describedby=\"caption-attachment-4630\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4630\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/1024px-Bacteroides_biacutis_01-1.jpg\" alt=\"15.5.1 Gut Microbes\" width=\"400\" height=\"269\"><figcaption id=\"caption-attachment-4630\" class=\"wp-caption-text\"><em>Figure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n<div>\n<h1>What Is It?<\/h1>\n<\/div>\n<p>Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n<div>\n<h1>Organs of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n<p>Most of the bacteria that normally live in the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4575\">lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4559\">small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4551\">chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n<div>\n<h1>Small Intestine<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4559\">small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m<sup>2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4576\">duodenum<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4605\">jejunum<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4606\">ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\n<figure id=\"attachment_4631\" aria-describedby=\"caption-attachment-4631\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4631\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0817_SmallIntestine_Anatomy-1.png\" alt=\"15.5.2\" width=\"600\" height=\"600\"><figcaption id=\"caption-attachment-4631\" class=\"wp-caption-text\"><em>Figure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4561\">mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4543\">villi<\/a>. In fact, each square inch (about 6.5 cm<sup>2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4583\">microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4582\">chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4593\">surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3743\">blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\n<figure id=\"attachment_4632\" aria-describedby=\"caption-attachment-4632\" style=\"width: 640px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-4632\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Human_jejunum_microvilli_1_-_TEM-1.jpg\" alt=\"15.5.3 Jejenum\" width=\"640\" height=\"480\"><figcaption id=\"caption-attachment-4632\" class=\"wp-caption-text\"><em>Figure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\n<h2>Duodenum<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4576\">duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4558\">stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4551\">chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\n<figure id=\"attachment_4635\" aria-describedby=\"caption-attachment-4635\" style=\"width: 326px\" class=\"wp-caption alignleft\"><img class=\" wp-image-4635\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-2422_Accessory_Organs-1.jpg\" alt=\"15.5.4 Pancreatic Duct and Common Bile Duct\" width=\"326\" height=\"406\"><figcaption id=\"caption-attachment-4635\" class=\"wp-caption-text\"><em>Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\n<p>The duodenum receives partially digested, semi-liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4582\">chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3197\">pancreas<\/a> through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3369\">pancreatic duct<\/a>, and it receives <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4599\">bile<\/a> from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2989\">liver<\/a> via the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4586\">gallbladder<\/a> through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3405\">common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4571\">mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\n<h3>Digestion of Carbohydrates in the Duodenum<\/h3>\n<p>Complex <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3563\">carbohydrates<\/a> (such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3367\">starch<span style=\"font-size: 1em\">es<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">) are broken down by the digestive enzyme <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4578\">amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_316\">sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\n<h3>Digestion of Proteins in the Duodenum<\/h3>\n<p>In the duodenum, the pancreatic enzymes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4597\">trypsin<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4598\">chymotrypsin<\/a> cleave <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3721\">protein<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0into peptides. Then, these smaller molecules are broken down into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3615\">amino acid<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\">. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\n<h3>Digestion of Lipids in the Duodenum<\/h3>\n<p>Pancreatic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4602\">lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4599\">bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\n<h2>Jejunum<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4605\">jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4553\">absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1655\">diffusion<\/a> (water and fatty acids), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3611\">facilitated diffusion<\/a> (the simple sugar fructose), or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3597\">active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2702\">blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4552\">lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n<ul>\n<li>Iron is absorbed in the duodenum.<\/li>\n<li>Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n<li>Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n<h2>Ileum<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4606\">ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4582\">chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n<div>\n<h1>Large Intestine<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4560\">large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4636\">feces<\/a>. Feces is stored in the large intestine until it leaves the body during <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4638\">defecation<\/a>.<\/p>\n<h2>Parts of the Large Intestine<\/h2>\n<p>Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4639\">colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4640\">cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4641\">rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4556\">anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\n<figure id=\"attachment_4642\" aria-describedby=\"caption-attachment-4642\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4642\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Blausen_0603_LargeIntestine_Anatomy-1.png\" alt=\"15.5.5 Large Intestine\" width=\"600\" height=\"600\"><figcaption id=\"caption-attachment-4642\" class=\"wp-caption-text\"><em>Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\n<p>The diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4643\">appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\n<figure id=\"attachment_4645\" aria-describedby=\"caption-attachment-4645\" style=\"width: 339px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-4645\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-Ds00070_an01934_im00887_divert_s_gif.webp-1-e1592580013661-1.png\" alt=\"15.5.6 Cecum and Appendix\" width=\"339\" height=\"369\"><figcaption id=\"caption-attachment-4645\" class=\"wp-caption-text\"><em>Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\n<h2>Functions of the Large Intestine<\/h2>\n<p>The removal of water from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4582\">chyme<\/a> to form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4636\">feces<\/a> starts in the ascending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4639\">colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\n<p>The large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n<div>\n<h1>Feature: My Human Body<\/h1>\n<\/div>\n<p>Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n<p>The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information <em>too<\/em> the brain, and only 10% of the fibres receiving information <em>from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\n<p>In a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">15.5 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4575\">lower GI tract<\/a> includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4559\">small intestine<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4560\">large intestine<\/a>. The small intestine is where most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4551\">chemical digestion<\/a> and virtually all <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4553\">absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n<li>The small intestine consists of three parts: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4576\">duodenum<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4605\">jejunum<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4606\">ileum<\/a>. All three parts of the small intestine are lined with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4561\">mucosa<\/a> that is highly enfolded and covered with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4543\">villi<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4583\">microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n<li>The duodenum secretes digestive enzymes, and also receives <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4599\">bile<\/a> from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2989\">liver<\/a> via the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4586\">gallbladder<\/a> and digestive enzymes and bicarbonate from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3197\">pancreas<\/a>. These digestive substances neutralize acidic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4582\">chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n<li>The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n<li>The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n<li>The large intestine consists of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4639\">colon<\/a> (which, in turn, includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4640\">cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4643\">appendix<\/a> is attached to the cecum of the colon.<\/li>\n<li>The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4636\">feces<\/a> from the body through the anus. The large intestine is also the site where trillions of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">15.5 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-186\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-186\" class=\"h5p-iframe\" data-content-id=\"186\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Plasma Membrane\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\n<li>What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\n<li>What is the main function of the jejunum?<\/li>\n<li>What roles does the\u00a0ileum play?<\/li>\n<li>How do beneficial bacteria in the large intestine help the human organism?<\/li>\n<li>When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\n<li>What causes intestinal gas, or flatulence?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">15.5 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/GTvnjaUU6Xk<\/p>\n<p style=\"text-align: center\">Why do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\n<p>https:\/\/youtu.be\/0IVO50DuMCs<\/p>\n<p style=\"text-align: center\">What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 15.5.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bacteroides_biacutis_01.jpg\" rel=\"cc:attributionURL\">1024px-Bacteroides_biacutis_01<\/a> by <a href=\"https:\/\/phil.cdc.gov\/Details.aspx?pid=3087\">CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain)<i>.<\/i><\/p>\n<p><strong>Figure 15.5.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0817_SmallIntestine_Anatomy.png\" rel=\"cc:attributionURL\">Blausen_0817_SmallIntestine_Anatomy<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 15.5.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Human_jejunum_microvilli_1_-_TEM.jpg\" rel=\"cc:attributionURL\">Human_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/<a href=\"https:\/\/www.dartmouth.edu\/emlab\/\"> E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain)<i>.<\/i><\/p>\n<p><strong>Figure 15.5.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:2422_Accessory_Organs.jpg\" rel=\"cc:attributionURL\">512px-2422_Accessory_Organs<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder\">OpenStax College<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 15.5.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0603_LargeIntestine_Anatomy.png\" rel=\"cc:attributionURL\">1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0<a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 15.5.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Ds00070_an01934_im00887_divert_s_gif.webp\" rel=\"cc:attributionURL\">512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by <a class=\"new\" title=\"User:Lfreeman04 (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Lfreeman04&amp;action=edit&amp;redlink=1\">Lfreeman04<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\"><span class=\"os-title-label\"><span class=\"search-highlight text\" data-timestamp=\"1597595846309\" data-highlight-id=\"ee98f411-eead-4047-a7fe-4767fec029a5\" data-highlighted=\"true\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span><span class=\"os-number\"><span class=\"search-highlight text\" data-timestamp=\"1597595846309\" data-highlight-id=\"ee98f411-eead-4047-a7fe-4767fec029a5\" data-highlighted=\"true\">23.24<\/span><\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"18088\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight text\" data-timestamp=\"1597595846309\" data-highlight-id=\"ee98f411-eead-4047-a7fe-4767fec029a5\" data-highlighted=\"true\">Accessory organs [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\n<p class=\"hanging-indent\">Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., &amp; Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. <em>Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4109\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4109\"><div tabindex=\"-1\"><div>\n<p><span style=\"text-align: initial;font-size: 1em\">Created by: Christine Miller<\/span><\/p>\n<h1 style=\"margin-top: 2.14286em;margin-bottom: 1.42857em;line-height: 1.28571em\">Definition<\/h1>\n<\/div>\n<p>In order to truly understand the concept of \u00a0Traditional Ecological Knowledge (TEK), it is important to gather as many definitions as possible- this gives us an accurate breadth of the term with all its nuances.\u00a0 Click through the images below to read several descriptions of TEK.<\/p>\n<div id=\"h5p-123\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-123\" class=\"h5p-iframe\" data-content-id=\"123\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Definitions of Traditional Ecological Knowledge.\"><\/iframe><\/div>\n<\/div>\n<h1>Value<\/h1>\n<p>People who have lived in a community for generations are often the first to notice any signs of environmental change. \u00a0Information about a particular region's climate and ecology is retained when the people from the region take on location as part of their cultural identity across generations. \u00a0This traditional knowledge is passed from generation to generation through story telling and mentorship.<\/p>\n<p>TEK shares some similarities with what is termed \"Western Science\". \u00a0Both recognize that knowledge is always growing and changing and that observations are critical to recognizing patterns and causalities in nature. \u00a0In addition, both TEK and Western Science recognize interdependence in biological systems and the need to treat ecology as\u00a0 a complex system. \u00a0TEK differs in some ways from Western Science: knowledge is passed on orally, partly through metaphor and story, and this learned knowledge is embedded into daily living. \u00a0TEK also differs from Western Science in that TEK is tied in to morality, spirituality and individual identity, making it more than just knowledge; it is sacred knowledge.<\/p>\n<h1>Examples<\/h1>\n<figure id=\"attachment_84\" aria-describedby=\"caption-attachment-84\" style=\"width: 200px\" class=\"wp-caption alignright\"><img class=\"wp-image-84 size-medium\" style=\"font-size: 1em;font-weight: bold\" title=\" By Walter Siegmund (talk) - Own work, CC BY-SA 3.0, https:\/\/commons.wikimedia.org\/w\/index.php?curid=25387182\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Avalanche-Lily-1.jpg\" alt=\" An Avalanche Lily in Bloom. The plant has two wide oval shaped leaves growing from the base of the plant. A single slender stem suspends a yellow flower with six yellow petals. The flower is tilted towards the ground and the anthers and stamen hang below the petals.\" width=\"200\" height=\"300\"><figcaption id=\"caption-attachment-84\" class=\"wp-caption-text\"><em>Figure 1.6.1 Avalanche Lily in bloom.<\/em><\/figcaption><\/figure>\n<p>People who are indigenous to the province of British Columbia have been managing natural resources in this area for time immemorial. \u00a0Numerous examples of sustainable harvesting methods can be found across the province, but our example, harvesting and management of the Avalanche Lily, comes from the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Secwepemc\">Secwepemc<\/a> peoples of the interior of British Columbia. \u00a0Many thanks to Nancy Turner, Marianne Boelscher Ignace and Ronald Ignace for their documentation of these practices in their paper: <a href=\"https:\/\/www.researchgate.net\/publication\/248146362_Traditional_Ecological_Knowledge_and_Wisdom_of_Aboriginal_Peoples_in_British_Columbia\" target=\"_blank\" rel=\"noopener noreferrer\">Traditional Ecological Knowledge and Wisdom of Aboriginal Peoples in British Columbia<\/a><\/p>\n<p>The Avalanche Lily is a yellow-flowered member of the lily family native to Western North America. \u00a0This flower grows from an edible \u00a0bulb which ranges in size from 3-5 centimetres. \u00a0The Secwepemc people have harvested these bulbs as an important food source for generations. \u00a0Oral transmission of knowledge allowed the Secwepemc people to use thousands of years of accumulated data around growing cycles, seasons and management practices to harvest these plants effectively while maintaining a healthy population of lilies for future use. \u00a0Very intentional conservation strategies were\/are practiced when harvesting the bulbs:<\/p>\n<ul>\n<li>Bulbs were harvested according to elevation in order to collect bulbs in the right stage of maturity and to spread out harvesting and yield over several months. \u00a0Bulbs picked early in the season were too soft, and bulbs picked late were too watery.<\/li>\n<li>Only the largest bulbs were selected by choosing stems with multiple fruiting bodies.<\/li>\n<li>As bulbs were harvested, smaller bulbs were replanted, seeds were scattered and the soil was tilled by the harvesters.<\/li>\n<li>Areas which had been extensively harvested were often left for several years to allow the lily population time to recover.<\/li>\n<\/ul>\n<p>The two videos below show how knowledge of a particular ecosystem is handed down through the traditions of mentorship and storytelling:<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: center\"><em>Modern science, native knowledge, by The Nature Conservancy, 2015<\/em><\/p>\n<p style=\"text-align: center\"><a href=\"https:\/\/www.nfb.ca\/film\/first_stories_nganawendaanan_ndeing\/\" target=\"_blank\" rel=\"noopener noreferrer\"><img class=\"wp-image-216 aligncenter\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/First-Stories-Sustainable-Harvesting-1-1.png\" alt=\"\" width=\"508\" height=\"237\"><\/a><br \/>\n<em>First Stories - Nganawendaanan Nde'ing (I Keep Them in My Heart), by Shannon Letande, 2006<\/em><\/p>\n<h1>TEK is Part of Place<\/h1>\n<p>The Traditional Ecological Knowledge held by Indigenous communities often includes very location specific knowledge.\u00a0 There are many diverse groups of First Peoples in British Columbia, each with expert knowledge about the ecology of their specific ancestral regions.\u00a0 The link to the map from the<a href=\"https:\/\/native-land.ca\/\"> Native Land Digital<\/a> website shows some of the traditional boundaries of the Indigenous people in British Columbia. Click on the areas to see where First Nations communities are located.<\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.6 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Traditional Ecological Knowledge (TEK) is an important and valuable body of knowledge<\/li>\n<li>People groups who have lived in an area over generations pass down TEK through storytelling and mentorship<\/li>\n<li>TEK and Western Science share certain characteristics, including use of observations, and identification of patterns and causalities in nature<\/li>\n<li>TEK and Western Science differ in that TEK is passed down through oral storytelling and is deeply rooted in morality, spirituality and individual identity<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.6 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>Type your exercises here.<\/p>\n<ol>\n<li>Define Traditional Ecological Knowledge.<\/li>\n<li>How is TEK passed down through generations?<\/li>\n<li>How does TEK differ from Western Science?<\/li>\n<li>What are some ways in which TEK can inform resource management?<\/li>\n<li>What are some of the ramifications of loss of TEK?\u00a0 How can TEK be maintained?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">1.6 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?time_continue=2&amp;amp;v=pHNlel72eQc&amp;amp;feature=emb_logo<\/p>\n<p style=\"text-align: center\">TEDxTC - Winona LaDuke - Seeds of Our Ancestors, Seeds of Life, by TEDx-TC, 2012<\/p>\n<\/div>\n<\/div>\n<h2 id=\"x-ck12-YzhkZDM1NjIyZWY2OTkxNDVlODQ3NzVmZGJkNzkyYmY.-wlj\"><strong>Attributions<\/strong><\/h2>\n<p><strong>Definitions of Traditional Ecological Knowledge<\/strong><\/p>\n<ul>\n<li><span class=\"_20NLS _1ByhS\"><span class=\"_20NLS _1ByhS\"><a href=\"https:\/\/unsplash.com\/photos\/oz07J1XUIVs\">Kamloops, BC, Canada<\/a> by <span class=\"_1JARO\"><a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@louispaulin\">Louis Paulin<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/span><\/span>\n<div class=\"_2BNtU\">\n<div class=\"_3wz4V _3g2uF _1c9QY _29Ao0\">\n<div class=\"_2LKaU\">\n<div class=\"_1fC6_\">\n<div class=\"_3x_oL _30-Kv\">\n<div class=\"_2Em44 _2KLco\">\n<div class=\"_3_xrB\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/SdzuR0GKizM\">The Totem Poles of Capilano, Vancouver<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@nookscribe\">Udayaditya Barua<\/a> <span class=\"_20NLS _1ByhS\"><span class=\"_1JARO\">on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/span><\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/oK5Sld0OLJs\">Alouette Lake<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@chloeevans\">Chloe Evans<\/a> <span class=\"_20NLS _1ByhS\"><span class=\"_1JARO\">on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/span><\/li>\n<\/ul>\n<p><strong>Figure 1.6.1<\/strong><\/p>\n<p id=\"firstHeading\" class=\"firstHeading\" lang=\"en\"><a href=\"https:\/\/commons.wikimedia.org\/w\/index.php?curid=25387182\">Erythronium grandiflorum 5077<\/a>, by <a href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Wsiegmund\">Walter Siegmund<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Herkes, J. (n.d.). Continuing studies: Traditional ecological knowledge. University of Northern British Columbia. Retrieved from https:\/\/www.unbc.ca\/continuing-studies\/courses\/traditional-ecological-knowledge<\/p>\n<p class=\"hanging-indent\"><span class=\"author\">Inglis, J. T.<\/span> (<span class=\"pubYear\">1993)<\/span>. <span class=\"articleTitle\"><em>Traditional ecological knowledge: Concepts and cases<\/em> (p. vi)<\/span><em>.<\/em> Canadian Museum of Nature, Ottawa, Ontario, Canada.<\/p>\n<p class=\"hanging-indent\">Letande, S. (2006). First Stories - Nganawendaanan Nde'ing (I keep them in my heart). https:\/\/www.nfb.ca\/film\/first_stories_nganawendaanan_ndeing\/<\/p>\n<p class=\"hanging-indent\">Minerals Management Service (n.d.). What is Traditional Knowledge [online]. Government of Alaska.https:\/\/web.archive.org\/web\/20030328053734\/http:\/\/www.mms.gov\/alaska\/native\/tradknow\/tk_mms2.htm<\/p>\n<p class=\"hanging-indent\">TEDx-TC. (2012, March 4). TEDxTC - Winona LaDuke - Seeds of our ancestors, seeds of life. https:\/\/www.youtube.com\/watch?v=pHNlel72eQc&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">The Nature Conservancy. (2015, February 25). Modern science, native knowledge. YouTube. https:\/\/www.youtube.com\/watch?v=1QRpnHoGivk<\/p>\n<p class=\"hanging-indent\">Turner, N., Ignace, M., &amp; Ignace, R. (2000, October 1). Traditional ecological knowledge and wisdom of Aboriginal peoples in British Columbia. <i>Ecological Applications,<\/i> <i>10<\/i>(5), 1275-1287. doi:10.2307\/2641283<\/p>\n<p class=\"hanging-indent\">Wakefield, A.J. (1999, September 11). MMR vaccination and autism. <em>Lancet<\/em>, 354(9182), 949-950.\u00a0 https:\/\/www.thelancet.com\/journals\/lancet\/article\/PIIS0140-6736(05)75696-8\/fulltext <span class=\"article-header__doi bulleted\"><span class=\"article-header__doi__label\">doi:<\/span>https:\/\/doi.org\/10.1016\/S0140-6736(05)75696-8<\/span><\/p>\n<p><cite id=\"cit384\">\u00a0<\/cite><\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_3949\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_3949\"><div tabindex=\"-1\"><p>A nervous system cell that provides support for neurons and helps them transmit nerve impulses.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4094\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4094\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_5024\" aria-describedby=\"caption-attachment-5024\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5024\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Cycling-to-Beat-Blood-Cancer-1.jpg\" alt=\"17.7.1 Cycling to Beat Blood Cancer\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-5024\" class=\"wp-caption-text\"><em>Figure 17.7.1 Be Unstoppable for Leukemia and Lymphoma Research.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Case Study Conclusion: Defending Your Defenses<\/h1>\n<\/div>\n<p>These people are participating in a bike ride to raise funds for leukemia and lymphoma research (Figure 17.7.1). Leukemia and lymphoma are blood cancers. In 2020,\u00a0 approximately 6,900 Canadians will be diagnosed with leukemia and 3,000 will die from this cancer. Lymphoma is the most common type of blood cancer. As a lymphoma patient, Hakeem, who you learned about in the beginning of this chapter, may eventually benefit from research funded by a bike ride like this one.<\/p>\n<p>What type of blood cell is affected in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5026\">lymphoma<\/a>? As the name implies, lymphoma is a cancer that affects lymphocytes, which are a type of leukocyte. As you have learned in this chapter, there are different types of lymphocytes, including the B and T cells of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4822\">adaptive immune system<\/a>. Different types of lymphoma affect different types of lymphocytes in different ways. It is important to correctly identify the type of lymphoma, so that patients can be treated appropriately.<\/p>\n<p>You may recall that one of Hakeem\u2019s symptoms was a swollen <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4831\">lymph node<\/a>, and he was diagnosed with lymphoma after a biopsy of that lymph node. Swollen lymph nodes are a common symptom of lymphoma. As you have learned, lymph nodes are distributed throughout the body along lymphatic vessels, as part of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2911\">lymphatic system<\/a>. The lymph nodes filter <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4552\">lymph<\/a> and store lymphocytes. Therefore, they play an important role in fighting infections. Because of this, they will often swell in response to an infection. In Hakeem\u2019s case, the swelling and other symptoms did not improve after several weeks and a course of antibiotics, which caused Dr. Hayes to suspect lymphoma instead. The biopsy showed that Hakeem did indeed have cancerous lymphocytes in his lymph nodes.<\/p>\n<p>But which type of lymphocytes were affected? Lymphoma most commonly affects B or T lymphocytes. The two major types of lymphoma are called Hodgkin (HL) or non-Hodgkin lymphoma (NHL). NHL is more common than HL. In 2020, the Canadian Cancer Society estimates 10,400 Canadians will be diagnosed with non-Hodgkin lymphoma, whereas 1,000 will be diagnosed with Hodgkin lymphoma. While HL is one distinct type of lymphoma, NHL has about 60 different subtypes, depending on which specific cells are affected and how.<\/p>\n<p>Hakeem was diagnosed with a type of NHL called diffuse large B-cell lymphoma (DLBCL) \u2014 the most common type of NHL. This type of lymphoma affects <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3447\">B cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\"> and causes them to appear large under the microscope. In addition to Hakeem\u2019s symptoms of fatigue, swollen lymph nodes, loss of appetite, and weight loss, common symptoms of this type of lymphoma include fever and night sweats. It is an aggressive and fast-growing type of lymphoma that is fatal if not treated. The good news is that with early detection and proper treatment, about 70% of patients with DLBCL can be cured.<\/span><\/p>\n<figure id=\"attachment_5028\" aria-describedby=\"caption-attachment-5028\" style=\"width: 299px\" class=\"wp-caption alignright\"><img class=\" wp-image-5028\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/antigen-stain-1.jpg\" alt=\"17.7.2 Antigen stain\" width=\"299\" height=\"267\"><figcaption id=\"caption-attachment-5028\" class=\"wp-caption-text\">Figure 17.7.2 A lab technician can apply stains that target specific antigens to help identify which type of lymphoma is present.<\/figcaption><\/figure>\n<p>How do physicians determine the specific type of lymphoma? Tissue obtained from a biopsy can be examined under a microscope to observe physical changes (such as abnormal cell size or shape) that are characteristic of a particular subtype of lymphoma. Additionally, tests can be performed on the tissue to determine which cell-surface antigens are present. Recall that antigens are molecules that bind to specific antibodies. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3567\">Antibodies<\/a> can be produced in the laboratory and labeled with compounds that can be identified by their colour under a microscope. When these antibodies are applied to a tissue sample, this colour will appear wherever the antigen is present, because it binds to the antibody. This technique was used in the photomicrograph in Figure 17.7.2 to identify the presence of a cell-surface antigen (shown as reddish-brown) in a sample of skin cells. This technique, called immunohistochemistry, is also commonly used to identify antigens in tissue samples from lymphoma patients.<\/p>\n<p>Why would identifying cell-surface antigens be important in diagnosing and treating lymphoma? As you have learned, the immune system uses antigens present on the surface of cells or pathogens to distinguish between self and non-self, and to launch adaptive immune responses. Cells that become cancerous often change their cell-surface antigens. This is one way that the immune system can identify and destroy them. Also, different cell types in the body can sometimes be identified by the presence of specific cell-surface antigens. Knowing the types of cell-surface antigens present in a tissue sample can help physicians identify which cells are cancerous, and possibly the specific subtype of cancer. Knowing this information can be helpful in choosing more tailored and effective treatments.<\/p>\n<p>One treatment for NHL is, in fact, the use of medications made from antibodies that bind to cell-surface antigens present on cells affected by the specific subtype of NHL. This is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5004\">immunotherapy<\/a>. These drugs can directly bind to and kill the cancerous cells. For patients with DLBCL like Hakeem, immunotherapy is often used in conjunction with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5029\">chemotherapy<\/a> and radiation as a course of treatment. Although Hakeem has a difficult road ahead, he and his medical team are optimistic that \u2014 given the high success rate when DLBCL is caught and treated early \u2014 he may be cured. More research into how the immune system functions may lead to even better treatments for lymphoma \u2014 and other types of cancers \u2014 in the future.<\/p>\n<div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 17 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>In this chapter, you learned about the immune system. Specifically, you learned that:<\/p>\n<ul>\n<li>Any agent that can cause disease is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3707\">pathogen<\/a>. Most human pathogens are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3683\">microorganisms<\/a>, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4283\">viruses<\/a>. The immune system is the\u00a0body\u00a0system that defends the human host from pathogens and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3513\">cancerous<\/a>\u00a0cells.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4820\">innate immune system<\/a> is a subset of the immune system that provides very quick, but non-specific responses to pathogens. It includes multiple types of barriers to pathogens, leukocytes that phagocytize pathogens, and several other general responses.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4822\">adaptive immune system<\/a> is a subset of the immune system that provides specific responses tailored to particular pathogens. It takes longer to put into effect, but it may lead to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4884\">immunity<\/a> to the pathogens.<\/li>\n<li>Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Most body cells have <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4825\">major histocompatibility complex (MHC)<\/a> proteins that identify them as self. Pathogens, infected cells, and tumor cells have non-self proteins called antigens that the immune system recognizes as foreign.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2704\">Antigens<\/a>\u00a0are proteins that bind to specific receptors on immune system cells and elicit an adaptive immune response. Some immune cells (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3447\">B cells<\/a>) respond to foreign antigens by producing antibodies that bind with antigens and target pathogens for destruction.<\/li>\n<li>An important role of the immune system is tumor surveillance. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4893\">Killer T cells<\/a>\u00a0of the adaptive immune system find and destroy tumor cells, which they can identify from their abnormal antigens.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4833\">neuroimmune system<\/a> that protects the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3841\">central nervous system<\/a> is thought to be distinct from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4834\">peripheral immune system<\/a> that protects the rest of the human body. The blood-brain and blood-spinal cord barriers are one type of protection of the neuroimmune system. Neuroglia also play a role in this system, for example, by carrying out <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1680\">phagocytosis<\/a>.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2911\">lymphatic system<\/a> is a human organ system that is a vital part of the adaptive immune system. It consists of several organs and a system of vessels that transport or filter the fluid called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4552\">lymph<\/a>. The main immune function of the lymphatic system is to produce, mature, harbor, and circulate white blood cells called lymphocytes, which are the main cells in the adaptive immune system, and are circulated in lymph.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>The return of lymph to the bloodstream is one of the functions of the lymphatic system. Lymph flows from tissue spaces, where it leaks out of blood vessels, to major veins in the upper chest. It is then returned to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3835\">cardiovascular system<\/a>. Lymph is similar in composition to blood <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4395\">plasma<\/a>. Its main cellular components are lymphocytes.<\/li>\n<li>Lymphatic vessels called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4607\">lacteals<\/a>\u00a0are found in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4543\">villi<\/a> that line the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4559\">small intestine<\/a>. Lacteals absorb fatty acids from the digestion of lipids in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3877\">digestive system<\/a>. The fatty acids are then transported through the network of lymphatic vessels to the bloodstream.<\/li>\n<li>Lymphocytes, which include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3447\">B cells<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4892\">T cells<\/a>, are the subset of leukocytes involved in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4822\">adaptive immune responses<\/a>. They may create a lasting memory of and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4884\">immunity<\/a> to specific pathogens.<\/li>\n<li>All lymphocytes are produced in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4021\">bone marrow<\/a> and then go through a process of maturation, in which they \u201clearn\u201d to distinguish self from non-self. B cells mature in the bone marrow, and T cells mature in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4829\">thymus<\/a>. Both the bone marrow and thymus are considered <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4845\">primary lymphatic organs<\/a>.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4847\">Secondary lymphatic organs<\/a> include the tonsils, spleen, and lymph nodes. There are four pairs of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4830\">tonsils<\/a>\u00a0that encircle the throat. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4497\">spleen<\/a> filters blood, as well as lymph. There are hundreds of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4831\">lymph nodes<\/a>\u00a0located in clusters along the lymphatic vessels. All of these secondary organs filter lymph and store lymphocytes, so they are sites where pathogens encounter and activate lymphocytes and initiate adaptive immune responses.<\/li>\n<\/ul>\n<\/li>\n<li>Unlike the adaptive immune system, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4820\">innate immune system<\/a> does not confer immunity. The innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4857\">Mechanical barriers<\/a>\u00a0\u2014\u00a0which include the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3521\">skin<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3538\">mucous membranes<\/a>, and fluids (such as tears and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4717\">urine<\/a>) \u2014 physically block pathogens from entering the body.<\/li>\n<li>Chemical barriers\u00a0\u2014\u00a0such as enzymes in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4711\">sweat<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4595\">saliva<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4885\">semen<\/a>\u00a0\u2014\u00a0kill pathogens on body surfaces.<\/li>\n<li>Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\n<\/ul>\n<\/li>\n<li>If pathogens breach the protective barriers, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4860\">inflammation<\/a> occurs. This creates a physical barrier against the spread of infection and repairs tissue damage. Inflammation is triggered by chemicals (such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4865\">cytokines<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4879\">histamines<\/a>), and it causes swelling, redness, and warmth.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4868\">complement system<\/a> is a complex biochemical mechanism that helps <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3567\">antibodies<\/a> kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3529\">cell membrane<\/a> of pathogens and bursting of the cells.<\/li>\n<li>Cellular responses of the innate immune system involve various types of leukocytes (white blood cells). For example, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3453\">neutrophils<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4340\">macrophages<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4875\">dendritic cells<\/a>\u00a0phagocytize pathogens. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3481\">Basophils<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4881\">mast cells<\/a>\u00a0release chemicals that trigger inflammation. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3423\">Natural killer cells<\/a>\u00a0destroy cancerous or virus-infected cells, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3437\">eosinophils<\/a>\u00a0kill parasites.<\/li>\n<li>Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\n<\/ul>\n<\/li>\n<li>The main cells of the adaptive immune system are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4832\">lymphocytes<\/a>. There are two major types of lymphocytes: T cells and B cells. Both types must be activated by foreign antigens to become functioning immune cells.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Most activated T cells become either <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4893\">killer T cells<\/a>\u00a0or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3445\">helper T cells<\/a>. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing cytokines that control other types of leukocytes.<\/li>\n<li>Activated B cells form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4902\">plasma cell<\/a>s that secrete antibodies, which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the complement system.<\/li>\n<\/ul>\n<\/li>\n<li>After an adaptive immune response occurs, long-lasting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4904\">memory B cells and memory T cells<\/a> may remain to confer <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4884\">immunity<\/a> to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\n<li>Immunity may be active or passive.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4906\">Active immunity<\/a> occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4910\">Passive immunity<\/a> occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\n<\/ul>\n<\/li>\n<li>Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4501\">HIV<\/a>) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4877\">allergy<\/a> is a disorder in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes allergies is called an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4999\">allergen<\/a>. Common allergens include pollen, dust mites, mold, specific foods (such as peanuts), insect stings, and certain medications (such as aspirin).<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>The prevalence of allergies has been increasing for decades, especially in developed countries, where they are much more common than in developing countries. The hygiene hypothesis posits that this has occurred because humans evolved to cope with more pathogens than we now typically face in our relatively sterile environments in developed countries. As a result, the immune system \u201ckeeps busy\u201d by attacking harmless antigens.<\/li>\n<li>Allergies occur when B cells are first activated to produce large amounts of antibodies to an otherwise harmless allergen, and the antibodies attach to mast cells. On subsequent exposures to the allergen, the mast cells immediately release cytokines and histamines that cause inflammation.<\/li>\n<li>Mild allergy symptoms are frequently treated with antihistamines that counter histamines and reduce allergy symptoms. A severe systemic allergic reaction, called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5002\">anaphylaxis<\/a>, is a medical emergency that is usually treated with injections of epinephrine. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5004\">Immunotherapy<\/a> for allergies involves injecting increasing amounts of allergens to desensitize the immune system to them.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5006\">Autoimmune diseases<\/a>\u00a0occur when the immune system fails to recognize the body\u2019s own molecules as self and attacks them, causing damage to tissues and organs. A family history of autoimmunity and female gender are risk factors for autoimmune diseases.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>In some autoimmune diseases, such as type I diabetes, the immune system attacks and damages specific body cells. In other autoimmune diseases, such as systemic lupus erythematosus, many different tissues and organs may be attacked and injured. Autoimmune diseases generally cannot be cured, but their symptoms can often be managed with drugs or other treatments.<\/li>\n<\/ul>\n<\/li>\n<li>Immunodeficiency occurs when the immune system is not working properly, generally because one or more of its components are inactive. As a result, the immune system is unable to fight off pathogens or cancers that a normal immune system would be able to resist.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5012\">Primary immunodeficiency<\/a> is present at birth and caused by rare genetic diseases. An example is severe combined immunodeficiency. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5013\">Secondary immunodeficiency<\/a> occurs because of some event or exposure experienced after birth. Possible causes include substance abuse, obesity, and malnutrition, among others.<\/li>\n<li>The most common cause of immunodeficiency in the world today is human immunodeficiency virus (HIV), which infects and destroys helper T cells. HIV is transmitted through mucous membranes or body fluids. The virus may eventually lead to such low levels of helper T cells that opportunistic infections occur. When this happens, the patient is diagnosed with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5016\">acquired immunodeficiency syndrome<\/a> (AIDS). Medications can control the multiplication of HIV in the human body, but\u00a0it can't\u00a0eliminate the virus completely.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p>Up to this point, this book has covered body systems that carry out processes within individuals, such as the digestive, muscular, and immune systems. Read the next chapter to learn about the body system that allows humans to produce new individuals \u2014 the reproductive system.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 17 Review<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-205\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-205\" class=\"h5p-iframe\" data-content-id=\"205\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"11.6 Quiz\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Compare and contrast a pathogen and an allergen.<\/li>\n<li>Describe three ways in which pathogens can enter the body.<\/li>\n<li>The complement system involves the activation of several proteins to kill pathogens. Why do you think this is considered part of the innate immune system, instead of the adaptive immune system?<\/li>\n<li>Why are innate immune responses generally faster than adaptive immune responses?<\/li>\n<li>Explain how an autoimmune disease could be triggered by a pathogen.<\/li>\n<li>What is an opportunistic infection?\u00a0Name two diseases or conditions that could result in opportunistic infections. Explain your answer.<\/li>\n<li>Which cell type in the immune system can be considered an \u201cantibody factory?\"<\/li>\n<li>Besides foreign pathogens, what is one thing that the immune system protects the body against?<\/li>\n<li>What cell type in the immune system is infected and killed by HIV?<\/li>\n<li>Name two types of cells that produce cytokines in the immune system.\u00a0What are two functions of cytokines in the immune system?<\/li>\n<li>Many pathogens evade the immune system by altering their outer surface in some way. Based on what you know about the functioning of the immune system, why is this often a successful approach?<\/li>\n<li>What is \u201cmissing self?\" How does this condition arise?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">17.7 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/Z3B-AaqjyjE<\/p>\n<p style=\"text-align: center\">What is leukemia? - Danilo Allegra and Dania Puggioni, TED-Ed, 2015.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 17.7.1<\/strong><\/p>\n<p><a href=\"https:\/\/flic.kr\/p\/eQCQyD\" rel=\"cc:attributionURL\">Cycling to Beat Blood Cancer<\/a> by <a href=\"https:\/\/www.flickr.com\/photos\/leukaemia_research\/\">Blood Cancer UK<\/a> (Formerly Bloodwise) on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/\">CC BY-NC-ND 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-nd\/2.0\/) license.<\/p>\n<p><strong>Figure 17.7.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Langerhans_Cells_in_Normal_Epidermis,_CD1a_Immunostain_(4435883030).jpg\" rel=\"cc:attributionURL\">antigen stain<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/78147607@N00\" rel=\"nofollow\">Ed Uthman<\/a> from Houston, TX, USA on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Hodgkin lymphoma statistics [online article]. (2020). Canadian Cancer Society. https:\/\/www.cancer.ca:443\/en\/cancer-information\/cancer-type\/hodgkin-lymphoma\/statistics\/?region=on<\/p>\n<p class=\"hanging-indent\">Non-Hodgkin lymphoma statistics [online article]. (2020). Canadian Cancer Society. https:\/\/www.cancer.ca:443\/en\/cancer-information\/cancer-type\/non-hodgkin-lymphoma\/statistics\/?region=on<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, April 30). What is leukemia? - Danilo Allegra and Dania Puggioni. YouTube. https:\/\/www.youtube.com\/watch?v=Z3B-AaqjyjE&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4096\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4096\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_5194\" aria-describedby=\"caption-attachment-5194\" style=\"width: 334px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5194\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Sperm-20051108-1.jpg\" alt=\"18.4.1 Sperm\" width=\"334\" height=\"330\"><figcaption id=\"caption-attachment-5194\" class=\"wp-caption-text\"><em>Figure 18.4.1 Sperm-ho!<\/em><\/figcaption><\/figure>\n<div>\n<h1>Colourful Sperm<\/h1>\n<\/div>\n<p>This false-colour image (Figure 18.4.1) shows real human sperm. The tiny gametes are obviously greatly magnified in the picture, because they are actually the smallest of all human cells. In fact, human sperm cells are small, even when compared with sperm cells of other animals. Mice sperm are about <em>twice<\/em> the length of human sperm! Human\u00a0sperm may be small in size, but in a normal, healthy man, huge numbers of them are usually released during each ejaculation. There may be hundreds of millions of sperm cells in a single teaspoon of semen. Producing sperm is one of the major\u00a0functions of the male reproductive system.<\/p>\n<div>\n<figure id=\"attachment_5195\" aria-describedby=\"caption-attachment-5195\" style=\"width: 356px\" class=\"wp-caption alignright\"><img class=\" wp-image-5195\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Sperm-Anatomy-1.png\" alt=\"18.4.2 Sperm Anatomy\" width=\"356\" height=\"516\"><figcaption id=\"caption-attachment-5195\" class=\"wp-caption-text\"><em>Figure 18.4.2 Each normal mature sperm cell has the structures labeled in this image.<\/em><\/figcaption><\/figure>\n<h1>Sperm Anatomy<\/h1>\n<\/div>\n<p>A mature sperm cell has several structures that help it reach and penetrate an egg. These are labeled in the drawing of a sperm shown in Figure 18.4.2.<\/p>\n<ul>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5196\">head<\/a><\/strong>\u00a0is the part of the sperm that contains the\u00a0nucleus\u00a0\u2014 and not much else. The nucleus, in turn, contains tightly coiled\u00a0DNA\u00a0that is the male parent\u2019s contribution to the genetic makeup of a zygote (if one forms). Each sperm is a haploid cell, containing half the chromosomal complement of a normal, diploid body cell.<\/li>\n<li>The front of the head is an area called the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5197\">acrosome<\/a><\/strong>. The acrosome contains enzymes that help the sperm penetrate an ovum (if it reaches one).<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5198\">midpiece<\/a><\/strong>\u00a0is the part of the sperm between the head and the flagellum. The midpiece is packed with\u00a0mitochondria\u00a0that produce the\u00a0energy\u00a0needed to move the flagellum.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3515\">flagellum<\/a><\/strong> (also called the tail) can rotate like a propeller, allowing the sperm to \u201cswim\u201d through the female reproductive tract to reach an ovum if one is present.<\/li>\n<\/ul>\n<div>\n<h1>Spermatogenesis<\/h1>\n<\/div>\n<p>The process of producing sperm is known as\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">spermatogenesis<\/a>. <\/strong>Spermatogenesis normally starts when a male reaches <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5155\">puberty<\/a>, and it usually continues uninterrupted until death, although a decrease in sperm production generally occurs at older ages. A young, healthy male may produce hundreds of millions of sperm a day! Only about half of these, however, are likely to become viable, mature sperm.<\/p>\n<h2>Where Sperm Are Produced<\/h2>\n<p>Spermatogenesis occurs in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5160\">seminiferous tubules<\/a> in the testes. Spermatogenesis requires high concentrations of testosterone. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">Testosterone<\/a> is secreted by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5171\">Leydig\u00a0cells<\/a>, which are adjacent to the seminiferous tubules in the testes.<\/p>\n<p>Sperm production in the seminiferous tubules is very sensitive to temperature. This may be the most important reason the testes are located outside the body in the scrotum. The temperature inside the scrotum is generally about 2 degrees Celsius cooler than core body temperature. This lower temperature is optimal for spermatogenesis. The scrotum regulates its internal temperature as needed by contractions of the smooth muscles lining the scrotum. When the temperature inside the scrotum becomes too low, the scrotal muscles contract. The contraction of the muscles pulls the scrotum higher against the body, where the temperature is warmer. The opposite occurs when the temperature inside the scrotum becomes too high.<\/p>\n<h2>Events of Spermatogenesis<\/h2>\n<figure id=\"attachment_5200\" aria-describedby=\"caption-attachment-5200\" style=\"width: 256px\" class=\"wp-caption alignright\"><img class=\"size-full wp-image-5200\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Spermatogenesis-1.jpg\" alt=\"18.4.3 Spermatogenesis\" width=\"256\" height=\"594\"><figcaption id=\"caption-attachment-5200\" class=\"wp-caption-text\"><em>Figure 18.4.3 Spermatogenesis includes one mitotic division and two meiotic divisions.<\/em><\/figcaption><\/figure>\n<p>Figure 18.4.3 summarizes the main cellular events that occur in the process of spermatogenesis. The process begins with a diploid stem cell called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5199\">spermatogonium<\/a><\/strong>\u00a0(plural, spermatogonia), and involves several cell divisions. The entire process takes at least ten weeks to complete, including maturation in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5090\">epididymis<\/a>.<\/p>\n<ol>\n<li>A spermatogonium undergoes\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1987\">mitosis<\/a>\u00a0to produce two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2103\">diploid<\/a> cells called primary spermatocytes. One of the primary spermatocytes goes on to produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a>. The other replenishes the reserve of spermatogonia.<\/li>\n<li>The primary spermatocyte undergoes\u00a0meiosis\u00a0I to produce two haploid daughter cells called secondary spermatocytes.<\/li>\n<li>The secondary spermatocytes rapidly undergo\u00a0meiosis\u00a0II to produce a total of four <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2484\">haploid<\/a> daughter cells called spermatids.<\/li>\n<li>The spermatids begin to form a tail, and their\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_277\">DNA<\/a>\u00a0becomes highly condensed. Unnecessary cytoplasm and\u00a0organelles\u00a0are removed from the cells, and they form a head, midpiece, and flagellum. The resulting cells are sperm (spermatozoa).<\/li>\n<\/ol>\n<div><\/div>\n<p>As shown in Figure 18.4.4, the events of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">spermatogenesis<\/a> begin near the wall of the seminiferous tubule \u2014 where spermatogonia are located \u2014 and continue inward toward the lumen of the tubule. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5169\">Sertoli cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0extend from the wall of the seminiferous tubule inward toward the lumen, so they are in contact with developing sperm at all stages of spermatogenesis. Sertoli cells play several roles in spermatogenesis:<\/span><\/p>\n<ul>\n<li>They secrete\u00a0endocrine <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3569\">hormones<\/a>\u00a0that help regulate spermatogenesis.<\/li>\n<li>They secrete substances that initiate\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2486\">meiosis<\/a>.<\/li>\n<li>They concentrate testosterone (from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5171\">Leydig cells<\/a>), which is needed at high levels to maintain spermatogenesis.<\/li>\n<li>They <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1680\">phagocytize<\/a> the extra cytoplasm that is shed from developing sperm cells.<\/li>\n<li>They secrete testicular fluid that helps carry sperm into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5090\">epididymis<\/a>.<\/li>\n<li>They maintain a blood-testis barrier, so immune system cells cannot reach and attack the sperm.<\/li>\n<\/ul>\n<figure id=\"attachment_5201\" aria-describedby=\"caption-attachment-5201\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5201\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Testis-cross-section-2-1.jpg\" alt=\"18.4.4 Testis Structure\" width=\"900\" height=\"511\"><figcaption id=\"caption-attachment-5201\" class=\"wp-caption-text\"><em>Figure 18.4.4 Cross-section of a testis and seminiferous tubules.<\/em><\/figcaption><\/figure>\n<h2>Maturation in the Epididymis<\/h2>\n<p>Although the sperm produced in the testes have tails, they are not yet motile (able to \u201cswim\u201d). The non-motile sperm are transported to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5090\">epididymis<\/a> in testicular fluid that is secreted by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5169\">Sertoli cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0with the help of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristaltic contractions<\/a>. In the epididymis, the sperm gain motility, so they are capable of swimming up the female genital tract and reaching an ovum. The mature sperm are stored in the epididymis until ejaculation occurs.<\/span><\/p>\n<div>\n<h1>Ejaculation<\/h1>\n<\/div>\n<p>Sperm are released from the body during\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5150\">ejaculation<\/a>,<\/strong>\u00a0which typically occurs during orgasm. Hundreds of millions of mature sperm \u2014 contained within a small amount of thick, whitish fluid called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4885\">semen<\/a>\u00a0\u2014\u00a0are propelled from the penis during a normal ejaculation.<\/p>\n<h2>How Ejaculation Occurs<\/h2>\n<p>Ejaculation occurs when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristalsis<\/a> of the muscle layers of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5095\"><em>vas deferens<\/em><\/a> and other accessory structures propel sperm from the epididymes, where mature sperm are stored. The muscle contractions force the sperm through the <em>vas deferens<\/em> and the ejaculatory ducts, and then out of the penis through the urethra. Due to the peristaltic action of the\u00a0muscles, the ejaculation occurs in a series of spurts.<\/p>\n<h2>The Role of Semen<\/h2>\n<p>As sperm travel through the ejaculatory ducts during ejaculation, they mix with secretions from the seminal vesicles, prostate gland, and bulbourethral glands to form semen (see Figure 18.4.5 ). The average amount of semen per ejaculate is about 3.7 mL, which is a little less than a teaspoonful. Most of this volume of semen consists of glandular secretions, with the hundreds of millions of sperm cells actually contributing relatively little to the total volume.<\/p>\n<figure id=\"attachment_5202\" aria-describedby=\"caption-attachment-5202\" style=\"width: 389px\" class=\"wp-caption alignleft\"><img class=\" wp-image-5202\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Human_semen_in_a_petri_dish-1.jpg\" alt=\"18.4.5\" width=\"389\" height=\"292\"><figcaption id=\"caption-attachment-5202\" class=\"wp-caption-text\"><em>Figure 18.4.5 This petri dish shows normal human semen in a typical ejaculate.<\/em><\/figcaption><\/figure>\n<p>The secretions in semen are important for the survival and motility of sperm. They provide a medium through which sperm can swim. They also include sperm-sustaining substances, such as high concentrations of the sugar fructose, which is the main source of\u00a0energy\u00a0for sperm. In addition, semen contains many alkaline substances that help neutralize the acidic environment in the female vagina. This protects the\u00a0DNA\u00a0in sperm from being denatured by\u00a0acid, and prolongs the life of sperm in the female reproductive tract.<\/p>\n<div>\n<h1>Erection<\/h1>\n<\/div>\n<p>Besides providing a way for sperm to leave the body, the main role of the penis in\u00a0reproduction\u00a0is\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5203\">intromission<\/a>,<\/strong>\u00a0or depositing sperm in the vagina of the female reproductive tract. Intromission depends on the ability of the penis to become stiff and erect, a state referred to as an\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5204\">erection<\/a>.<\/strong>\u00a0The human penis, unlike that of most other mammals, contains no erectile bone. Instead, in order to reach its erect state, it relies entirely on engorgement with\u00a0blood\u00a0of its columns of spongy tissue. During sexual arousal, the arteries that supply blood to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5091\">penis<\/a> dilate, allowing more blood to fill the spongy tissue. The now-engorged spongy tissue presses against and constricts the veins that carry blood away from the penis. As a result, more blood enters than leaves the penis, until a constant erectile size is achieved.<\/p>\n<p>In addition to sperm, the penis also transports urine out of the body. These two functions cannot occur simultaneously. During an erection, the sphincters that prevent urine from leaving the bladder are controlled by centres in the brain so they cannot relax and allow urine to enter the urethra.<\/p>\n<div>\n<h1>Testosterone Production<\/h1>\n<\/div>\n<p>The final major function of the male reproductive system is the production of the male sex\u00a0hormone\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">testosterone<\/a>. In mature males, this occurs mainly in the testes. Testosterone production is under the control of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5205\">luteinizing hormone<\/a> (LH) from the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a>. LH stimulates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5171\">Leydig cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3486\">testes<\/a> to secrete testosterone.<\/span><\/p>\n<p>Testosterone is important for male sexual\u00a0development\u00a0at\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5155\">puberty<\/a>. It stimulates maturation of the\u00a0male reproductive organs, as well as the development of secondary male sex characteristics (such as facial hair). Testosterone is also needed in mature males for normal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">spermatogenesis<\/a> to be maintained in the testes. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5206\">Follicle stimulating\u00a0hormone<\/a>\u00a0(FSH) from the\u00a0pituitary gland\u00a0is also needed for spermatogenesis to occur, in part because it helps <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5169\">Sertoli cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0in the testes concentrate testosterone to high enough levels to maintain sperm production. Testosterone is also needed for proper functioning of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5096\">prostate gland<\/a>. In addition, testosterone plays a role in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5204\">erection<\/a>, allowing sperm\u00a0to\u00a0be deposited within the female reproductive tract.<\/span><\/p>\n<div>\n<h1>Feature: My\u00a0Human Body<\/h1>\n<\/div>\n<figure id=\"attachment_5207\" aria-describedby=\"caption-attachment-5207\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5207\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Laptop-by-logan-weaver-b76PEyeIptQ-unsplash-scaled-2.jpg\" alt=\"18.4.6 Laptop and Sperm Production\" width=\"400\" height=\"267\"><figcaption id=\"caption-attachment-5207\" class=\"wp-caption-text\"><em>Figure 18.4.6 The heat emitted by a laptop could decrease sperm production.<\/em><\/figcaption><\/figure>\n<p>If you\u2019re a man and you use a laptop computer on your lap for long periods of time, you may be decreasing your fertility. The reason? A laptop computer generates considerable\u00a0heat, and its proximity to the scrotum during typical use results in a significant rise in\u00a0temperature\u00a0inside the scrotum. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">Spermatogenesis<\/a> is very sensitive to high temperatures, so it may be adversely affected by laptop computer use. If you want to avoid the potentially fertility-depressing effect of laptop computer use, you might want to consider using your laptop computer on a table or other surface rather than on your lap \u2014 at least when you log on for long computer sessions. Other activities that raise scrotal temperature and have the potential to reduce spermatogenesis including soaking in hot tubs, wearing tight clothing, and biking. Although the effects of short-term scrotal heating on fertility seem to be temporary, years of such heat exposure may cause irreversible effects on sperm production.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.4 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Parts of a mature sperm include the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5196\">head<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5197\">acrosome<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5198\">midpiece<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3515\">flagellum<\/a>. The process of producing sperm is called spermatogenesis. This normally starts during\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5155\">puberty<\/a>, and continues uninterrupted until death.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">Spermatogenesis<\/a> occurs in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5160\">seminiferous tubules<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3486\">testes<\/a>, and requires high concentrations of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3409\">testosterone<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5169\">Sertoli cells<\/a>\u00a0in the testes play many roles in spermatogenesis, including concentrating testosterone under the influence of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5206\">follicle stimulating\u00a0hormone<\/a>\u00a0from the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a>.<\/li>\n<li>Spermatogenesis begins with a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2103\">diploid<\/a> stem cell called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5199\">spermatogonium<\/a>, which undergoes\u00a0mitosis\u00a0to produce a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2484\">haploid<\/a> secondary spermatocytes, and these cells in turn undergo meiosis II to produce spermatids. After the spermatids grow a tail and undergo other changes, they become <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a>.<\/li>\n<li>Before sperm are able to \u201cswim,\u201d they must mature in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5090\">epididymis<\/a>. The mature sperm are then stored in the epididymis until <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5150\">ejaculation<\/a> occurs.<\/li>\n<li>Ejaculation is the process in which <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4885\">semen<\/a> is propelled by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristalsis<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5095\"><em>vas deferens<\/em><\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5174\">ejaculatory ducts<\/a>\u00a0from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4732\">urethra<\/a> in the penis. Semen is a whitish fluid that contains sperm and secretions from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5097\">seminal vesicles<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5096\">prostate gland<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5183\">bulbourethral glands<\/a>. These alkaline secretions are important for sperm survival and motility.<\/li>\n<li>Besides ejaculating sperm, another reproductive role of the penis is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5203\">intromission<\/a>, which is depositing sperm in the female <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a>. This requires the penis to become stiff and erect, a state referred to as an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5204\">erection<\/a>. Erection usually occurs with sexual arousal as the columns of spongy tissue inside the penis become engorged with\u00a0blood.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5171\">Leydig cells<\/a>\u00a0in the testes secrete testosterone under the control of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5205\">luteinizing hormone<\/a> (LH) from the pituitary gland. Testosterone is needed for male sexual\u00a0development\u00a0at puberty, and to maintain normal spermatogenesis after puberty. It also plays a role in prostate function and\u00a0penis's ability\u00a0to become erect.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.4 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>&nbsp;<\/p>\n<ol>\n<li>\n<div id=\"h5p-209\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-209\" class=\"h5p-iframe\" data-content-id=\"209\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.13 Review Questions\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Compare and contrast the terms: erection, ejaculation, and intromission.<\/li>\n<li>Describe semen and its components.<\/li>\n<li>Explain how erection occurs.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.4 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/gNHSTa0Yct4<\/p>\n<p style=\"text-align: center\">How You're Destroying Your Sperm! Seeker, 2014.<\/p>\n<p>https:\/\/youtu.be\/krSMZDsjLuU<\/p>\n<p style=\"text-align: center\">Human Physiology - Reproduction: Spermatogenesis, Janux, 2015.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 18.4.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sperm-20051108.jpg\" rel=\"cc:attributionURL\">Sperm-20051108<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/93087247@N00\" rel=\"nofollow\">Gilberto Santa Rosa<\/a>\u00a0from\u00a0<a class=\"extiw\" title=\"w:Rio de Janeiro\" href=\"https:\/\/en.wikipedia.org\/wiki\/Rio_de_Janeiro\">Rio de Janeiro<\/a>,\u00a0<a class=\"extiw\" title=\"w:Brazil\" href=\"https:\/\/en.wikipedia.org\/wiki\/Brazil\">Brazil<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\" rel=\"license\">CC BY 2.0<\/a>\u00a0(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<p><strong>Figure 18.4.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sperm_Anatomy.png\" rel=\"cc:attributionURL\">Sperm Anatomy<\/a> by <a title=\"User:Christinelmiller\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Christinelmiller\">Christinelmiller<\/a> on Wikimedia Commons is used under a \u00a0<a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 18.4.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_28_01_04.jpg\" rel=\"cc:attributionURL\">Spermatogenesis<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-1-anatomy-and-physiology-of-the-male-reproductive-system\">OpenStax College<\/a> is used and adapted by Christine Miller under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 18.4.4<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/20.4\/\" rel=\"cc:attributionURL\">Testis-cross-section<\/a>\u00a0by <a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/20.4\/\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\">CC BY-NC 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<\/p>\n<div><img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/?_ga=2.264717007.1988667202.1598996862-1392032960.1592870652\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><\/div>\n<p><strong>Figure 18.4.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Human_semen_in_a_petri_dish.jpg\" rel=\"cc:attributionURL\">Human_semen_in_a_petri_dish<\/a> by <a title=\"User:Digitalkil\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Digitalkil\">Digitalkil<\/a> on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/en:public_domain).<\/p>\n<p><strong>Figure 18.4.6<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/b76PEyeIptQ\" rel=\"cc:attributionURL\">Laptop by logan-weaver-b76PEyeIptQ-unsplash<\/a> [photo] by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@lgnwvr\">LOGAN WEAVER<\/a> on <a href=\"http:\/\/unsplash.com\">Unsplash<\/a> is used under the <a href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\"><span class=\"os-title-label\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><span class=\"os-number\">27.5<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"81306\" class=\"os-title\" data-type=\"title\">Spermatogenesis [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 27.1). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-1-anatomy-and-physiology-of-the-male-reproductive-system<\/span><\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). <span id=\"81306\" class=\"os-title\" data-type=\"title\">F<\/span>igure 4 Cross-section of a testis and seminiferous tubules [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 20.4) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/20.4\/<\/p>\n<p class=\"hanging-indent\">Janux. (2015, January 10). Human physiology - Reproduction: spermatogenesis. YouTube. https:\/\/www.youtube.com\/watch?v=krSMZDsjLuU&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Seeker. (2014, June 16). How you're destroying your sperm! YouTube. https:\/\/www.youtube.com\/watch?v=gNHSTa0Yct4&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4098\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4098\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_5234\" aria-describedby=\"caption-attachment-5234\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5234\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/1024px-Cattien_stone_yoni-1.png\" alt=\"18.6.1\" width=\"400\" height=\"302\"><figcaption id=\"caption-attachment-5234\" class=\"wp-caption-text\"><em>Figure 18.6.1 Yep, that's a vagina.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Fertility Symbol<\/h1>\n<\/div>\n<p>The geometric design on the ancient stone carving in Figure 18.6.1 represents a powerful symbol of female fertility: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a>. The symbol is called <em>yoni<\/em> in Hindu, and it reflects the value placed by Hindu culture on the ability of females to give birth. The vagina is one of several organs in the\u00a0female reproductive system.<\/p>\n<div>\n<h1>Female Reproductive Organs<\/h1>\n<\/div>\n<p>The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called ova (or oocytes), secrete female sex hormones (such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a>), and carry and give birth to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5086\">fetus<\/a>. The internal female reproductive organs include the vagina, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviduct<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a>. The external organs \u2014 collectively called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5152\">vulva<\/a> \u2014 include the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5153\">clitoris<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5154\">labia<\/a>.<\/span><\/p>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a><\/strong> is an elastic, muscular canal leading from its opening in the vulva to the neck of the uterus, called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5235\">cervix<\/a>. It is about 7.5 cm (about 3 in) long at the front, and about 9 cm (3.5 in) long at the back. The vagina accommodates the penis and is the site where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a> are usually ejaculated during sexual intercourse. In the context of pregnancy and natural (vaginal) childbirth, the vagina is referred to as the birth canal. In addition, it channels the flow of menstrual blood from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>.<\/p>\n<h2>Structure of the Vagina<\/h2>\n<p>Muscles and ligaments support the vagina within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2992\">pelvic cavity<\/a>. The vagina itself is made up of several layers of fibrous and muscular tissues and lined with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3538\">mucous membrane<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">. Folds in the mucosa provide the vagina with extra surface area so it can stretch in both length and width during intercourse or childbirth. The elasticity of the vagina and the extra mucosa allow it to stretch to many times its normal diameter in order to deliver a baby.<\/span><\/p>\n<h2>Bacteria\u00a0and\u00a0pH\u00a0in the Vagina<\/h2>\n<p>A healthy <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a> is home to many <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3727\">symbiotic<\/a>\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> that help prevent pathogens (such as yeast) from colonizing the vagina. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4330\">pH<\/a> in the vagina is normally between 3.8 and 4.5, and this acidity also helps keep pathogenic microorganisms from colonizing it. The vagina constantly sheds its epithelium, so it is considered self-cleaning. As a consequence, there is no need for douching to clean it. Physicians actually discourage the practice, as it may upset the normal bacterial and pH balance in the vagina, although washing the vulva with a mild soap is good practice.<\/p>\n<div>\n<h1>Uterus<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a><\/strong> (commonly called the womb) is a pear-shaped, muscular organ that is about 7.6 cm (about 3 in) long. It is located above the vagina and behind the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4731\">bladder<\/a> in the centre of the pelvis. The position of the uterus in the pelvis is stabilized by several ligaments and bands of supportive tissue. The uterus is where a fetus develops during gestation, and the organ provides mechanical protection and support for the developing offspring. Contractions of the muscular wall of the uterus are responsible for pushing the fetus out of the uterus during childbirth.<\/p>\n<figure id=\"attachment_5240\" aria-describedby=\"caption-attachment-5240\" style=\"width: 368px\" class=\"wp-caption alignright\"><img class=\" wp-image-5240\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1000px-Gray1167.svg_-1.png\" alt=\"18.6.2 Uterus and Vagina\" width=\"368\" height=\"400\"><figcaption id=\"caption-attachment-5240\" class=\"wp-caption-text\"><em>Figure 18.6.2 The cervix of the uterus opens into the vagina. The body of the uterus lies above the cervix.<\/em><\/figcaption><\/figure>\n<h2>Parts of the Uterus<\/h2>\n<p>As shown in Figure 18.6.2, the lower end of the uterus forms the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5235\">cervix<\/a>,<\/strong> which is also called the neck of the uterus. The cervix is about 2.5 cm (almost 1 in) long and protrudes downward into the vagina. A small canal runs the length of the cervix, connecting the uterine cavity with the lumen of the vagina. This allows semen deposited in the vagina to enter the uterus, and a baby to pass from the uterus into the vagina during birth. Glands in the cervix secrete mucus that varies in water content and thickness, so it can function either as a barrier to keep <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3683\">microorganism<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0out of the uterus during\u00a0pregnancy, or as a transport medium to help\u00a0sperm\u00a0enter the uterus around the time of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a>. The rest of the uterus above the cervix is called the body of the uterus. The upper end of the uterus is connected with the two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviduct<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\">.<\/span><\/p>\n<h2>Tissues of the Uterus<\/h2>\n<p>As indicated in Figure 18.6.3, the uterus consists of three tissue layers, called the endometrium, myometrium, and perimetrium.<\/p>\n<ul>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a><\/strong> is the innermost tissue layer of the uterus. It consists of epithelial tissue, including mucous membranes. This layer thickens during each menstrual cycle and, unless an egg is fertilized, sloughs off during the following menstrual period. If an ovum is fertilized, the thickened endometrium is maintained by hormones and provides nourishment to the embryo. As gestation progresses, the endometrium develops into the maternal portion of the placenta.<\/li>\n<li>The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5242\">placenta<\/a><\/strong>\u00a0is a temporary organ that consists of a mass of maternal and fetal\u00a0blood vessels\u00a0through which the mother\u2019s and fetus\u2019s\u00a0blood\u00a0exchange substances.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5243\">myometrium<\/a><\/strong>\u00a0is the middle layer of the uterus. It consists mostly of a thick layer of smooth muscle tissue. Powerful contractions of the smooth muscle allow the uterus to contract and expel an infant during childbirth.<\/li>\n<li>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5244\">perimetrium<\/a><\/strong>\u00a0is the outermost layer of the uterus. It covers\u00a0the\u00a0outer surface of the uterus. This layer actually consists of two layers of epithelium that secrete fluid into the space between them. The fluid allows for small movements of the uterus within the pelvis, without\u00a0causing it to rub\u00a0against other organs.<\/li>\n<\/ul>\n<figure id=\"attachment_5245\" aria-describedby=\"caption-attachment-5245\" style=\"width: 380px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5245 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Uterine_anatomy.-e1593113913396-1.jpg\" alt=\"18.6.3 Layers of the Uterus\" width=\"380\" height=\"260\"><figcaption id=\"caption-attachment-5245\" class=\"wp-caption-text\"><em>Figure 18.6.3 The thick walls of the uterus are composed of layers of tissues known as endometrium, myometrium, and perimetrium (not shown in this image).<\/em><\/figcaption><\/figure>\n<div>\n<p><span style=\"font-size: 1.424em\">Oviducts<\/span><\/p>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviduct<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a><\/strong><span style=\"text-align: initial;font-size: 1em\"> (often referred to as Fallopian tubes) are two thin tubes that lie between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>. The oviducts are not attached to the ovaries, but their broad upper ends \u2014 called infundibula \u2014 lie very close to the ovaries. The infundibula also have fringe-like extensions called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5247\">fimbriae<\/a> that move in a waving motion to help guide eggs from the ovaries into the oviducts. The lower ends of the oviducts are attached to the upper part of the body of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a> on either side of the body. They open into the uterus.<\/span><\/p>\n<p>The oviducts are made up of multiple tissue layers. The innermost layer consists of mucosal epithelium. The epithelium is covered with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1597\">cilia<\/a>, which can move in a sweeping motion to help ova move through the tube from the ovary to the uterus. In between the ciliated cells of the epithelium are cells that secrete a fluid called tubular fluid. This fluid contains nutrients for sperm, ova, and zygotes. The secretions in tubular fluid also remove certain molecules from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3397\">plasma membrane<\/a> of sperm so they are better able to penetrate an egg. Other layers of the oviducts consist of connective tissue and smooth muscle. Contractions of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2982\">smooth muscle<\/a> allow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristalsis<\/a> to help move eggs through the tubes.<\/p>\n<div>\n<h1>Ovaries<\/h1>\n<\/div>\n<p>Like the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3486\">testes<\/a> in males, the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a><\/strong>\u00a0in females are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3408\">gonad<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0that produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3931\">gamete<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\"> and secrete sex <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3569\">hormones<\/a>. The gametes produced by the ovaries are called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5099\">ova<\/a>, or oocytes. The main sex hormone secreted by the ovaries is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a>. The position of the paired ovaries relative to the other reproductive system organs is shown in Figure 18.6.4. Each ovary lies along one side of the uterus and is about 4 cm (a little more than 1.5 in) long. Fibrous ligaments attach one end of each ovary to its nearby oviduct and the other and to its side of the uterus. These ligaments keep the ovaries in place within the pelvis.<\/span><\/p>\n<figure id=\"attachment_5251\" aria-describedby=\"caption-attachment-5251\" style=\"width: 689px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5251 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Sites_of_tubo_ovarian_abscess-e1593120204450-1.png\" alt=\"18.6.4 Placement of the Ovaries\" width=\"689\" height=\"482\"><figcaption id=\"caption-attachment-5251\" class=\"wp-caption-text\"><em>Figure 18.6.4 The placement of the two ovaries within the pelvis allows eggs from each ovary to enter a Fallopian tube and travel to the uterus.<\/em><\/figcaption><\/figure>\n<h2>Ovarian Follicles<\/h2>\n<p>The ovary consists of two main layers, called the ovarian medulla (the inner layer) and the ovarian cortex (the outer layer). The ovary also contains\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3743\">blood<\/a>\u00a0and lymphatic vessels. The ovarian cortex consists primarily of the functional units of the ovaries, which are called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">ovarian follicle<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a>.<\/strong><span style=\"text-align: initial;font-size: 1em\"> The follicles are nests of epithelial cells, within each of which is an ovum. The photomicrograph in Figure 18.6.5 shows an ovarian follicle and the developing ovum inside it. If an ovum and follicle complete maturation, the follicle ruptures and the ovum is released from the ovary. This event is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a>.<\/span><\/p>\n<figure id=\"attachment_5254\" aria-describedby=\"caption-attachment-5254\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5254\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ovarian_follicle-1.png\" alt=\"18.6.4 Ovarian Follicle\" width=\"800\" height=\"480\"><figcaption id=\"caption-attachment-5254\" class=\"wp-caption-text\"><em>Figure 18.6.5 An ovum within its nest of follicular cells inside an ovary.<\/em><\/figcaption><\/figure>\n<h2>Ova in the Ovaries<\/h2>\n<p>Whereas the male <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3486\">testes<\/a> produce sperm continuously after\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5155\">puberty<\/a>, the female ovary already contains all the ova it will ever produce by the time a female is born. At birth, a baby girl\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a> contain at least a million eggs, each of which is contained within a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">follicle<\/a>. Only about 500 of these eggs will eventually mature and be ovulated. This process starts at puberty and typically continues at monthly intervals until <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5255\">menopause<\/a> occurs around age 52. The remaining eggs never mature, and their number declines as the woman ages. By menopause, a woman\u2019s reserve of eggs is nearly depleted, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a> no longer occurs.<\/p>\n<div>\n<h1>Vulva<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5152\">vulva<\/a><\/strong> is a general term for all of the external female reproductive organs. The vulva includes the clitoris, labia, and external openings for the urethra and vagina.<\/p>\n<h2>Labia<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5154\">labia<\/a><\/strong> (singular, labium) refer to the \u201clips\u201d of the vulva, which are folds of tissue that contain and protect the other, more delicate structures of the vulva (as shown in Figure 18.6.6). There are two pairs of labia: the outer and larger labia majora, and the inner and smaller labia minora. The labia minora contain numerous sebaceous glands. These glands release secretions that help lubricate the labia and vulvar area.<\/p>\n<figure id=\"attachment_5256\" aria-describedby=\"caption-attachment-5256\" style=\"width: 361px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5256\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/HumanVulva-NewText-PhiloViv-1.jpg\" alt=\"18.6.5 Vulva Labeled\" width=\"361\" height=\"256\"><figcaption id=\"caption-attachment-5256\" class=\"wp-caption-text\"><em>Figure 18.6.6 The vulva includes the external features of the female reproductive system including the labia, clitoris and clitoral hood, and the openings from the urethra and to the vagina.<\/em><\/figcaption><\/figure>\n<h2>Clitoris<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5153\">clitoris<\/a>,<\/strong>\u00a0is located at the front of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5152\">vulva<\/a> where the labia minora meet. The visible portion of the clitoris is called the glans clitoris. It is roughly the size and shape of a pea. It is highly sensitive, because it contains many nerve endings. A hood of tissue called the clitoral hood (shown in Figure 18.6.5 above), or prepuce, normally covers and protects the clitoris. The clitoris is the homologue to the male penis, and they both contain spongy tissue.\u00a0\u00a0<span style=\"font-size: 1em\">Stimulation of the glans clitoris during sexual activity generally results in sexual arousal in females, and may lead to orgasm.<\/span><span style=\"font-size: 1em\"> The glans clitoris is the only part of the overall clitoris visible externally, but this spongy tissue extends down either side of the openings to the urethra and vagina, as seen in Figure 18.6.7.<\/span><\/p>\n<figure id=\"attachment_5257\" aria-describedby=\"caption-attachment-5257\" style=\"width: 729px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5257\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Vulva-1.jpg\" alt=\"18.6.6 Clitoris\" width=\"729\" height=\"428\"><figcaption id=\"caption-attachment-5257\" class=\"wp-caption-text\"><em>Figure 18.6.7 While the glans clitoris is the only externally visible part of the clitoris, this spongy tissue extends dorsally into the corpus cavernosum, which flanks the left and right sides of the opening to the vagina.<\/em><\/figcaption><\/figure>\n<h2>Other Vulvar Structures<\/h2>\n<p>The area between the two labia minora is called the vestibule of the vulva. Both the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4732\">urethra<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a> have openings to the outside of the body in the vestibule. As you can see in Figure 18.6.7 above, the urethral opening (or meatus) is located just in front of, and is much smaller than, the vaginal opening. Both openings are protected by the labia. Two glands \u2014 called Bartholin\u2019s glands \u2014 open on either side of the vaginal opening. These glands secrete mucus and a vaginal and vulvar lubricant.<\/p>\n<div>\n<h1>Breasts<\/h1>\n<\/div>\n<figure id=\"attachment_5258\" aria-describedby=\"caption-attachment-5258\" style=\"width: 350px\" class=\"wp-caption alignright\"><img class=\"size-full wp-image-5258\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Breast-Diagram-2.gif\" alt=\"18.6.7 Breasts\" width=\"350\" height=\"274\"><figcaption id=\"caption-attachment-5258\" class=\"wp-caption-text\"><em>Figure 18.6.8 The breasts are not really reproductive organs, but they play a reproductive role as mammary glands that can produce milk to feed an infant.<\/em><\/figcaption><\/figure>\n<p>The breasts are not directly involved in\u00a0reproduction, but because they contain\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3432\">mammary gland<\/strong><strong style=\"font-size: 1em\">s<\/strong><strong style=\"text-align: initial;font-size: 1em\"><\/a>,<\/strong><span style=\"text-align: initial;font-size: 1em\"> they can provide nourishment to an infant after birth. The breasts overlay major muscles in the chest from which they project outward in a conical shape. The two main types of tissues in the breast are adipose (fat) tissue and glandular tissue that produces milk. As shown in Figure 18.6.8, each mature breast contains many lobules, where milk is produced and stored during pregnancy. During breastfeeding (or lactation), the milk drains into ducts and sacs, which in turn converge at the nipple. Milk exits the breast through the nipple in response to the suckling action of an infant and is regulated by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2962\">positive feedback loop<\/a>. The nipple is surrounded by a more darkly pigmented area called the areola. The areola contains glands that secrete an oily fluid, which lubricates and protects the nipple during breastfeeding.<\/span><\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.6 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The\u00a0female reproductive system\u00a0is made up of internal and external organs that function to produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2484\">haploid<\/a> female <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3931\">gametes<\/a> called ova, secrete female sex hormones (such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a>), and carry and give birth to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5086\">fetus<\/a>.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a> is an elastic, muscular canal that can accommodate the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5091\">penis<\/a>. It is also where sperm are usually ejaculated during <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5149\">sexual intercourse<\/a>. The vagina is the birth canal, and it channels the flow of menstrual blood from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>. A healthy vagina has a balance of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3727\">symbiotic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2588\">bacteria<\/a> and an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3599\">acidic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4330\">pH<\/a>.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a> is a muscular organ above the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a> where a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5086\">fetus<\/a> develops. Its muscular walls contract to push out the fetus during childbirth. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5235\">cervix<\/a> is the neck of the uterus that extends down into the vagina. It contains a canal connecting the vagina and uterus for sperm, or for an infant to pass through. The innermost layer of the uterus \u2014 the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a> \u2014 thickens each month in preparation for an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5262\">embryo<\/a>, but is shed in the following menstrual period if\u00a0fertilization does not occur.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviducts<\/a>\u00a0extend from the uterus to the ovaries. Waving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5247\">fimbriae<\/a> at the ovary ends of the oviducts guide ovulated eggs into the tubes where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3917\">fertilization<\/a> may occur as the ova travel to the uterus. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_1597\">Cilia<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2890\">peristalsis<\/a> help ova move through the tubes. Tubular fluid helps nourish sperm as they swim up the tubes toward ova.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a> are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3408\">gonads<\/a> that produce ova and secrete sex hormones, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a>. Nests of cells called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">follicles<\/a> in the ovarian cortex are the functional units of ovaries. Each follicle surrounds an immature ovum. At birth, a baby girl\u2019s ovaries contain at least a million ova, and they will not produce any more during her lifetime. During a woman\u2019s reproductive years, one ova typically matures and is ovulated each month.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5152\">vulva<\/a> is a general term for external female reproductive organs. The vulva includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5153\">clitoris<\/a>, two pairs of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5154\">labia<\/a>, and openings for the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4732\">urethra<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5143\">vagina<\/a>. Secretions from mucosal glands near the vaginal opening lubricate the vulva.<\/li>\n<li>The breasts are not technically reproductive organs, but their <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3432\">mammary glands<\/a> produce milk to feed an infant after birth. Milk drains through ducts and sacs, and out through the nipple when a\u00a0 baby sucks during breastfeeding.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.6 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-211\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-211\" class=\"h5p-iframe\" data-content-id=\"211\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Mitosis and Cytokinesis True\/False\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>State the general functions of the female reproductive system.<\/li>\n<li>Describe the vagina and its reproductive functions.<\/li>\n<li>Outline the structure and basic functions of the uterus.<\/li>\n<li>What is the endometrium? How does it change during the monthly cycle?<\/li>\n<li>Why are breasts included in discussions of reproduction, if they are not organs of the female reproductive system?<\/li>\n<li>What is the function of the folds in the mucous membrane lining of the vagina?<\/li>\n<li>What are two ways in which the female reproductive system protects itself from pathogens?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.6 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/SkB4gG8ke7Q<\/p>\n<p style=\"text-align: center\">The uncomplicated truth about women's sexuality | Sarah Barmak, TED, 2019.<\/p>\n<p>https:\/\/youtu.be\/9rs2gNchQig<\/p>\n<p style=\"text-align: center\">Human Physiology - Functional Anatomy of the Female Reproductive System, Janux, 2015.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"content\" class=\"site-content\">\n<section class=\"standard post-956 chapter type-chapter status-publish hentry focusable\" data-type=\"chapter\">\n<h2>Attributes<\/h2>\n<p><strong>Figure 18.6.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Cattien_stone_yoni.png\" rel=\"cc:attributionURL\">1024px-Cattien_stone_yoni<\/a> by <a title=\"User:Binh Giang\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Binh_Giang\">Binh Giang<\/a> on Wikimedia Commons is in the \u00a0<a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/en:public_domain).<\/p>\n<p><strong>Figure 18.6.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Gray1167.svg\" rel=\"cc:attributionURL\">1000px-Gray1167.svg<\/a> by <a href=\"https:\/\/en.wikipedia.org\/wiki\/Henry_Vandyke_Carter\">Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0<a class=\"extiw\" title=\"en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/public_domain\">public domain<\/a>\u00a0(https:\/\/en.wikipedia.org\/wiki\/public_domain). (<a class=\"extiw\" title=\"en:Bartleby.com\" href=\"https:\/\/en.wikipedia.org\/wiki\/Bartleby.com\">Bartleby.com<\/a>:\u00a0<a class=\"external text\" href=\"http:\/\/www.bartleby.com\/107\/\" rel=\"nofollow\">Gray\u2019s Anatomy<\/a>, <a class=\"external text\" href=\"http:\/\/www.bartleby.com\/107\/illus1167.html\" rel=\"nofollow\">Plate 1167<\/a>).<\/p>\n<p><strong>Figure 18.6.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Uterine_anatomy..jpg\" rel=\"cc:attributionURL\">Uterine_anatomy.<\/a> from <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/books\/NBK27042\/bin\/Teixeira01.jpg\">Uterine Stem cells<\/a> by <a href=\"http:\/\/www.stembook.org\">The Stem Cell Research Community, StemBook<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 18.6.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Sites_of_tubo_ovarian_abscess.PNG\" rel=\"cc:attributionURL\">Sites_of_tubo_ovarian_abscess<\/a> by <a title=\"User:Bfpage\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Bfpage\">Bfpage<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\" rel=\"license\">CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong>Figure 18.6.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Ovarian_follicle.png\" rel=\"cc:attributionURL\">Ovarian_follicle<\/a> by <a title=\"User:TiagoLubiana\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:TiagoLubiana\">TiagoLubiana<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" rel=\"license\">CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 18.6.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:HumanVulva-NewText-PhiloViv.jpg#file\" rel=\"cc:attributionURL\">HumanVulva-NewText-PhiloViv<\/a> by <a title=\"User:Amphis\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Amphis\">Amphis<\/a> (edited) on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en\" rel=\"license\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\/deed.en) license. (<span style=\"text-align: initial;font-size: 1em\">Original \u00a0<a class=\"extiw\" title=\"en:Image:HumanVulva-NoText-PhiloVivero.jpg\" href=\"https:\/\/en.wikipedia.org\/wiki\/Image:HumanVulva-NoText-PhiloVivero.jpg\">en:Image:HumanVulva-NoText-PhiloVivero.jpg<\/a> by <\/span><a class=\"extiw\" style=\"text-align: initial;font-size: 1em\" title=\"en:user:PhiloVivero\" href=\"https:\/\/en.wikipedia.org\/wiki\/user:PhiloVivero\">en:user:PhiloVivero<\/a>)<\/p>\n<p><strong>Figure 18.6.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_28_02_02.jpg\" rel=\"cc:attributionURL\">Vulva<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system\">OpenStax College<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 18.6.8<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Breast-Diagram.gif\" rel=\"cc:attributionURL\">Breast-Diagram<\/a> by <a href=\"https:\/\/www.womenshealth.gov\/cancer\/breast-cancer\">Women's Health<\/a> (NCI\/ NIH) on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:en:Public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/en:Public_domain).<\/p>\n<p>&nbsp;<\/p>\n<div class=\"media-atttributions\">\n<p><span style=\"font-size: 1.424em;font-weight: bold;color: #333333\">References<\/span><\/p>\n<\/div>\n<\/section>\n<\/div>\n<p class=\"hanging-indent\"><span class=\"os-title-label\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><span class=\"os-number\">27.10<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"33336\" class=\"os-title\" data-type=\"title\">The vulva [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/span><\/p>\n<p class=\"hanging-indent\">Janux. (2015, January 10). Human physiology - Functional anatomy of the female reproductive system. YouTube. https:\/\/www.youtube.com\/watch?v=9rs2gNchQig&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED. (2019, March 22). The uncomplicated truth about women's sexuality | Sarah Barmak. YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=SkB4gG8ke7Q&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Teixeira, J., Rueda, B.R., and Pru, J.K. (September 30, 2008). Figure 1 Uterine anatomy. In <em>Uterine Stem Cells<\/em> (StemBook, ed.). The Stem Cell Research Community, StemBook, doi\/10.3824\/stembook.1.16.1, http:\/\/www.stembook.org<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4100\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4100\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_5295\" aria-describedby=\"caption-attachment-5295\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5295\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/WaterforPeople_Uganda-1.jpg\" alt=\"18.8.1 Menstruation Awareness\" width=\"400\" height=\"300\"><figcaption id=\"caption-attachment-5295\" class=\"wp-caption-text\"><em>Figure 18.8.1 It's healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n<div>\n<h1>Taboo Topic<\/h1>\n<\/div>\n<p>The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n<div>\n<h1>What Is the Menstrual Cycle?<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5293\">menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a> for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5280\">pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5280\">pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\n<h2>Menarche and Menopause<\/h2>\n<p>The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5300\">menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\n<p>The cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5255\">menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\n<h2>Variation in the Menstrual Cycle<\/h2>\n<p>The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n<div>\n<h1>Ovarian Cycle<\/h1>\n<\/div>\n<p>The events of the menstrual cycle that take place in the ovaries make up the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5301\">ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">follicles<\/a> of one of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\n<figure id=\"attachment_5302\" aria-describedby=\"caption-attachment-5302\" style=\"width: 919px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5302 \" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ovarian-Cycle-e1593185427416-1.png\" alt=\"18.8.2 Ovarian Cycle\" width=\"919\" height=\"473\"><figcaption id=\"caption-attachment-5302\" class=\"wp-caption-text\"><em>Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\n<h2>Follicular Phase<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5303\">follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">ovarian follicle<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\"> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5206\">follicle stimulating hormone<\/a> (FSH), which is secreted by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a>. The maturing follicle releases <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\n<figure id=\"attachment_5304\" aria-describedby=\"caption-attachment-5304\" style=\"width: 724px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5304 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Figure_43_04_04-e1593185848433-1.png\" alt=\"18.8.3 Pituitary and Ovarian Hormones\" width=\"724\" height=\"457\"><figcaption id=\"caption-attachment-5304\" class=\"wp-caption-text\"><em>Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\n<h2>Ovulation<\/h2>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">Ovulation<\/a><\/strong>\u00a0is the second phase of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5301\">ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5205\">luteinizing hormone<\/a> (LH) from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5303\">follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5274\">oocyte<\/a>.<\/p>\n<h2>Luteal Phase<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5305\">luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5206\">FSH<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5306\">LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5277\">corpus luteum<\/a>. The corpus luteum secretes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n<ul>\n<li>If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n<li>If\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3917\">fertilization<\/a>\u00a0has occurred so a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2471\">zygote<\/a> forms and then divides to become a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5279\">blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5307\">human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n<div>\n<h1>Uterine Cycle<\/h1>\n<\/div>\n<p>The events of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5293\">menstrual cycle<\/a> that take place in the uterus make up the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5308\">uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\n<figure id=\"attachment_5309\" aria-describedby=\"caption-attachment-5309\" style=\"width: 488px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5309 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ovarian-and-menstrual-cycle-e1593188761715-1.jpg\" alt=\"18.8.4 Uterine cycle\" width=\"488\" height=\"206\"><figcaption id=\"caption-attachment-5309\" class=\"wp-caption-text\"><em>Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\n<h2>Menstruation<\/h2>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5310\">Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3917\">fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\n<h2>Proliferative Phase<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5311\">proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5235\">cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\n<h2>Secretory Phase<\/h2>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5312\">secretory phase<\/a><\/strong>\u00a0is the third and final phase of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5308\">uterine cycle<\/a>. During this phase, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">progesterone<\/a> produced by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5277\">corpus luteum<\/a> in the ovary stimulates further changes in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5241\">endometrium<\/a> so it is more receptive to implantation of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5279\">blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2982\">smooth muscle<\/a> tissue in the uterine wall.<\/p>\n<p>&nbsp;<\/p>\n<h1>Bringing it All Together<\/h1>\n<p>It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\n<figure id=\"attachment_5313\" aria-describedby=\"caption-attachment-5313\" style=\"width: 1000px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5313\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1000px-MenstrualCycle2_en.svg_-1.png\" alt=\"18.8.8 Overview of ovarian and uterine cycle\" width=\"1000\" height=\"977\"><figcaption id=\"caption-attachment-5313\" class=\"wp-caption-text\"><em>Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.8 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5293\">menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a> for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5280\">pregnancy<\/a>. It involves changes in both the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a> and uterus, and is controlled by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a> hormones (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5206\">FSH<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5306\">LH<\/a>) and ovarian hormones (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3903\">estrogen<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3411\">progesterone<\/a>).<\/li>\n<li>The female reproductive period is delineated by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5300\">menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5255\">menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n<li>The events of the menstrual cycle that take place in the ovaries make up the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5301\">ovarian cycle<\/a>. It includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5303\">follicular phase<\/a> (when a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5253\">follicle<\/a> and its ovum mature due to rising levels of FSH), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a> (when the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5099\">ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5305\">luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n<li>The events of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5293\">menstrual cycle<\/a> that take place in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a> make up the uterine cycle. It includes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5310\">menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5311\">proliferative phase<\/a>, during which the endometrium builds up again until <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a> occurs; and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5312\">secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5262\">embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.8 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-213\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-213\" class=\"h5p-iframe\" data-content-id=\"213\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Eukaryote and Prokaryote Drag and Drop\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\n<li>What organs are involved in the menstrual cycle?<\/li>\n<li>Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\n<li>Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\n<li>If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\n<li>Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.8 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/cjbgZwgdY7Q<\/p>\n<p style=\"text-align: center\">Why do women have periods? TED-Ed, 2015.<\/p>\n<p>https:\/\/youtu.be\/5B3Abpv0ysM<\/p>\n<p style=\"text-align: center\">Girl's Rite of Passage | National Geographic, 2007.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 18.8.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:WaterforPeople_Uganda.jpg\" rel=\"cc:attributionURL\">WaterforPeople_Uganda<\/a> by <a href=\"https:\/\/www.flickr.com\/photos\/gtzecosan\/17511016219\/in\/album-72157626959211983\/\">WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\" rel=\"license\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.<\/p>\n<p><strong>Figure 18.8.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_43_04_04.png%20andhttps:\/\/commons.wikimedia.org\/wiki\/File:MenstrualCycle2_en.svg\" rel=\"cc:attributionURL\">Ovarian Cycle<\/a> by <a href=\"https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction\">CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" rel=\"license\">CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.<\/p>\n<p><strong>Figure 18.8.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_43_04_04.png\" rel=\"cc:attributionURL\">Figure_43_04_04<\/a>\u00a0by <a href=\"https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction\">CNX OpenStax<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" rel=\"license\">CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)<\/p>\n<p><strong>Figure 18.8.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_28_02_07.jpg\" rel=\"cc:attributionURL\">Ovarian and menstrual cycle<\/a>\u00a0by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system\">OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 <a href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.<\/p>\n<p><strong>Figure 18.8.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:MenstrualCycle2_en.svg\" rel=\"cc:attributionURL\">1000px-MenstrualCycle2_en.svg<\/a> by <a title=\"User:Isometrik\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Isometrik\">Isometrik<\/a> on Wikimedia Common is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/3.0\" rel=\"license\">CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). <span class=\"os-title-label\">Figure\u00a0<\/span><span class=\"os-number\">27.15<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"78432\" class=\"os-title\" data-type=\"title\">Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\n<p class=\"hanging-indent\">National Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In <em>Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&amp;feature=youtu.be<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_5483\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_5483\"><div tabindex=\"-1\"><p>A synovial joint in which an oval-shaped process of one bone fits into a roughly elliptical cavity of the other, allowing movement in two planes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4093\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4093\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4998\" aria-describedby=\"caption-attachment-4998\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4998\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Oedema-1.jpg\" alt=\"17.6.1 Allergies\" width=\"400\" height=\"243\"><figcaption id=\"caption-attachment-4998\" class=\"wp-caption-text\"><em>Figure 17.6.1 Oh m-eye!<\/em><\/figcaption><\/figure>\n<div>\n<h1>Allergy Eyes<\/h1>\n<\/div>\n<p>Eyes that are red, watery, and itchy are typical of an allergic reaction known as allergic rhinitis. Commonly called hay fever, allergic rhinitis is an immune system reaction, typically to the pollen of certain plants. Your immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system itself can develop problems. Sometimes, it responds to harmless foreign substances as though they were pathogens. This is the basis of allergies like hay fever.<\/p>\n<div>\n<h1>Allergies<\/h1>\n<\/div>\n<p>An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4877\">allergy<\/a><\/strong>\u00a0is a disorder in which the immune system makes an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4860\">inflammatory response<\/a> to a harmless <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2704\">antigen<\/a>. It occurs when the immune system is hypersensitive to an antigen in the environment that causes little or no response in most people. Allergies are strongly familial. Allergic parents are more likely to have allergic children, and those children\u2019s allergies are likely to be more severe, which is evidence that there is a heritable tendency to develop allergies. Allergies are more common in children than adults, because many children outgrow their allergies by adulthood.<\/p>\n<h2>Allergens<\/h2>\n<p>Any antigen that causes an allergy is called an\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4999\">allergen<\/a>.<\/strong> Common allergens are plant pollens, dust mites, mold, specific foods (such as peanuts or shellfish), insect stings, and certain common medications (such as aspirin and penicillin). Allergens may be inhaled or ingested, or they may come into contact with the skin or eyes. Symptoms vary depending on the type of exposure, and the severity of the immune system response. Some of the most common causes of allergies are shown in Figure 17.6.2: latex, pollen, dust mites, pet dander, insect stings and various foods. Inhaling pollen may cause symptoms of allergic rhinitis, such as sneezing and red itchy eyes. Insect stings may cause an itchy rash. This type of allergy is called contact dermatitis.<\/p>\n<div>\n<div id=\"h5p-203\">\n<div class=\"h5p-content\" data-content-id=\"203\"><\/div>\n<\/div>\n<p><em>Figure 17.6.2 Common allergens include latex, pollen, dust mites, pet dander, insect stings, and foods.<\/em><\/p>\n<\/div>\n<h2>Prevalence of Allergies<\/h2>\n<p>There has been a significant increase in the prevalence of allergies over the past several decades, especially in the rich nations of the world, where allergies are now very common disorders. In the developed countries, about 20% of people have or have had hay fever, another 20% have had contact dermatitis, and about 6% have food allergies. In the poorer nations of the world, on the other hand, allergies of all types are much less common.<\/p>\n<p>One explanation for the rise in allergies in the developed world is the hygiene hypothesis. According to this hypothesis, people in developed countries live in relatively sterile environments because of hygienic practices and sanitation systems. As a result, people in these countries are exposed to fewer pathogens than their immune system evolved to cope with. To compensate, their immune system \u201ckeeps busy\u201d by attacking harmless antigens in allergic responses.<\/p>\n<h2>How Allergies Occur<\/h2>\n<p>The diagram in Figure 17.6.3 shows how an allergic reaction occurs. At the first exposure to an allergen, B cells are activated to form plasma cells that produce large amounts of antibodies to the allergen. These antibodies attach to leukocytes called mast cells. Subsequently, every time the person encounters the allergen again, the mast cells are already primed and ready to deal with it. The primed mast cells immediately release cytokines and histamines, which in turn cause inflammation and recruitment of leukocytes, among other responses. These responses are responsible for the signs and symptoms of allergies.<\/p>\n<figure id=\"attachment_5000\" aria-describedby=\"caption-attachment-5000\" style=\"width: 341px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5000\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-Mast_cells-1.jpg\" alt=\"17.3 Allergic Reaction Pathway\" width=\"341\" height=\"612\"><figcaption id=\"caption-attachment-5000\" class=\"wp-caption-text\"><em>Figure 17.6.3 This diagram shows how the adaptive immune system is activated by an otherwise harmless antigen on ragweed pollen, responding to the allergen as though it was a pathogen.<\/em><\/figcaption><\/figure>\n<h2>Treating Allergies<\/h2>\n<p>The symptoms of allergies can range from mild to life-threatening. Mild allergy symptoms are often treated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3377\"><strong>antihistamine<span style=\"font-size: 1em\">s<\/span><\/strong><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">. These are drugs that reduce or eliminate the effects of the histamines that produce allergy symptoms.<\/span><\/p>\n<h3>Treating Anaphylaxis<\/h3>\n<p>The most severe allergic reaction is a systemic reaction called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5002\">anaphylaxis<\/a>.<\/strong> This is a life-threatening response caused by a massive release of histamines. Many of the signs and symptoms of anaphylaxis are shown in Figure 17.6.4. Some of them include a drop in blood pressure, changes in heart rate, shortness of breath, and swelling of the tongue and throat, which may threaten the patient with suffocation unless emergency treatment is given. People who have had anaphylactic reactions may carry an epinephrine autoinjector (widely known by its brand name EpiPen\u00ae) so they can inject themselves with epinephrine if they start to experience an anaphylactic response. The epinephrine helps control the immune reaction until medical care can be provided. Epinephrine constricts blood vessels to increase blood pressure, relaxes smooth muscles in the lungs to reduce wheezing and improve breathing, modulates heart rate, and works to reduce swelling that may otherwise block the airways.<\/p>\n<figure id=\"attachment_5003\" aria-describedby=\"caption-attachment-5003\" style=\"width: 745px\" class=\"wp-caption alignnone\"><img class=\" wp-image-5003\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Signs_and_symptoms_of_anaphylaxis-1.png\" alt=\"17.6.4 Signs and Symptoms of Anaphylaxis\" width=\"745\" height=\"627\"><figcaption id=\"caption-attachment-5003\" class=\"wp-caption-text\"><em>Figure 17.6.4 Anaphylaxis is a rapid, systemic reaction to allergens that may lead to life-threatening symptoms.<\/em><\/figcaption><\/figure>\n<h3>Immunotherapy for Allergies<\/h3>\n<figure id=\"attachment_5005\" aria-describedby=\"caption-attachment-5005\" style=\"width: 303px\" class=\"wp-caption alignleft\"><img class=\" wp-image-5005\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Allergy-Tests-1.jpg\" alt=\"17.6.5 Allergy Tests\" width=\"303\" height=\"378\"><figcaption id=\"caption-attachment-5005\" class=\"wp-caption-text\"><em>Figure 17.6.5 Skin testing for common allergens is one way to identify the cause(s) of a patient\u2019s allergic symptoms.<\/em><\/figcaption><\/figure>\n<p>Another way to treat allergies is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5004\">immunotherapy<\/a>,<\/strong>\u00a0commonly called \u201callergy shots.\u201d This approach may actually\u00a0<em>cure<\/em> specific allergies, at least for several years if not permanently. It may be particularly beneficial for allergens that are difficult or impossible to avoid (such as pollen). First, however, patients must be tested to identify the specific allergens that are causing their allergies. As shown in Figure 17.6.5, this may involve scratching tiny amounts of common allergens into the skin, and then observing whether there is a localized reaction to any of them. Each allergen is applied in a different numbered location on the skin, so if there is a reaction \u2014 such as redness or swelling \u2014 the responsible allergens can be identified. Then, through periodic injections (usually weekly or monthly), patients are gradually exposed to larger and larger amounts of the allergens. Over time, generally from months to years, the immune system becomes desensitized to the allergens. This method of treating allergies is often effective for allergies to pollen or insect stings, but its usefulness for allergies to food is unclear.<\/p>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Autoimmune Diseases<\/span><\/p>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5006\">Autoimmune diseases<\/a><\/strong>\u00a0occur when the immune system fails to recognize the body\u2019s own molecules as self. As a result, instead of ignoring the body\u2019s healthy cells, it attacks them, causing damage to tissues and\u00a0altering\u00a0organ growth and function. Most often, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3447\">B cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0are at fault for autoimmune responses. They are generally the cells that lose tolerance for self. Why does this occur? Some autoimmune diseases are thought to be caused by exposure to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3707\">pathogen<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\">\u00a0that have antigens similar to the body\u2019s own molecules. After this exposure, the immune system responds to body cells as though they were pathogens, as well.<\/span><\/p>\n<p>Certain individuals are genetically susceptible to developing autoimmune diseases. These individuals are also more likely to develop more than one such disease. Gender is a risk factor for autoimmunity \u2014 females are much more likely than males to develop autoimmune diseases. This is likely due, in part, to gender differences in sex hormones.<\/p>\n<p>At a population level, autoimmune diseases are less common where infectious diseases are more common. The hygiene hypothesis has been proposed to explain the inverse relationship between infectious and autoimmune diseases, as well as the prevalence of allergies. According to the hypothesis, without infectious diseases to \u201ckeep it busy,\u201d the immune system may attack the body\u2019s own cells instead.<\/p>\n<h2>Common Autoimmune Diseases<\/h2>\n<p>An estimated 15 million or more people worldwide have one or more autoimmune diseases. Two of the most common autoimmune diseases are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3507\">type I diabetes<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5007\">multiple sclerosis<\/a>. In terms of the specific body cells that are attacked by the immune system, both are localized diseases. In the case of type I diabetes, the immune system attacks and destroys <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2590\">insulin<\/a>-secreting islet cells in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3197\">pancreas<\/a>. In the case of multiple sclerosis, the immune system attacks and destroys the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3028\">myelin sheath<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0that normally insulate the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3809\">axon<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\">\u00a0of <\/span><span style=\"font-size: 1em;text-align: initial\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2984\">neuron<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0and allow rapid transmission of nerve impulses.<\/span><\/p>\n<p>Some relatively common autoimmune diseases are systemic \u2014 or body-wide \u2014 diseases. They include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5008\">rheumatoid arthritis<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5009\">systemic lupus erythematosus<\/a> (SLE). In these diseases, the immune system may attack and injure many tissues and organs. For example, as you can see in Figure 17.6.6, symptoms of SLE may involve the muscular, skeletal, integumentary, respiratory, and cardiovascular systems.<\/p>\n<figure id=\"attachment_5010\" aria-describedby=\"caption-attachment-5010\" style=\"width: 549px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5010\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Symptoms_of_SLE-1.png\" alt=\"Figure 17.6.6 Symptoms of Lupus\" width=\"549\" height=\"687\"><figcaption id=\"caption-attachment-5010\" class=\"wp-caption-text\"><em>Figure 17.6.6 Systemic lupus erythematosus is an autoimmune disease that may cause symptoms of body-wide tissue damage.<\/em><\/figcaption><\/figure>\n<h2>Treatment for Autoimmune Diseases<\/h2>\n<p>None of these common autoimmune diseases can be cured, although all of them have treatments that may help relieve symptoms and prevent some of the long-term damage they may cause. Traditional treatments for autoimmune diseases include immunosuppressive drugs to block the immune response, as well as anti-inflammatory drugs to quell inflammation. Hormone replacement may be another option.\u00a0Type I diabetes, for example, is treated with injections of the hormone insulin, because islet cells in the pancreas can no longer secrete it.<\/p>\n<div>\n<h1>Immunodeficiency<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5011\">Immunodeficiency<\/a><\/strong>\u00a0occurs when the immune system is not working properly, generally because one or more components of the immune system are inactive. As a result, the immune system may be unable to fight off pathogens or cancers that a normal immune system would be able to resist. Immunodeficiency may occur for a variety of reasons.<\/p>\n<h2>Causes of Immunodeficiency<\/h2>\n<p>Dozens of rare genetic diseases can result in a defective immune system. This type of immunodeficiency is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5012\">primary immunodeficiency<\/a><\/strong>. One is born with one of these diseases, rather than acquiring it after birth. Probably the best known of these primary immunodeficiency diseases is <a href=\"https:\/\/www.niaid.nih.gov\/diseases-conditions\/severe-combined-immunodeficiency-scid\">severe combined immunodeficiency<\/a> (SCID). It is also known as \u201cbubble boy disease,\u201d because people with this disorder are extremely vulnerable to infectious diseases, and some of them have become well known for living inside a bubble that provides a sterile environment. SCID is most often caused by an X-linked <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2479\">recessive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2381\">mutation<\/a> that interferes with normal B cell and T cell production.<\/p>\n<p>Other types of immunodeficiency are not present at birth, but are acquired due to experiences or exposures that occur after birth. Acquired immunodeficiency is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5013\">secondary immunodeficiency<\/a><\/strong>\u00a0because it is secondary to some other event or exposure. Secondary immunodeficiency may occur for a number of different reasons:<\/p>\n<ul>\n<li style=\"margin-top: 0px\"><strong>Some pathogens attack and destroy immune system cells.<\/strong>\u00a0An example is the virus known as HIV, which attacks and destroys T cells.<\/li>\n<li><strong>The immune system naturally becomes less effective as people get older.<\/strong>\u00a0This age-related decline \u2014 called immunosenescence \u2014 generally begins around the age of 50 and worsens with increasing age. Immunosenescence is\u00a0the reason\u00a0older people are generally more susceptible to disease than younger people.<\/li>\n<li><strong>The immune system may be damaged by another disorder, such as obesity, alcoholism, or the abuse of other drugs.<\/strong><\/li>\n<li><strong>In developing countries, malnutrition is the most common cause of immune system damage and immunodeficiency.<\/strong>\u00a0Inadequate protein intake is especially damaging to the immune system. It can lead to impaired complement system activity, phagocyte malfunction, and lower-than-normal production of antibodies and cytokines.<\/li>\n<li><strong>Certain medications can suppress the immune system.<\/strong>\u00a0This is the intended effect of immunosuppressant drugs given to people with transplanted organs so they do not reject them. In many cases, however, immunosuppression is an unwanted side effect of drugs used to treat other disorders.<\/li>\n<\/ul>\n<div>\n<h1>Focus on HIV<\/h1>\n<\/div>\n<p><strong>Human immunodeficiency virus (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4501\">HIV<\/a>)<\/strong> is the most common cause of immunodeficiency in the world today. HIV infections of human hosts are a relatively recent phenomenon. Scientists think that the virus originally infected monkeys, but then jumped to human populations. most likely from a bite, probably sometime during the early to mid-1900s. This most likely occurred in West Africa, but the virus soon spread around the world. HIV was first identified by medical researchers in 1981. Since then, HIV has killed almost 40 million people worldwide, and its economic toll has also been enormous. The hardest hit countries are in Africa, where the virus has infected human populations the longest, and medications to control the virus are least available.\u00a0 In 2016, over 63,000 Canadians were living with HIV.<\/p>\n<h2>HIV Transmission<\/h2>\n<p>HIV is transmitted through direct contact of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3538\">mucous membrane<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\"> or body fluids such as blood, semen, or breast milk. As shown in Figure 17.6.7, transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted from an infected mother\u2019s blood during late pregnancy or childbirth, or through breast milk after birth. In the past, HIV was also transmitted occasionally through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.<\/span><\/p>\n<figure id=\"attachment_5014\" aria-describedby=\"caption-attachment-5014\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5014\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/HIV-transmission-1.jpg\" alt=\"17.6.7 HIV Transmission\" width=\"500\" height=\"351\"><figcaption id=\"caption-attachment-5014\" class=\"wp-caption-text\"><em>Figure 17.6.7 HIV may be transmitted in all of the ways shown here: through sexual activities, from mother to child (in utero, during childbirth, through breastmilk), and by sharing needles.<\/em><\/figcaption><\/figure>\n<h2>HIV and the Immune System<\/h2>\n<p>HIV infects and destroys <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3445\">helper T cell<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">, the type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4832\">lymphocyte<\/span><span style=\"font-size: 1em\">s<\/span><span style=\"font-size: 1em;text-align: initial\"><\/a><\/span><span style=\"font-size: 1em;text-align: initial\"> that regulate the immune response. This process is illustrated in the diagram in Figure 17.6.8. The virus injects its own DNA into a helper T cell and uses the T cell\u2019s \u201cmachinery\u201d to make copies of itself. In the process, the helper T cell is destroyed, and the virus copies go on to infect other helper T cells. HIV is able to evade the immune system and keep destroying helper T cells by mutating frequently so its surface antigens keep changing, and by using the host cell\u2019s membrane to hide its own antigens.<\/span><\/p>\n<figure id=\"attachment_5015\" aria-describedby=\"caption-attachment-5015\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5015\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/HIV-life-cycle-1.jpg\" alt=\"17.6.8 HIV and Helper T Cells\" width=\"500\" height=\"467\"><figcaption id=\"caption-attachment-5015\" class=\"wp-caption-text\"><em>Figure 17.6.8 This diagram shows how HIV infects and destroys helper T cells.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5016\">Acquired immunodeficiency syndrome<\/a> (AIDS)<\/strong> may result from years of damage to the immune system by HIV. It occurs when helper T cells fall to a very low level and opportunistic diseases occur. Opportunistic diseases are infections and tumors that are rare, except in people with a damaged immune system. The diseases take advantage of the \u201copportunity\u201d presented by people whose immune system cannot fight back. Opportunistic diseases are usually the direct cause of death for people with AIDS.<\/p>\n<h2>Treating HIV\/AIDS<\/h2>\n<p>For patients who have access to HIV medications, infection with the virus is no longer the death sentence\u00a0that it once was. By 1995, combinations of drugs called \u201chighly active antiretroviral therapy\u201d were developed. For some patients, these drugs can reduce the amount of virus they are carrying to undetectable levels. However, some\u00a0level\u00a0of virus always hides in the body\u2019s immune cells, and it will multiply again if a patient stops taking the medications. Researchers are trying to develop drugs to kill these hidden viruses, as well. If their efforts are successful, it could end AIDS.<\/p>\n<div>\n<h1>Feature: Human Biology in the News<\/h1>\n<\/div>\n<p>EpiPens\u00ae and their sole manufacturer (pharmaceutical company Mylan) were featured in headlines in 2016, but not for a good reason. The media\u00a0outburst\u00a0was triggered by a drastic price hike in EpiPens\u00ae \u2014 and Mylan\u2019s apparent greed.<\/p>\n<figure id=\"attachment_5017\" aria-describedby=\"caption-attachment-5017\" style=\"width: 290px\" class=\"wp-caption alignleft\"><img class=\"wp-image-5017\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Epipen-by-Stock-Catalog-on-flickr-e1592884964782-1.jpg\" alt=\"17.6.9 Epipens\" width=\"290\" height=\"375\"><figcaption id=\"caption-attachment-5017\" class=\"wp-caption-text\"><em>Figure 17.6.9 An Epipen\u00ae is a hypodermic device that administers a dose of epinephrine, used for the emergency treatment of an acute allergic reaction.<\/em><\/figcaption><\/figure>\n<p>EpiPens\u00ae are auto-injectable syringes preloaded with a measured dose of epinephrine, a drug that can rapidly stop a life-threatening anaphylactic response to an allergen. Using the device is easy and does not require any special training. The injector just needs to be jammed against the thigh, which can be done through clothing or on bare skin. Each year, doctors write millions of prescriptions for EpiPens\u00ae. Many people with severe allergies always carry two of the devices with them, just in case they experience anaphylaxis, although most of them never need to use them. Other people with severe allergies have literally had their lives saved multiple times by EpiPens\u00ae when they had anaphylactic reactions. Even when the devices haven\u2019t been used, they must be replaced each year due to expiration of the epinephrine.<\/p>\n<p>You might think that EpiPens\u00ae would be relatively inexpensive, given their life-saving potential. As recently as 2009, a two-pack of EpiPens\u00ae cost about $100. However, in just seven years, the cost of the same two-pack of EpiPens\u00ae skyrocketed by an incredible 400%! By 2016, the cost was $600 or more. Mylan apparently raised the price for the sole purpose of increasing profits. The company also raised prices significantly on many other drugs. The price hike in EpiPens\u00ae alone was certainly profitable. In 2015, the sale of EpiPens\u00ae earned Mylan $1 billion. Mylan\u2019s CEO took home almost $19 million the same year, which was an increase of more than 600% over her prior salary.<\/p>\n<p>News coverage of the price hike in EpiPens\u00ae began in the summer of 2016 after a price increase in May of that year. Both private citizens and elected officials expressed outrage over the price increase, especially when coupled with the gluttonous profits of the company and its CEO. By late August, Mylan responded to the backlash by offering discount coupons for EpiPens\u00ae. A few days later, the company promised to introduce a cheaper, generic version of the device. Analysts quickly determined that selling a generic version would allow Mylan to make more money on the product than reducing the price of the name-brand device, which they still declined to do. By September of 2016, Mylan was being investigated for antitrust violations\u00a0related\u00a0to sales of EpiPens\u00ae to public schools in New York City.<\/p>\n<p>The Mylan\/EpiPen\u00ae story may still be making the news. But whatever its outcome, the story has already added fuel to public and private debates about important ethical issues\u00a0\u2014\u00a0issues such as the excessive costs of life-saving drugs and the huge profits of big pharma. What is the most recent news on EpiPens\u00ae and Mylan? If you are interested, you can check the headlines online to find out. What are your views on the ethical issues they raise?<\/p>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">17.6 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4877\">allergy<\/a> is a disorder in which the immune system makes an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4860\">inflammatory response<\/a> to a harmless antigen. Any <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2704\">antigen<\/a> that causes allergies is called an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4999\">allergen<\/a>. Common allergens include pollen, dust mites, mold, specific foods (such as peanuts), insect stings, and certain medications (such as aspirin).<\/li>\n<li>The prevalence of allergies has been increasing for decades, especially in developed countries, where they are much more common than in developing countries. The hygiene hypothesis posits that this has occurred because humans evolved to cope with more pathogens than we now typically face in our relatively sterile environments in developed countries. As a result, the immune system \u201ckeeps busy\u201d by attacking harmless antigens.<\/li>\n<li>Allergies occur when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3447\">B cells<\/a>\u00a0are first activated to produce large amounts of antibodies to an otherwise harmless allergen, and the antibodies attach to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4881\">mast cells<\/a>. On subsequent exposures to the allergen, the mast cells immediately release <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4865\">cytokines<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4879\">histamines<\/a>\u00a0that cause inflammation.<\/li>\n<li>Mild allergy symptoms are frequently treated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3377\">antihistamines<\/a>\u00a0that counter histamines and reduce allergy symptoms. A severe systemic allergic reaction, called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5002\">anaphylaxis<\/a>, is a medical emergency that is usually treated with injections of epinephrine. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5004\">Immunotherapy<\/a> for allergies involves injecting increasing amounts of allergens to desensitize the immune system to them.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5006\">Autoimmune diseases<\/a> occur when the immune system fails to recognize the body\u2019s own molecules as self and attacks them, causing damage to tissues and organs. A family history of autoimmunity and female gender are risk factors for autoimmune diseases.<\/li>\n<li>In some autoimmune diseases, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3507\">type I diabetes<\/a>, the immune system attacks and damages specific body cells. In other autoimmune diseases, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5009\">systemic lupus erythematosus<\/a>, many different tissues and organs may be attacked and injured. Autoimmune diseases generally cannot be cured, but their symptoms can often be managed with drugs or other treatments.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5011\">Immunodeficiency<\/a> occurs when the immune system is not working properly, generally because one or more of its components are inactive. As a result, the immune system is unable to fight off pathogens or cancers that a normal immune system would be able to resist.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5012\">Primary immunodeficiency<\/a> is present at birth and caused by rare genetic diseases. An example is severe combined immunodeficiency. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5013\">Secondary immunodeficiency<\/a> occurs because of some event or exposure experienced after birth. Possible causes include aging, certain medications, infections with pathogens, and other disorders, such as obesity or malnutrition.<\/li>\n<li>The most common cause of immunodeficiency in the world today is human immunodeficiency virus (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4501\">HIV<\/a>), which infects and destroys helper T cells. HIV is transmitted through mucous membranes or body fluids. The virus may eventually lead to such low levels of helper T cells that opportunistic infections occur. When this happens, the patient is diagnosed with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5016\">acquired immunodeficiency syndrome<\/a> (AIDS). Medications can control the multiplication of HIV in the human body \u2014 but they don't eliminate the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4283\">virus<\/a> completely.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">17.6 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>\n<div id=\"h5p-204\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-204\" class=\"h5p-iframe\" data-content-id=\"204\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Fermentation\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>How does immunotherapy for allergies work?<\/li>\n<li>What are autoimmune diseases?<\/li>\n<li>Identify two risk factors for autoimmune diseases.<\/li>\n<li>Autoimmune diseases may be specific to particular tissues, or they may be systemic. Give an example of each type of autoimmune disease.<\/li>\n<li>What is immunodeficiency? Compare and contrast primary and secondary immunodeficiency. Give an example of each.<\/li>\n<li>What is the most common cause of immunodeficiency in the world today? How does this affect the immune system?<\/li>\n<li>Distinguish between HIV and AIDS.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">17.6 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/-q7Fz7NIMWM<\/p>\n<p style=\"text-align: center\">Why do people have seasonal allergies? - Eleanor Nelsen, TED-Ed, 2016.<\/p>\n<p>https:\/\/youtu.be\/0TipTogQT3E<\/p>\n<p style=\"text-align: center\">Why it\u2019s so hard to cure HIV\/AIDS - Janet Iwasa, TED-Ed, 2015.<\/p>\n<p>https:\/\/youtu.be\/pJa6KVLwl9U<\/p>\n<p style=\"text-align: center\">The Boy in the Bubble | Retro Report | The New York Times, 2015.<\/p>\n<p>https:\/\/youtu.be\/Mjr9h_QmdeM<\/p>\n<p style=\"text-align: center\">Why Are Peanut Allergies Becoming So Common? Seeker, 2014.<\/p>\n<p>https:\/\/youtu.be\/RiMSmDBvgto<\/p>\n<p style=\"text-align: center\">What Are Tonsil Stones? | Gross Science, 2015.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h2>Attributions<\/h2>\n<p><strong>Figure 17.6.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Oedema.JPG\" rel=\"cc:attributionURL\">Oedema<\/a> by <a class=\"new\" title=\"User:Championswimmer (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Championswimmer&amp;action=edit&amp;redlink=1\">Championswimmer<\/a> on Wikimedia Commons is in the <a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/en:public_domain).<\/p>\n<p><strong>Figure 17.6.2<\/strong><\/p>\n<ul>\n<li><a href=\"http:\/\/pngimg.com\/download\/81708\">Medical (latex) gloves<\/a> from pngimg.com is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/\">CC BY-NC 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/) license.<\/li>\n<li><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:House_dust_mites_(5247996458).jpg\">House dust mites (5247996458)<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/9082612@N05\" rel=\"nofollow\">Gilles San Martin<\/a> from Namur, Belgium on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/deed.en\">CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/deed.en) license.<\/li>\n<li><a href=\"https:\/\/www.flickr.com\/photos\/krayker\/4312985916\/\">\u00a0Honey bee macro<\/a> by <a class=\"owner-name truncate no-outline\" title=\"Go to Karunakar Rayker's photostream\" href=\"https:\/\/www.flickr.com\/photos\/krayker\/\" data-track=\"attributionNameClick\">Karunakar Rayker<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a<a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\"> CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/li>\n<li><a href=\"https:\/\/www.pexels.com\/photo\/brown-nuts-on-white-surface-4202959\/\">Peanuts<\/a> by <a href=\"https:\/\/www.pexels.com\/@karolina-grabowska\">Karolina Grabowska<\/a> on <a href=\"http:\/\/pexels.com\">Pexels<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\">CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/).<\/li>\n<li><a href=\"https:\/\/pxhere.com\/en\/photo\/1597775\">Photo of dog and grey cat<\/a> by <a class=\"profile-link\" href=\"https:\/\/pxhere.com\/en\/photographer\/2204651\">ERC4N51<\/a> on pxhere is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\">CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/).<\/li>\n<li><a href=\"https:\/\/pixabay.com\/photos\/pollen-allergy-spring-nature-4845841\/\">Tags: Pollen Allergy Spring<\/a>\u00a0by <a class=\"hover_opacity\" href=\"https:\/\/pixabay.com\/users\/Castagnari53-15149183\/\">Castagnari53<\/a> on <a href=\"http:\/\/pixabay.com\">Pixabay<\/a>, is used under the <a href=\"https:\/\/pixabay.com\/service\/license\/\">Pixabay License<\/a> (https:\/\/pixabay.com\/service\/license\/).<\/li>\n<\/ul>\n<p><strong>Figure 17.6.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Mast_cells.jpg\" rel=\"cc:attributionURL\">512px-Mast_cells<\/a> by <a href=\"https:\/\/scholarworks.iupui.edu\/bitstream\/handle\/1805\/748\/Understanding%20the%20immune%20system.pdf?sequence=1&amp;isAllowed=y\">National Institute of Allergy and Infectious Diseases (U.S.) &amp; National Cancer Institute<\/a> (p.29) is in\u00a0the <a class=\"extiw\" title=\"w:en:public domain\" href=\"https:\/\/en.wikipedia.org\/wiki\/en:public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/en:public_domain).<\/p>\n<p><strong>Figure 17.6.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Signs_and_symptoms_of_anaphylaxis.png\" rel=\"cc:attributionURL\">Signs_and_symptoms_of_anaphylaxis<\/a> by <a title=\"User:Mikael H\u00e4ggstr\u00f6m\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Mikael_H%C3%A4ggstr%C3%B6m\">Mikael H\u00e4ggstr\u00f6m<\/a> on Wikimedia Commons is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\">CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/).<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 17.6.5<\/strong><\/p>\n<p><a href=\"https:\/\/flic.kr\/p\/5XtivM\" rel=\"cc:attributionURL\">Allergy Tests<\/a> by <a class=\"owner-name truncate\" title=\"Go to Dan Pupius's photostream\" href=\"https:\/\/www.flickr.com\/photos\/dpup\/\" data-track=\"attributionNameClick\">Dan Pupius<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/\" rel=\"license\">CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.<\/p>\n<p><strong>Figure 17.6.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Symptoms_of_SLE.png\" rel=\"cc:attributionURL\">1024px-Symptoms_of_SLE<\/a> by <a class=\"extiw\" title=\"en:User:Mikael H\u00e4ggstr\u00f6m\" href=\"https:\/\/en.wikipedia.org\/wiki\/User:Mikael_H%C3%A4ggstr%C3%B6m\">Mikael H\u00e4ggstr\u00f6m<\/a> on Wikimedia Commons\u00a0is used under a <a href=\"https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\">CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/).<\/p>\n<p><strong>Figure 17.6.7<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/\" rel=\"cc:attributionURL\">HIV transmission<\/a>\u00a0by <a href=\"https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\">CC BY NC 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<\/p>\n<div><img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/?_ga=2.264717007.1988667202.1598996862-1392032960.1592870652\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><\/div>\n<p><strong>Figure 17.6.8<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/\" rel=\"cc:attributionURL\">HIV life cycle<\/a> by <a href=\"https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/\">CK-12 Foundation<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\">CC BY NC 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.<\/p>\n<p><img src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> <span style=\"font-size: 1em\">\u00a9<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/\">CK-12 Foundation<\/a> <span style=\"font-size: 1em\">Licensed under\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\"><\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/terms-of-use\/?_ga=2.264717007.1988667202.1598996862-1392032960.1592870652\">Terms of Use<\/a><span style=\"font-size: 1em\">\u00a0\u2022\u00a0<\/span><a style=\"font-size: 1em\" href=\"http:\/\/www.ck12.org\/about\/attribution\/\">Attribution<\/a><\/p>\n<p><strong>Figure 17.6.9<\/strong><\/p>\n<p><a href=\"https:\/\/flic.kr\/p\/23Ju8FC\" rel=\"cc:attributionURL\">Epipen by Stock Catalog on flickr<\/a> by <a class=\"owner-name truncate\" title=\"Go to Stock Catalog's photostream\" href=\"https:\/\/www.flickr.com\/photos\/stockcatalog\/\" data-track=\"attributionNameClick\">Stock Catalog<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/2.0\/\">CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). Figure 8 HIV may be transmitted in all of the ways shown here [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 19.6) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/<\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). Figure 9 This diagram shows how HIV infects and destroys helper T cells [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 19.6) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-human-biology\/section\/19.6\/<\/p>\n<p class=\"hanging-indent\">CBS News. (2016, August 16). Rising cost of potentially life-saving EpiPen puts pinch on families [online article]. CBS Interactive Inc. https:\/\/www.cbsnews.com\/news\/allergy-medication-epipen-epinephrine-rising-costs-impact-on-families\/<\/p>\n<p class=\"hanging-indent\">Gross Science. (2015, June 29). What are tonsil stones? | Gross Science. YouTube. https:\/\/www.youtube.com\/watch?v=RiMSmDBvgto&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">H\u00e4ggstr\u00f6m, M. (2014). Medical gallery of Mikael H\u00e4ggstr\u00f6m 2014.\u00a0<i>WikiJournal of Medicine<\/i>\u00a0<b>1<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.008.\u00a0ISSN\u00a02002-4436<\/p>\n<p class=\"hanging-indent\">National Institute of Allergy and Infectious Diseases (NIAID). (n.d.). Severe combined immunodeficiency (SCID) [online article]. National Institute of Health (NIH). https:\/\/www.niaid.nih.gov\/diseases-conditions\/severe-combined-immunodeficiency-scid<\/p>\n<p class=\"hanging-indent\">National Institute of Allergy and Infectious Diseases (U.S.) &amp; National Cancer Institute (U.S.). (2003, September). Understanding the immune system and how it works [NIH Publication No. 03-5423]. <em>Scholar Works<\/em> - \u00a0Indiana University. https:\/\/scholarworks.iupui.edu\/handle\/1805\/748<\/p>\n<p class=\"hanging-indent\">[The] New York Times. (2015, December 15). The boy in the bubble | Retro Report | The New York Times. YouTube. https:\/\/www.youtube.com\/watch?v=pJa6KVLwl9U&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Seeker. (2014, October 3). Why are peanut allergies becoming so common? YouTube. https:\/\/www.youtube.com\/watch?v=Mjr9h_QmdeM&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Summary: Estimates of HIV incidence, prevalence, and Canada's progress on meeting the 90-90-90 HIV targets 2016. (2018, July). Public Health Agency of Canada. https:\/\/www.canada.ca\/content\/dam\/phac-aspc\/documents\/services\/publications\/diseases-conditions\/summary-estimates-hiv-incidence-prevalence-canadas-progress-90-90-90\/pub-eng.pdf<\/p>\n<p class=\"hanging-indent\">Swetlitz, I., Silverman, E. (2016, August 25). Mylan may have violated antitrust law in its EpiPen sales to schools, legal experts say [online article]. STATNews.com. https:\/\/www.statnews.com\/2016\/08\/25\/mylan-antitrust-epipen-schools\/<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2016, May 26). Why do people have seasonal allergies? - Eleanor Nelsen. YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=-q7Fz7NIMWM&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, March 16). Why it\u2019s so hard to cure HIV\/AIDS - Janet Iwasa, https:\/\/www.youtube.com\/watch?v=0TipTogQT3E&amp;feature=youtu.be<\/p>\n<p>&nbsp;<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4971_4102\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4971_4102\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<div id=\"h5p-215\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-215\" class=\"h5p-iframe\" data-content-id=\"215\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Cell Biology Multiple Choice\"><\/iframe><\/div>\n<\/div>\n<p><em>Figure 18.10.1 Families all have something in common.<\/em><\/p>\n<div>\n<h1>Family Portrait<\/h1>\n<\/div>\n<p>What do all these families (Figure 18.10.1) have in common?\u00a0 They were born!\u00a0 Every person on this planet was conceived, carried in utero and then born.\u00a0 While families come in all shapes, sizes and styles, we all came into existence in the same way.\u00a0 Virtually all human societies past and present \u2014 value having children. Indeed, for many people, parenthood is an important life goal. Unfortunately, some people are unable to achieve that goal because of infertility.<\/p>\n<div>\n<h1>What Is Infertility?<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4813\">Infertility<\/a><\/strong> is the inability of a sexually mature adult to reproduce by natural means. For scientific and medical purposes, infertility is generally defined as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse. Infertility may be primary or secondary. Primary infertility applies to cases in which an individual has never achieved a successful pregnancy. Secondary infertility applies to cases in which an individual has had at least one successful pregnancy, but fails to achieve another after trying for at least a year. Infertility is a common problem. The government of Canada reported that in 2019, 16% of Canadian couples experience infertility, a number which has doubled since the 1980s. If you look around at the couples you know, that means that almost 1 in 6 of them are having issues with fertility.<\/p>\n<div>\n<h1>Causes of Infertility<\/h1>\n<\/div>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5280\">Pregnancy<\/a> is the result of a multi-step process. In order for a normal pregnancy to occur, a woman must release an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5099\">ovum<\/a> from one of her <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a>, the ovum must go through an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviduct<\/a>, a man\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a> must fertilize the ovum as it passes through the oviduct, and then the resulting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2471\">zygote<\/a> must implant in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>. If there is a problem with any of these steps, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4813\">infertility<\/a> can result.<\/p>\n<p>A couple\u2019s infertility may be due to a problem with either the male or the female partner. As shown in the circle graph below (Figure 18.10.2), about 40% of infertility cases are due to female infertility, and about 30% are due to male infertility. The remaining 30% of cases are due to a combination of male and female problems or unknown causes.<\/p>\n<figure id=\"attachment_5342\" aria-describedby=\"caption-attachment-5342\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-5342 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Causes-of-infertility-in-Canada-1.png\" alt=\"18.10.2 Causes of Infertility\" width=\"900\" height=\"515\"><figcaption id=\"caption-attachment-5342\" class=\"wp-caption-text\"><em>Figure 18.10.2 This graph shows that infertility affects males as often as females, and that the cause of infertility often is unexplained.<\/em><\/figcaption><\/figure>\n<h2>Causes of Male Infertility<\/h2>\n<p>Male infertility occurs when there are no, or too few, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a>, or when the sperm are not healthy and motile and cannot travel through the female reproductive tract to fertilize an egg. A common cause of inadequate numbers or motility of sperm is varicocele, which is enlargement of blood vessels in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5089\">scrotum<\/a>. This may raise the temperature of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3486\">testes<\/a> and adversely affect sperm production. In other cases, there is no problem with the sperm, but there is a blockage in the male reproductive tract that prevents the sperm from being ejaculated.<\/p>\n<p>Factors that increase a man\u2019s risk of infertility include heavy alcohol use, drug abuse, cigarette smoking, exposure to environmental toxins (such as pesticides or lead), certain medications, serious diseases (such as kidney disease), and radiation or chemotherapy for cancer. Another risk factor is advancing age. Male fertility normally peaks in the mid-twenties and gradually declines after about age 40, although it may never actually drop to zero.<\/p>\n<h2>Causes of Female Infertility<\/h2>\n<p>Female infertility generally occurs due to one of two problems: failure to produce viable <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5099\">ova<\/a> by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3487\">ovaries<\/a>, or structural problems in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5101\">oviduct<span style=\"font-size: 1em\">s<\/span><span style=\"text-align: initial;font-size: 1em\"><\/a><\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5102\">uterus<\/a>. The most common cause of female infertility is a problem with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a>. Without ovulation, there are no ova to be fertilized. Anovulatory cycles (menstrual cycles in which ovulation does not occur) may be associated with no or irregular menstrual periods, but even regular menstrual periods may be anovulatory for a variety of reasons. The most common cause of anovulatory cycles is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_3485\"><strong>polycystic ovary syndrome (PCOS)<\/strong><\/a>, which causes hormone imbalances that can interfere with normal ovulation. Another relatively common cause of anovulation is primary ovarian insufficiency. In this condition, the ovaries stop working normally and producing viable eggs at a relatively early age, generally before the age of 40.<\/span><\/p>\n<p>Structural problems with the oviducts or uterus are less common causes of female infertility. The oviducts may be blocked as a result of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5333\">endometriosis<\/a>. Another possible cause is pelvic inflammatory disease, which occurs when sexually transmitted infections spread to the oviducts or other female reproductive organs (see Figure 18.10.3). The infection may lead to scarring and blockage of the oviducts. If an ovum is produced and the oviducts are functioning \u2014 and a woman has a condition such as uterine fibroids \u2014 implantation in the uterus may not be possible. Uterine fibroids are non-cancerous clumps of tissue and muscle that form on the walls of the uterus.<\/p>\n<figure id=\"attachment_5344\" aria-describedby=\"caption-attachment-5344\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-5344\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Blausen_0719_PelvicInflammatoryDisease-1.png\" alt=\"18.10.3 Pelvic inflammatory disease\" width=\"1024\" height=\"768\"><figcaption id=\"caption-attachment-5344\" class=\"wp-caption-text\"><em>Figure 18.10.3 An infection of the Fallopian tubes may cause scarring and blockage of the tubes, so sperm cannot reach eggs for fertilization.<\/em><\/figcaption><\/figure>\n<p>Factors that increase a woman\u2019s risk of infertility include tobacco smoking, excessive use of alcohol, stress, poor diet, strenuous athletic training, and being overweight or underweight. Advanced age is even more problematic for females than males. Female fertility normally peaks in the mid-twenties, and continuously declines after age 30 and until menopause around the age of 52, after which the ovary no longer releases eggs. About 1\/3 of couples in which the woman is over age 35 have fertility problems. In older women, more cycles are likely to be anovulatory, and the eggs may not be as healthy.<\/p>\n<h1>Diagnosing Causes of Infertility<\/h1>\n<p>Diagnosing the cause(s) of a couple\u2019s infertility often requires testing both the man and the woman for potential problems. In men, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4885\">semen<\/a> is likely to be examined for the number, shape, and motility of sperm. If problems are found with sperm, further studies are likely to be done, such as medical imaging to look for structural problems with the testes or ducts.<\/p>\n<p>In women, the first step is most often determining whether <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5239\">ovulation<\/a> is occurring. This can be done at home by carefully monitoring body temperature (it rises slightly around the time of ovulation) or using a home ovulation test kit, which is available over the counter at most drugstores. Whether or not ovulation is occurring can also be detected with blood tests or ultrasound imaging of the ovaries. If ovulation is occurring normally, then the next step may be an X-ray of the oviducts and uterus to see if there are any blockages or other structural problems. Another approach to examining the female reproductive tract for potential problems is laparoscopy. In this surgical procedure, a tiny camera is inserted into the woman\u2019s abdomen through a small incision. This allows the doctor to directly inspect the reproductive organs.<\/p>\n<div>\n<h1>Treating Infertility<\/h1>\n<\/div>\n<p>Infertility often can be treated successfully. The type of treatment depends on the cause of infertility.<\/p>\n<h2>Treating Male Infertility<\/h2>\n<p>Medical problems that interfere with sperm production may be treated with medications or other interventions that may lead to the resumption of normal sperm production.\u00a0If, for example,\u00a0an infection is interfering with sperm production, then antibiotics that clear up the infection may resolve the problem. If there is a blockage in the male reproductive tract that prevents the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5150\">ejaculation<\/a> of sperm, surgery may be able to remove the blockage. Alternatively, the man\u2019s sperm may be removed from his body and then used for artificial insemination of his partner. In this procedure, the sperm are injected into the woman\u2019s reproductive tract.<\/p>\n<h2>Treating Female Infertility<\/h2>\n<p>In females, it may be possible to correct blocked Fallopian tubes or uterine fibroids with surgery. Ovulation problems, on the other hand, are usually treated with hormones that act either on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2938\">pituitary gland<\/a> or on the ovaries. Hormonal treatments that stimulate ovulation often result in more than one egg being ovulated at a time, thus increasing the chances of a woman having twins, triplets, or even higher multiple births. Multiple fetuses are at greater risk of being born too early or having health and developmental problems. The mother is also at greater risk of complications arising during pregnancy. Therefore, the possibility of multiple fetuses should be weighed in making a decision about this type of infertility treatment.<\/p>\n<h2>Assisted Reproductive Technology<\/h2>\n<p>Some cases of infertility are treated with\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5345\">assisted reproductive technology (ART)<\/a><\/strong>. This is a collection of medical procedures in which ova are removed from the woman\u2019s body and sperm are taken from the man\u2019s body to be manipulated in ways that increase the chances of fertilization occurring. The eggs and sperm may be injected into one of the woman\u2019s oviducts for fertilization to take place <em>in vivo<\/em>\u00a0(in the body). More commonly, however, the eggs and sperm are mixed together outside the body so fertilization takes place\u00a0<em>in vitro<\/em> (in a test tube or dish in a lab). The latter approach is illustrated in Figure 18.10.4. With <em>in vitro <\/em>fertilization, the fertilized eggs may be allowed to develop into embryos before being placed in the woman\u2019s uterus.<\/p>\n<figure id=\"attachment_5346\" aria-describedby=\"caption-attachment-5346\" style=\"width: 508px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-5346\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Blausen_0060_AssistedReproductiveTechnology-1.png\" alt=\"18.10.4 Assisted Reproductive Technology\" width=\"508\" height=\"381\"><figcaption id=\"caption-attachment-5346\" class=\"wp-caption-text\"><em>Figure 18.10.4 This figure shows one way ART procedures may be used to treat infertility. An egg is removed from the female reproductive tract, injected with sperm from her partner, and allowed to develop into an embryo in the lab. Then, the embryo is placed inside the woman\u2019s uterus for implantation and development during the remainder of gestation.<\/em><\/figcaption><\/figure>\n<p>ART has about a 40% chance of leading to a live birth in women under the age of 35, but only about a 20%t chance of success in women over the age of 35. Some studies have found a higher-than-average risk of birth defects in children produced by ART procedures, but this may be due to the generally higher ages of the parent \u2014 not the technologies used.<\/p>\n<h2>Other Approaches<\/h2>\n<p>Other approaches for certain causes of infertility include the use of a surrogate mother, a gestational carrier, or sperm donation.<\/p>\n<ul>\n<li>A\u00a0<strong>surrogate mother<\/strong>\u00a0is a woman who agrees to become pregnant using the man\u2019s sperm and her own egg. The child, who will be the biological offspring of the surrogate and the male partner, is given up at birth for adoption by the couple. Surrogacy might be selected by women with no eggs or unhealthy eggs.\u00a0A\u00a0woman who carries a mutant gene for a serious genetic disorder might choose this option to ensure that the defective gene is not passed on to the offspring.<\/li>\n<li>A\u00a0<strong>gestational carrier<\/strong>\u00a0is a woman who agrees to receive a transplanted embryo from a couple and carry it to term. The child, who will be the biological offspring of the couple, is given to the parents at birth. A gestational carrier might be used by women who have normal ovulation but no uterus, or who cannot safely carry a fetus to term because of a serious health problem (such as kidney disease or cancer).<\/li>\n<li><strong>Sperm donation<\/strong>\u00a0is the use of sperm from a fertile man (generally through artificial insemination) for cases in which the male partner in a couple is infertile, or in which a woman seeks to become pregnant without a male partner.\u00a0A\u00a0lesbian couple may use donated sperm to enable one of them to become pregnant and have a child.\u00a0Sperm can be obtained from a sperm bank, which buys and stores sperm for artificial insemination, or a male friend or other individual may donate sperm to a specific woman.<\/li>\n<\/ul>\n<div>\n<h1>Social and Ethical Issues Relating to Infertility<\/h1>\n<\/div>\n<p>For people who have a strong desire for children of their own, infertility may lead to deep disappointment and depression. Individuals who are infertile may even feel biologically inadequate. Partners in infertile couples may argue and feel resentment toward each other, and married couples may get divorced because of infertility. Infertility treatments \u2014 especially ART procedures \u2014 are generally time-consuming and expensive. The high cost of the treatments can put them out of financial reach of many couples.<\/p>\n<h2>Ethical Concerns<\/h2>\n<p>Some people question whether the allocation of medical resources to infertility treatments is justified, and whether the resources could be better used in other ways. The status of embryos that are created\u00a0<em>in vitro<\/em>\u00a0and then not used for a pregnancy is another source of debate. Some people oppose their destruction on religious grounds, and couples may sometimes argue about what should be done with their extra embryos. Ethical issues are also raised by procedures that increase the chances of multiple births, because of the medical and developmental risks associated with multiple births.<\/p>\n<h2>Infertility in Developing Countries<\/h2>\n<p>Infertility is an under-appreciated problem in the poorer nations of the world, because of assumptions about overpopulation problems and high birth rates in developing countries. In fact, infertility is at least as great a problem in developing as in developed countries. High rates of health problems and inadequate health care in the poorer nations increase the risk of infertility. At the same time, infertility treatments are usually not available \u2014 or are far too expensive \u2014 for the vast majority of people who may need them. In addition, in many developing countries, the production of children is highly valued. Children may be needed for family income generation and economic security of the elderly. It is not uncommon for infertility to lead to social stigmatization, psychological problems, and abandonment by spouses.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.10 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_4813\">Infertility<\/a> is the inability of a sexually mature adult to reproduce by natural means. It is defined scientifically and medically as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse.<\/li>\n<li>About 40% of infertility in couples is due to female infertility, and another 30% is due to male infertility. In the remaining cases, a couple\u2019s infertility is due to problems in both partners, or to unknown causes.<\/li>\n<li>Male infertility occurs when there are no, or too few, healthy, motile <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2506\">sperm<\/a>. This may be caused by problems with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_2507\">spermatogenesis<\/a>, or by blockage of the male reproductive tract that prevents sperm from being ejaculated. Risk factors for male infertility include heavy alcohol use, smoking, certain medications, and advancing age, to name just a few.<\/li>\n<li>Female infertility occurs due to failure to produce viable ova by the ovaries, or structural problems in the oviducts or uterus. Polycystic ovary syndrome (PCOS) is the most common cause of failure to produce viable ova. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5333\">Endometriosis<\/a> and uterine fibroids are possible causes of structural problems in the oviducts and uterus. Risk factors for female infertility include smoking, stress, poor diet, and older age, among others.<\/li>\n<li>Diagnosing the cause(s) of a couple\u2019s infertility generally requires testing both the man and the woman for potential problems. For men, semen is likely to be examined for adequate numbers of healthy, motile sperm. For women, signs of ovulation are monitored, for example, with an ovulation test kit or ultrasound of the ovaries. For both partners, the reproductive tract may be medically imaged to look for blockages or other abnormalities.<\/li>\n<li>Treatments for infertility depend on the cause. For example, if a medical problem is interfering with sperm production, medication may resolve the underlying problem so sperm production is restored. Blockages in either the male or the female reproductive tract can often be treated surgically. If there are problems with ovulation, hormonal treatments may stimulate ovulation.<\/li>\n<li>Some cases of infertility are treated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4971_5345\">assisted reproductive technology (ART)<\/a>. This is a collection of medical procedures in which ova and sperm are taken from the couple and manipulated in a lab to increase the chances of fertilization occurring and an embryo forming. Other approaches for certain causes of infertility include the use of a surrogate mother, gestational carrier, or sperm donation.<\/li>\n<li>Infertility can negatively impact a couple socially and psychologically, and it may be a major cause of marital friction or even divorce. Infertility treatments may raise ethical issues relating to the costs of the procedures and the status of embryos that are created\u00a0<em>in vitro, <\/em>but not used for pregnancy. Infertility is an under-appreciated problem in developing countries, where birth rates are high and children have high economic \u2014 as well as social \u2014 value. In these countries, poor health care is likely to lead to more problems with infertility and fewer options for treatment.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.10 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is infertility? How is infertility defined scientifically and medically?<\/li>\n<li>What percentage of infertility in couples is due to male infertility? What percentage is due to female infertility?<\/li>\n<li>Identify causes of and risk factors for male infertility.<\/li>\n<li>Identify causes of and risk factors for female infertility.<\/li>\n<li>How are causes of infertility in couples diagnosed?<\/li>\n<li>How is infertility treated?<\/li>\n<li>Discuss some of the social and ethical issues associated with infertility or its treatment.<\/li>\n<li>Why is infertility an under-appreciated problem in developing countries?<\/li>\n<li>Describe two similarities between causes of male and female infertility.<\/li>\n<li>Explain the difference between males and females in terms of how age affects fertility.<\/li>\n<li>\n<div id=\"h5p-216\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-216\" class=\"h5p-iframe\" data-content-id=\"216\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"12.2 Quiz\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Do you think that taking medication to stimulate ovulation is likely to improve fertility in cases where infertility is due to endometriosis? Explain your answer.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">18.10 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/P27waC05Hdk<\/p>\n<p>How <em>in vitro<\/em> fertilization (IVF) works - Nassim Assefi and Brian A. Levine, TED-Ed, 2015<\/p>\n<p>https:\/\/youtu.be\/6BBmMtVfZ4Y<\/p>\n<p style=\"text-align: center\">A journey through infertility -- over terror's edge | Camille Preston | TEDxBeaconStreet, TEDx Talks, 2014.<\/p>\n<p>https:\/\/youtu.be\/iqA8uAjvEdM<\/p>\n<p style=\"text-align: center\">Smoking Marijuana May Lower Sperm Count by 33%, David Pakman Show, 2015.<\/p>\n<p>https:\/\/youtu.be\/V6-v4eF9dyA<\/p>\n<p style=\"text-align: center\">ivf embryo developing over 5 days by fertility Dr Raewyn Teirney, Fertility Specialist Sydney, 2014.<\/p>\n<p>https:\/\/youtu.be\/4Khn_z9FPmU<\/p>\n<p style=\"text-align: center\">Homosexuality: It's about survival - not sex | James O'Keefe | TEDxTallaght, 2016.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 18.10.1<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/www.flickr.com\/photos\/bobjagendorf\/9177582651\">Gay Pride Parade NYC 2013 - Happy Family<\/a> by <a class=\"owner-name truncate\" title=\"Go to Bob Jagendorf's photostream\" href=\"https:\/\/www.flickr.com\/photos\/bobjagendorf\/\" data-track=\"attributionNameClick\">Bob Jagendorf<\/a> on <a href=\"http:\/\/flickr.com\">Flickr<\/a> is used under a <a href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/\">CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/kf3dLxBql6U\">#beaches #summer #family #blue #water<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@theuntangled_\">Jove Duero<\/a> on <a href=\"https:\/\/unsplash.com\/\">Unsplash<\/a> is used under the <a class=\"ICezk _2GAZm _2WvKc\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/MOqAfi6GvVU\">Photograph of five men near outdoor<\/a> by <a href=\"https:\/\/unsplash.com\/@dollargill\">Dollar Gill<\/a> on <a style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/\">Unsplash<\/a><span style=\"font-size: 1em\"> is used under the <\/span><a class=\"ICezk _2GAZm _2WvKc\" style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a><span style=\"font-size: 1em\"> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/JYJxLjvIGWY\">Fam\u00edlia<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@laerciocavalcanti\">Laercio Cavalcanti<\/a> on <a style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/\">Unsplash<\/a><span style=\"font-size: 1em\"> is used under the <\/span><a class=\"ICezk _2GAZm _2WvKc\" style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a><span style=\"font-size: 1em\"> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/GmDVGjqeVEk\">Happiness \ud83d\ude42<\/a> by <a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@suicide_chewbacca\">Ashwini Chaudhary<\/a> on <a style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/\">Unsplash<\/a><span style=\"font-size: 1em\"> is used under the <\/span><a class=\"ICezk _2GAZm _2WvKc\" style=\"font-size: 1em\" href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a><span style=\"font-size: 1em\"> (https:\/\/unsplash.com\/license).\u00a0<\/span><\/li>\n<\/ul>\n<p><strong>Figure 18.10.2<\/strong><\/p>\n<p>Causes of infertility in Canada by Christine Miller is in the <a href=\"https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/\" rel=\"license\">Public Domain<\/a> (https:\/\/creativecommons.org\/publicdomain\/mark\/1.0\/).<\/p>\n<p><strong>Figure 18.10.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0719_PelvicInflammatoryDisease.png\" rel=\"cc:attributionURL\">1024px-Blausen_0719_PelvicInflammatoryDisease<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a><span style=\"text-align: initial;font-size: 1em\"> on Wikimedia Commons is used under a <a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span><\/p>\n<p><strong>Figure 18.10.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0060_AssistedReproductiveTechnology.png\" rel=\"cc:attributionURL\">1024px-Blausen_0060_AssistedReproductiveTechnology<\/a> by <a title=\"User:BruceBlaus\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:BruceBlaus\">BruceBlaus<\/a><span style=\"text-align: initial;font-size: 1em\"> on Wikimedia Commons is used under a <a style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/creativecommons.org\/licenses\/by\/3.0\" rel=\"license\">CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span><\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\">Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. <em>WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\n<p class=\"hanging-indent\">David Pakman Show. (2015, September 1). Smoking marijuana may lower sperm count by 33%. YouTube. https:\/\/www.youtube.com\/watch?v=iqA8uAjvEdM<\/p>\n<p class=\"hanging-indent\">Fertility Specialist Sydney. (2014, April 11). ivf embryo developing over 5 days by fertility Dr Raewyn Teirney. YouTube. https:\/\/www.youtube.com\/watch?v=V6-v4eF9dyA&amp;t=5s<\/p>\n<p class=\"hanging-indent\">Public Health Agency of Canada. (2019, May 28). Fertility. Government of Canada. https:\/\/www.canada.ca\/en\/public-health\/services\/fertility\/fertility.html<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, May 7). How <em>in vitro<\/em> fertilization (IVF) works - Nassim Assefi and Brian A. Levine.\u00a0 YouTube. https:\/\/www.youtube.com\/watch?v=P27waC05Hdk&amp;t=4s<\/p>\n<p class=\"hanging-indent\">TEDx Talks. (2014, June 26). A journey through infertility -- over terror's edge | Camille Preston | TEDxBeaconStreet. YouTube. https:\/\/www.youtube.com\/watch?v=6BBmMtVfZ4Y&amp;t=2s<\/p>\n<p class=\"hanging-indent\">TEDx Talks. (2016, November 15). Homosexuality: It's about survival - not sex | James O'Keefe | TEDxTallaght. YouTube. https:\/\/www.youtube.com\/watch?v=4Khn_z9FPmU&amp;t=1s<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":32,"menu_order":8,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by-nc"},"chapter-type":[48],"contributor":[],"license":[55],"class_list":["post-4971","chapter","type-chapter","status-publish","hentry","chapter-type-numberless","license-cc-by-nc"],"part":4907,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters\/4971","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/users\/32"}],"version-history":[{"count":4,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters\/4971\/revisions"}],"predecessor-version":[{"id":6465,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters\/4971\/revisions\/6465"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/parts\/4907"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters\/4971\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/media?parent=4971"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapter-type?post=4971"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/contributor?post=4971"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/license?post=4971"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}