{"id":4943,"date":"2019-06-24T16:27:19","date_gmt":"2019-06-24T16:27:19","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/13-4-structure-of-bone-3\/"},"modified":"2023-11-30T18:56:58","modified_gmt":"2023-11-30T18:56:58","slug":"13-4-structure-of-bone-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/13-4-structure-of-bone-3\/","title":{"raw":"11.4\u00a0Structure of Bone","rendered":"11.4\u00a0Structure of Bone"},"content":{"raw":"&nbsp;\r\n\r\n[caption id=\"attachment_4020\" align=\"aligncenter\" width=\"400\"]<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_19098148350-2.jpg\" alt=\"11.4.1 Roasted Bone Marrow\" width=\"400\" height=\"267\" \/> <em>Figure 11.4.1 Roasted bone marrow.<\/em>[\/caption]\r\n\r\n<div>\r\n<h1>Roasted Bone Marrow<\/h1>\r\n<\/div>\r\nDo you recognize the food item in the top left of Figure 11.4.1? It\u2019s roasted [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary], 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.\r\n<div>\r\n<h1>What Are Bones?<\/h1>\r\n<\/div>\r\n<strong>[pb_glossary id=\"5913\"]Bones[\/pb_glossary]<\/strong>\u00a0are organs that consist primarily of bone tissue, also called osseous tissue.\u00a0<strong>[pb_glossary id=\"4022\"]Bone tissue[\/pb_glossary]<\/strong>\u00a0is a type of connective tissue consisting mainly of a [pb_glossary id=\"3970\"]collagen[\/pb_glossary] 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.\r\n<div>\r\n<h1>Bone Anatomy<\/h1>\r\n<\/div>\r\nThere 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.\r\n<h2>Types of Osseous Tissue<\/h2>\r\nThe 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.\r\n\r\nFlat bones are typically enveloped by compact bone, with a center of spongy bone.\r\n<table class=\"no-lines aligncenter\" style=\"border-collapse: collapse; width: 48.1291%; height: 620px;\" border=\"0\">\r\n<tbody>\r\n<tr style=\"height: 305px;\">\r\n<td style=\"width: 50%; height: 305px;\">\r\n\r\n[caption id=\"attachment_4027\" align=\"aligncenter\" width=\"588\"]<img class=\"wp-image-4027\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cross-section.svg_-2.png\" alt=\"11.4.2 Bone Cross Section\" width=\"588\" height=\"294\" \/> <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>[\/caption]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 50%;\">\r\n<div class=\"mceTemp\"><\/div>\r\n\r\n[caption id=\"attachment_4034\" align=\"aligncenter\" width=\"678\"]<img class=\"wp-image-4034\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anatomy_of_a_Flat_Bone-2.jpg\" alt=\"11.4.3 Anatomy of a flat bone\" width=\"678\" height=\"343\" \/> <em>Figure 11.4.3 Flat bones are typically enveloped by compact bone, with a center of spongy bone.<\/em>[\/caption]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div style=\"text-align: center;\">\r\n\r\n&nbsp;\r\n\r\n<\/div>\r\n<strong>[pb_glossary id=\"4030\"]Compact bone tissue[\/pb_glossary]<\/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>[pb_glossary id=\"4031\"]Spongy bone tissue[\/pb_glossary]<\/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.\r\n<table class=\"aligncenter\" style=\"border-collapse: collapse; width: 100%; height: 322px;\" border=\"0\">\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 50%;\">\r\n\r\n[caption id=\"attachment_4028\" align=\"aligncenter\" width=\"414\"]<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\" \/> <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>[\/caption]<\/td>\r\n<td style=\"width: 50%;\">\r\n\r\n[caption id=\"attachment_4033\" align=\"aligncenter\" width=\"253\"]<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-2.png\" alt=\"11.4.5 Spongy Bone Diagram\" width=\"253\" height=\"257\" \/> <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>[\/caption]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\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>\r\n\r\n<h2>Other Tissues in Bones<\/h2>\r\nBesides compact and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum.\r\n<ul>\r\n \t<li><strong>[pb_glossary id=\"4021\"]Bone marrow[\/pb_glossary]<\/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 [pb_glossary id=\"3961\"]hematopoiesis[\/pb_glossary].<\/li>\r\n \t<li><strong>[pb_glossary id=\"4039\"]Periosteum[\/pb_glossary]<\/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>\r\n<\/ul>\r\n<h2>Bone Cells<\/h2>\r\nAs shown in Figure 11.4.6, bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.\r\n<ul>\r\n \t<li><strong>[pb_glossary id=\"4040\"]Osteoblasts[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4041\"]Osteocytes[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4042\"]Osteoclasts[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4043\"]Osteogenic cells[\/pb_glossary]<\/strong>\u00a0are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4048\" align=\"alignnone\" width=\"807\"]<img class=\"wp-image-4048 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cells-2.jpg\" alt=\"11.4.6 Bone Cells\" width=\"807\" height=\"567\" \/> <em>Figure 11.4.6 Different types of bones cells have different functions.<\/em>[\/caption]\r\n\r\nBone 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] of mineral ions in the blood.\r\n<div>\r\n<h1>Types of Bones<\/h1>\r\n<\/div>\r\nThere 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.\r\n<ul>\r\n \t<li><strong>[pb_glossary id=\"4049\"]Long bones[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4050\"]Short bones[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4052\"]Flat bones[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4053\"]Sesamoid bones[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4054\"]Sutural bones[\/pb_glossary]<\/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>\r\n \t<li><strong>[pb_glossary id=\"4055\"]Irregular bones[\/pb_glossary]<\/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>\r\n<\/ul>\r\n[caption id=\"attachment_4056\" align=\"aligncenter\" width=\"2048\"]<img class=\"wp-image-4056 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0229_ClassificationofBones-2.png\" alt=\"11.4.7 Bone Types of the Body\" width=\"2048\" height=\"2048\" \/> <em>Figure 11.4.7 This diagram shows an example of each of six types of bones classified by shape or location.<\/em>[\/caption]\r\n\r\n<div>\r\n<h1>Feature: Reliable Sources<\/h1>\r\n<\/div>\r\nDiseased 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.\r\n\r\nLearn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:\r\n<ol>\r\n \t<li>How does one become a potential bone marrow donor?<\/li>\r\n \t<li>Who can and who cannot donate bone marrow?<\/li>\r\n \t<li>How is a bone marrow donation made?<\/li>\r\n \t<li>What risks are there in donating bone marrow?<\/li>\r\n<\/ol>\r\n<div>\r\n<div class=\"textbox textbox--key-takeaways\"><header class=\"textbox__header\">\r\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">11.4 Summary<\/span><\/h1>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<ul>\r\n \t<li>[pb_glossary id=\"5913\"]Bones[\/pb_glossary]\u00a0are organs that consist mainly of [pb_glossary id=\"4022\"]bone tissue[\/pb_glossary] (or osseous tissue). Osseous tissue is a type of connective tissue consisting of a [pb_glossary id=\"3970\"]collagen[\/pb_glossary] matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and\u00a0minerals\u00a0makes bone hard, without making it brittle.<\/li>\r\n \t<li>There are two types of osseous tissues: [pb_glossary id=\"4030\"]compact bone[\/pb_glossary] tissue and [pb_glossary id=\"4031\"]spongy bone[\/pb_glossary] 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>\r\n \t<li>Besides osseous tissues, bones also contain [pb_glossary id=\"3011\"]nerves[\/pb_glossary],\u00a0[pb_glossary id=\"5835\"]blood vessels[\/pb_glossary], [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\u00a0human 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<\/ul>\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;\">11.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<ol>\r\n \t<li>Describe osseous tissue.<\/li>\r\n \t<li>Why are bones hard, but not brittle?<\/li>\r\n \t<li>Compare and contrast the compact and spongy bone.<\/li>\r\n \t<li>What non-osseous tissues are found in bones?<\/li>\r\n \t<li>List four types of bone cells and their functions.<\/li>\r\n \t<li>Identify six types of bones. Give an example of each type.<\/li>\r\n \t<li>[h5p id=\"595\"]<\/li>\r\n \t<li>Compare and contrast yellow bone marrow and red bone marrow.<\/li>\r\n \t<li>Which type of bone cell divides to produce new bone cells?\u00a0Where is this cell type located?<\/li>\r\n \t<li>Where do osteoblasts and osteocytes come from? How are they related to each other?<\/li>\r\n \t<li>Which type of bone is embedded in tendons?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--examples\"><header class=\"textbox__header\">\r\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff;\">11.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=rDGqkMHPDqE&amp;feature=emb_logo\r\n<p style=\"text-align: center;\">The Skeletal System: Crash Course A&amp;P #19, CrashCourse, 2015.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=0dV1Bwe2v6c&amp;feature=emb_logo\r\n<p style=\"text-align: center;\">Bone Remodeling and Modeling, Amgen, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=1Qfmkd6C8u8\r\n<p style=\"text-align: center;\">How bones make blood - Melody Smith, TED-Ed, 2020.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<h2>Attributions<\/h2>\r\n<strong>Figure 11.4.1<\/strong>\r\n\r\n<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.\r\n\r\n<strong>Figure 11.4.2<\/strong>\r\n\r\n<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.\r\n\r\n<strong>Figure 11.4.3<\/strong>\r\n\r\n<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.\r\n\r\n<strong>Figure 11.4.4<\/strong>\r\n\r\n<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>.\r\n\r\n<strong>Figure 11.4.5<\/strong>\r\n\r\n<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.\r\n\r\n<strong>Figure 11.4.6<\/strong>\r\n\r\n<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.\r\n\r\n<strong>Figure 11.4.7<\/strong>\r\n\r\n<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.\r\n\r\n<span style=\"font-size: 1.424em; font-weight: bold;\">\r\nReferences<\/span>\r\n<p class=\"hanging-indent\">Amgen. (2012, January 19). Bone remodeling and modeling. YouTube. https:\/\/www.youtube.com\/watch?v=0dV1Bwe2v6c<\/p>\r\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>\r\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>\r\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>\r\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>\r\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>\r\n\r\n<\/div>","rendered":"<p>&nbsp;<\/p>\n<figure id=\"attachment_4020\" aria-describedby=\"caption-attachment-4020\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" 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_19098148350-2.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_4943_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_4943_5913\">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_4943_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_4943_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;\">\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 loading=\"lazy\" decoding=\"async\" class=\"wp-image-4027\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cross-section.svg_-2.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 loading=\"lazy\" decoding=\"async\" class=\"wp-image-4034\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anatomy_of_a_Flat_Bone-2.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_4943_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_4943_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;\">\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 loading=\"lazy\" decoding=\"async\" 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 loading=\"lazy\" decoding=\"async\" class=\"wp-image-4033\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Spongy_bone_-_Trabecular_bone_2_-_Smart-Servier-2.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_4943_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_4943_3961\">hematopoiesis<\/a>.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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 loading=\"lazy\" decoding=\"async\" class=\"wp-image-4048 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_cells-2.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_4943_5761\">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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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 loading=\"lazy\" decoding=\"async\" class=\"wp-image-4056 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0229_ClassificationofBones-2.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_4943_5913\">Bones<\/a>\u00a0are organs that consist mainly of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_4030\">compact bone<\/a> tissue and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3011\">nerves<\/a>,\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_5835\">blood vessels<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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-595\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-595\" class=\"h5p-iframe\" data-content-id=\"595\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"11.4 Quiz\"><\/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><iframe loading=\"lazy\" id=\"oembed-1\" title=\"The Skeletal System: Crash Course Anatomy &amp; Physiology #19\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/rDGqkMHPDqE?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p style=\"text-align: center;\">The Skeletal System: Crash Course A&amp;P #19, CrashCourse, 2015.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-2\" title=\"Bone Remodeling and Modeling\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/0dV1Bwe2v6c?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p style=\"text-align: center;\">Bone Remodeling and Modeling, Amgen, 2012.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-3\" title=\"How bones make blood - Melody Smith\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/1Qfmkd6C8u8?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p style=\"text-align: center;\">How bones make blood &#8211; 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 &#8211; Melody Smith. YouTube. https:\/\/www.youtube.com\/watch?v=1Qfmkd6C8u8<\/p>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_4943_4021\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_3573\">cells<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2801\">tissues<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2803\">organs<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_2955\">negative feedback<\/a> to maintain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3669\">homeostasis<\/a>.<\/p>\n<h2>Maintaining Homeostasis<\/h2>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3777\">adrenal glands<\/a>\u00a0may also be stimulated to secrete the\u00a0hormone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_2956\">negative feedback loop<\/a> that includes a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2950\">stimulus<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2951\">sensor<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3859\">control centre<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_5913\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_4022\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_4022\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<div>\n<figure id=\"attachment_2974\" aria-describedby=\"caption-attachment-2974\" style=\"width: 993px\" class=\"wp-caption alignnone\"><img class=\"wp-image-2974\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/3D_Medical_Illustration_Meninges_Details-1.jpg\" alt=\"\" width=\"993\" height=\"559\"><figcaption id=\"caption-attachment-2974\" class=\"wp-caption-text\"><em>Figure 7.9.1 Meninges: Dura Mater, Arachnoid, and Pia Mater.<\/em><\/figcaption><\/figure>\n<\/div>\n<p>As you learned in this chapter, the human body consists of many complex systems that normally work together efficiently \u2014 like a well-oiled machine \u2014 to carry out life\u2019s functions. For example, the image above (Figure 7.9.1) illustrates how the brain and spinal cord are protected by layers of membrane called meninges and fluid that flows between the meninges and in spaces called ventricles inside the brain. This fluid is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3847\">cerebrospinal fluid<\/a>, and as you have learned, one of its important functions is to cushion and protect the brain and spinal cord, which make up most of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3841\">central nervous system<\/a> (CNS). Additionally, cerebrospinal fluid circulates nutrients and removes waste products from the CNS. Cerebrospinal fluid is produced continually in the ventricles, circulates throughout the CNS, and is then reabsorbed by the bloodstream. If too much cerebrospinal fluid is produced, its flow is blocked, or not enough is reabsorbed, the system becomes out of balance and it can build up in the ventricles. This causes an enlargement of the ventricles called hydrocephalus that can put pressure on the brain, resulting in the types of neurological problems that former professional football player Jayson, described in the beginning of this chapter, is suffering from.<\/p>\n<p>Recall that Jayson\u2019s symptoms included loss of bladder control, memory loss, and difficulty walking. The cause of his symptoms was not immediately clear, although his doctors suspected that it related to the nervous system, since the nervous system acts as the control centre of the body, controlling and regulating many other organ systems. Jayson\u2019s memory loss directly implicated the brain's involvement, since that is the site of thoughts and memory. The urinary system is also controlled in part by the nervous system, so the inability to hold urine appropriately can also be a sign of a neurological issue. Jayson\u2019s trouble walking involved the muscular system, which works alongside the skeletal system to enable movement of the limbs. In turn, the contraction of muscles is regulated by the nervous system. You can see why a problem in the nervous system can cause a variety of different symptoms by affecting multiple organ systems in the human body.<\/p>\n<p>To try to find the exact cause of Jayson\u2019s symptoms, his doctors performed a lumbar puncture (or spinal tap), which is the removal of some cerebrospinal fluid through a needle inserted into the lower part of the spinal canal. They then analyzed Jayson\u2019s cerebrospinal fluid for the presence of pathogens (such as bacteria) to determine whether an infection was the cause of his neurological symptoms. When no evidence of infection was found, they used an MRI to observe the structures of his brain. This is when they discovered his enlarged ventricles, which are a hallmark of hydrocephalus.<\/p>\n<p>To treat Jayson\u2019s hydrocephalus, a surgeon implanted a device called a shunt in his brain to remove the excess fluid. An illustration of a brain shunt is shown in Figure 9.7.2 . One side of the shunt consists of a small tube, called a catheter, which was inserted into Jayson\u2019s ventricles. Excess cerebrospinal fluid is then drained through a one-way valve to the other end of the shunt, which was threaded under his skin to his abdominal cavity, where the fluid is released and can be reabsorbed by the bloodstream.<\/p>\n<figure id=\"attachment_2976\" aria-describedby=\"caption-attachment-2976\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-2976 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Hydrocephalus-with-Shunt-1-1.png\" alt=\"\" width=\"400\" height=\"413\"><figcaption id=\"caption-attachment-2976\" class=\"wp-caption-text\"><em>Figure 7.9.2 An illustration of a brain shunt.<\/em><\/figcaption><\/figure>\n<p>Implantation of a shunt is the most common way to treat hydrocephalus, and for some people, it can allow them to recover almost completely. However, there can be complications associated with a brain shunt. The shunt can have mechanical problems or cause an infection. Also, the rate of draining must be carefully monitored and adjusted to balance the rate of cerebrospinal fluid removal with the rate of its production. If it is drained too fast, it is called overdraining, and if it is drained too slowly, it is called underdraining. In the case of underdraining, the pressure on the brain and associated neurological symptoms will persist. In the case of overdraining, the ventricles can collapse, which can cause serious problems, such as the tearing of blood vessels and hemorrhaging. To avoid these problems, some shunts have an adjustable pressure valve, where the rate of draining can be adjusted by placing a special magnet over the scalp. You can see how the proper balance between cerebrospinal fluid production and removal is so critical \u2013 both in the causes of hydrocephalus and in its treatment.<\/p>\n<p>In what other ways does your body regulate balance, or maintain a state of homeostasis? In this chapter you learned about the feedback loops that keep body temperature and blood glucose within normal ranges. Other important examples of homeostasis in the human body are the regulation of the pH in the blood and the balance of water in the body. You will learn more about homeostasis in different body systems in the coming chapters.<\/p>\n<p>Thanks to Jayson\u2019s shunt, his symptoms are starting to improve, but he has not fully recovered. Time may tell whether the removal of the excess cerebrospinal fluid from his ventricles will eventually allow him to recover normal functioning or whether permanent damage to his nervous system has already been done. The flow of cerebrospinal fluid might seem simple, but when it gets out of balance, it can easily wreak havoc on multiple organ systems because of the intricate interconnectedness of the systems within the human \u201cmachine.\"<\/p>\n<p>To learn more about hydrocephalus and its treatment, watch this video from Boston Children's Hospital:<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=bHD8zYImKqA<\/p>\n<p style=\"text-align: center\">Hydrocephalus and its treatment | Boston Children\u2019s Hospital, 2011.<\/p>\n<p>&nbsp;<\/p>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">Chapter 7 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>This chapter provided an overview of the organization and functioning of the human body. You learned that:<\/p>\n<ul>\n<li>The human body consists of multiple parts that function together to maintain life. The biology of the human body incorporates the body\u2019s structure \u2014 or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3793\">anatomy<\/a> \u2014 and the body\u2019s functioning, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2979\">physiology<\/a>.<\/li>\n<li>The organization of the human body is a hierarchy of increasing size and complexity, starting at the level of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3619\">atoms<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3687\">molecules<\/a>\u00a0and ending at the level of the entire <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2980\">organism<\/a>.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3573\">Cells<\/a> are the level of organization above atoms and molecules, and they are the basic units of structure and function of the human body. Each cell carries out basic life functions, as well as other specific roles. Cells of the human body show a lot of variation.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Variations in cell function are generally reflected in variations in cell structure.<\/li>\n<li>Some cells are unattached to other cells and can move freely. Others are attached to each other and cannot move freely. Some cells can divide readily and form new cells, and others can divide only under exceptional circumstances. Many cells are specialized to produce and secrete particular substances.<\/li>\n<li>All the different cell types within an individual have the same genes. Cells can vary because different genes are expressed depending on the cell type.<\/li>\n<li>Many common types of human cells consist of several subtypes of cells, each of which has a special structure and function. For example, subtypes of bone cells include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3545\">osteocytes<\/a>, osteoblasts, osteogenic cells, and osteoclasts.<\/li>\n<\/ul>\n<\/li>\n<li>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2801\">tissue <\/a>is a group of connected cells that have a similar function. There are four basic types of human tissues that make up all the organs of the human body: epithelial, muscle, nervous, and connective tissues.<\/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_4943_2815\">Connective tissues<\/a>, such as bone, tendons and blood, are made up of a scattering of living cells that are separated by non-living material, called extracellular matrix.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2814\">Epithelial tissues<\/a>, such as skin and mucous membranes, protect the body and its internal organs and secrete or absorb substances.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2817\">Muscular tissues<\/a>\u00a0are made up of cells that have the unique ability to contract. They include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2981\">skeletal<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2982\">smooth<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3833\">cardiac muscle<\/a> tissues.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2818\">Nervous tissues<\/a>\u00a0are made up of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2984\">neurons<\/a>, which transmit messages, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2985\">neuroglia<\/a> of various types, which play supporting roles.<\/li>\n<\/ul>\n<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2803\">organ <\/a>is a structure that consists of two or more types of tissues that work together to do the same job.\u00a0The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3823\">brain<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2987\">heart<\/a> are two examples.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>Many organs are composed of a major tissue that performs the organ\u2019s main function, as well as other tissues that play supporting roles.<\/li>\n<li>The human body contains five organs that are considered vital for survival: the heart, brain, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2988\">kidneys<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2989\">liver<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2990\">lungs<\/a>. If any of these five organs stops functioning, death of the organism is imminent without medical intervention.<\/li>\n<\/ul>\n<\/li>\n<li>An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2804\">organ system<\/a> is a group of organs that work together to carry out a complex overall function. For example, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2906\">skeletal system<\/a> provides structure to the body and protects internal organs.<\/li>\n<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li>There are 11 major organ systems in the human organism. They are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2905\">integumentary<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2906\">skeletal<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2907\">muscular<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2908\">nervous<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3893\">endocrine<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3835\">cardiovascular<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2911\">lymphatic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2912\">respiratory<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3877\">digestive<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2914\">urinary<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2916\">reproductive systems<\/a>. Only the reproductive system varies significantly between males and females.<\/li>\n<\/ul>\n<\/li>\n<li>The human body is divided into a number of body cavities. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3819\">body cavity<\/a> is a fluid-filled space in the body that holds and protects internal organs. The two largest human body cavities are the ventral cavity and dorsal cavity.<\/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_4943_2923\">ventral cavity<\/a> is at the anterior (or front) of the trunk. It is subdivided into the<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2925\"> thoracic cavity<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3763\">abdominal cavity<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2992\">pelvic cavity<\/a>.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3883\">dorsal cavity<\/a> is at the posterior (or back) of the body, and includes the head and the back of the trunk. It is subdivided into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3869\">cranial cavity<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2928\">spinal cavity<\/a>.<\/li>\n<\/ul>\n<\/li>\n<li>Organ systems of the human body must work together to keep the body alive and functioning normally. This requires communication among organ systems. This is controlled by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3807\">autonomic nervous system<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3893\">endocrine system<\/a>. The autonomic nervous system controls involuntary body functions, such as heart rate and digestion. The endocrine system secretes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3569\">hormones<\/a> into the blood that travel to body cells and influence their activities.<\/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_4943_3633\">Cellular respiration<\/a> is a good example of organ system interactions, because it is a basic life process that occurs in all living cells. It is the intracellular process that breaks down <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3359\">glucose<\/a> with oxygen to produce carbon dioxide and energy. Cellular respiration requires the interaction of the digestive, cardiovascular, and respiratory systems.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3921\">fight-or-flight response<\/a> is a good example of how the nervous and endocrine systems control other organ system responses. It is triggered by a message from the brain to the endocrine system and prepares the body for flight or a fight. Many organ systems are stimulated to respond, including the cardiovascular, respiratory, and digestive systems.<\/li>\n<li>Playing softball or doing other voluntary physical activities may involve the interaction of nervous, muscular, skeletal, respiratory, and cardiovascular systems.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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<\/ul>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\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_4943_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_4943_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_4943_2956\">negative feedback loop<\/a> that includes a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2950\">stimulus<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2951\">sensor<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3859\">control centre<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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>Sometimes homeostatic mechanisms fail, resulting in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2968\">homeostatic imbalance<\/a>. Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body\u2019s control system, making the elderly more susceptible to disease.<\/li>\n<\/ul>\n<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2962\">Positive feedback loops<\/a> are 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<\/ul>\n<p>The severe and broad impact of hydrocephalus on the body\u2019s systems highlights the importance of the nervous system and its role as the master control system of the body. In the next chapter, you will learn much more about the structures and functioning of this fascinating and important 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 7 Review<\/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 tissues and organs.<\/li>\n<li>\n<div id=\"h5p-113\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-113\" class=\"h5p-iframe\" data-content-id=\"113\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"7.8 Homeostasis and Feedback\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>\n<div id=\"h5p-114\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-114\" class=\"h5p-iframe\" data-content-id=\"114\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"7.8 Interactions of Organ Systems\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Which type of tissue lines the inner and outer surfaces of the body?<\/li>\n<li>What is a vital organ? What happens if a vital organ stops working?<\/li>\n<li>Name three organ systems that transport or remove wastes from the body.<\/li>\n<li>Name two types of tissue in the digestive system.<\/li>\n<li>\n<div id=\"h5p-222\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-222\" class=\"h5p-iframe\" data-content-id=\"222\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Timeline of Pivotal Events in DNA Research\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Describe one way in which the integumentary and cardiovascular systems work together to regulate homeostasis in the human body.<\/li>\n<li>\n<div id=\"h5p-115\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-115\" class=\"h5p-iframe\" data-content-id=\"115\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Nature of Science\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li><em>True or False:\u00a0<\/em>Body cavities are filled with air.<\/li>\n<li>In which organ system is the pituitary gland?\u00a0Describe how the pituitary gland increases metabolism.<\/li>\n<li>When the level of thyroid hormone in the body gets too high, it acts on other cells to reduce production of more thyroid hormone. What type of feedback loop does this represent?<\/li>\n<li>Hypothetical organ A is the control centre in a feedback loop that helps maintain homeostasis. It secretes molecule A1 which reaches organ B, causing organ B to secrete molecule B1. B1 negatively feeds back onto organ A, reducing the production of A1 when the level of B1 gets too high.\n<ol type=\"a\">\n<li>What is the stimulus in this feedback loop?<\/li>\n<li>If the level of B1 falls significantly below the set point, what do you think happens to the production of A1? Why?<\/li>\n<li>What is the effector in this\u00a0feedback loop?<\/li>\n<li>If organs A and B are part of the endocrine system, what type of molecules do you think A1 and B1 are likely to be?<\/li>\n<\/ol>\n<\/li>\n<li>What are the two main systems that allow various organ systems to communicate with each other?<\/li>\n<li>What are two functions of the hypothalamus?<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 7.9.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:3D_Medical_Illustration_Meninges_Details.jpg\">3D Medical Illustration Meninges Details<\/a> by <a href=\"https:\/\/www.scientificanimations.com\">Scientific Animations<\/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>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/deed.en) license.<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">Figure 7.9.2<\/strong><\/p>\n<p><a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/9.8\/\" rel=\"cc:attributionURL\">Hydrocephalus with Shunt<\/a> from <a href=\"https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/9.8\/\">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 style=\"font-size: 1em\" src=\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\" alt=\"\"> \u00a0<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 <\/span><a style=\"border: none;text-decoration: none\" href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/\"><img class=\"alignnone size-full wp-image-8217\" style=\"border: none;vertical-align: bottom\" title=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" src=\"https:\/\/img2.ck12.org\/media\/images\/icon_licence.png\" alt=\"CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0)\" width=\"80\"><\/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<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, April 25).\u00a0<span class=\"os-title-label\">Figure\u00a0<\/span><span class=\"os-number\">1.3<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"42861\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight first text last\" data-timestamp=\"1594951484732\" data-highlight-id=\"6aeb8ede-949a-42a7-8c5b-539b9c5c7092\" data-highlighted=\"true\">Levels of<\/span> structural organization of the human body<\/span>\u00a0[digital image]. In\u00a0<em>Anatomy and Physiology<\/em>\u00a0(Section 1.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-2-structural-organization-of-the-human-body<\/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\u00a0<\/span><span class=\"os-number\">1.4<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"7375\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight first text last\" data-timestamp=\"1594955809917\" data-highlight-id=\"528f84b2-6176-4d78-a9ff-058c8cf1e4df\" data-highlighted=\"true\">Organ systems<\/span> of the human body [digital image]. In <em>Anatomy and Physiology<\/em>\u00a0(Section 1.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-2-structural-organization-of-the-human-body\u00a0<\/span><\/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\u00a0<\/span><span class=\"os-number\">1.15<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"26938\" class=\"os-title\" data-type=\"title\">Dorsal and ventral body cavities<\/span><span class=\"os-divider\"> <span id=\"7375\" class=\"os-title\" data-type=\"title\">[digital image]. In\u00a0<em>Anatomy and Physiology<\/em> (Section 1.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-6-anatomical-terminology<\/span><\/span><\/p>\n<p class=\"hanging-indent\">Boston Children's Hospital. (2011, ). Hydrocephalus and its treatment | Boston Children\u2019s Hospital. YouTube. https:\/\/www.youtube.com\/watch?v=bHD8zYImKqA&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Brainard, J\/ CK-12 Foundation. (2016). Figure 2 An illustration of a brain shunt [digital image]. In <em>CK-12 College Human Biology<\/em> (Section 9.8) [online Flexbook]. CK12.org. https:\/\/www.ck12.org\/book\/ck-12-college-human-biology\/section\/9.8\/<\/p>\n<p class=\"hanging-indent\">File:Body cavities lateral view labeled.jpg. (2018, January 4). <i>Wikimedia Commons<\/i>.\u00a0 https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Body_Cavities_Lateral_view_labeled.jpg&amp;oldid=276851269. (Original image: <span class=\"os-title-label\">Figure\u00a0<\/span><span class=\"os-number\">1.15<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"26938\" class=\"os-title\" data-type=\"title\">Dorsal and ventral <span class=\"search-highlight first text last\" data-timestamp=\"1597187104310\" data-highlight-id=\"dd6bb77b-db10-4510-a74c-807b0111f516\" data-highlighted=\"true\">body cavities,<\/span><\/span> from OpenStax, <em>Anatomy and Physiology<\/em>.)<\/p>\n<p class=\"hanging-indent\">File:Body cavities lateral view labeled.jpg. (2018, January 4). <i>Wikimedia Commons. <\/i>\u00a0https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Body_Cavities_Lateral_view_labeled.jpg&amp;oldid=276851269. (Original image: OpenStax [Version 8.25 from the textbook OpenStax <em>Anatomy and Physiology<\/em>] adapted for Review questions by Christine Miller].<\/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_4943_3970\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3415\">chemical equations<\/a>. According to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4030\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3178\">Agonists<\/a><\/strong>\u00a0are drugs that increase the activity of particular <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_4031\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_3004\">voluntary<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3841\">central nervous system<\/a> (CNS) and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3014\">somatic nervous system<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3015\">sympathetic division<\/a> (which controls the fight-or-flight response), the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3016\">parasympathetic division<\/a> (which controls most routine involuntary responses), and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_3839\">cell body<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3873\">dendrites<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3049\">action potentials<\/a>\u00a0occur at gaps in the myelin sheath, called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3823\">brain <\/a>and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3040\">synapse<\/a>. At a chemical synapse, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3056\">neurotransmitter<\/a> chemicals are released from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3821\">bones<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2929\">meninges<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3849\">cerebrum<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3095\">thalamus<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2937\">hypothalamus<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3096\">hippocampus<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3011\">nerves<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3124\">mechanoreceptors<\/a>\u00a0(mechanical forces), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3125\">thermoreceptors<\/a>\u00a0(temperature), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3126\">nociceptors<\/a>\u00a0(pain), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3127\">photoreceptors<\/a>\u00a0(light), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3128\">chemoreceptors<\/a>\u00a0(chemicals).\n<ul>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3144\">myopia<\/a> (nearsightedness), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3145\">hyperopia<\/a> (farsightedness), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3171\">stimulants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3172\">depressants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3173\">anxiolytics<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3174\">euphoriants<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3175\">hallucinogens<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3178\">agonists<\/a>, which enhance the activity of particular neurotransmitters, or as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_3961\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_3961\"><div tabindex=\"-1\"><p>Created by CK-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\">So Many Species!<\/span><\/h1>\n<figure id=\"attachment_237\" aria-describedby=\"caption-attachment-237\" style=\"width: 300px\" class=\"wp-caption alignleft\"><img class=\"wp-image-237 size-medium\" style=\"color: #373d3f;font-weight: bold;font-size: 1em\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/6-Kingdoms-1.jpg\" alt=\"\" width=\"300\" height=\"300\"><figcaption id=\"caption-attachment-237\" class=\"wp-caption-text\"><em>Figure 2.4.1 The classification of species from each of the six kingdoms.<\/em><\/figcaption><\/figure>\n<p>The collage shows a single\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3723\">species<\/a><\/strong>\u00a0in each of the six kingdoms into which all of Earth's living things are commonly classified. How many species are there in each\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3677\">kingdom<\/a><\/strong>? In a word:\u00a0<em>millions<\/em>. A total of almost two million living\u00a0species\u00a0have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million\u00a0unique\u00a0species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.<\/p>\n<h1>What Is\u00a0Biodiversity?<\/h1>\n<p>Biological diversity, or\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3627\">biodiversity<\/a><\/strong><strong>,<\/strong>\u00a0refers to all of the variety of life that exists on Earth. Biodiversity can be described and measured at three different levels: species diversity, genetic diversity, and ecosystem diversity.<\/p>\n<ul>\n<li>Species diversity\u00a0refers to the number of different species in an ecosystem or on Earth as a whole. This is the\u00a0most common\u00a0way to measure biodiversity.\u00a0Current\u00a0estimates for Earth's total number of living species range from 5 to 30 million species.<\/li>\n<li>Genetic diversity\u00a0refers to the variation in genes within all of these species.<\/li>\n<li>Ecosystem diversity\u00a0refers to the variety of\u00a0ecosystems\u00a0on Earth. An\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3651\">ecosystem<\/a><\/strong>\u00a0is a system formed by populations of many different species interacting with each other and their environment.<\/li>\n<\/ul>\n<div>\n<p>https:\/\/www.youtube.com\/watch?v=GK_vRtHJZu4<\/p>\n<p style=\"text-align: center\">Why is Biodiversity So Important? - Kim Preshoff, TEDEd, 2015<\/p>\n<h1>Defining\u00a0a\u00a0Species<\/h1>\n<\/div>\n<p>Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think.\u00a0Formally, a\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3723\">species<\/a><\/strong>\u00a0is defined as a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species,\u00a0<em>Homo sapiens.<\/em>\u00a0All humans can potentially interbreed with each other, but not with members of any other species.<\/p>\n<p>In the real world, it isn't always possible to make the observations necessary to determine whether or not different organisms can interbreed. For one thing, many species reproduce asexually, so individuals never interbreed \u2014 even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know if different organisms could interbreed. Keep in mind that 99 per cent of all species that have ever existed are now extinct! In practice, many biologists and virtually\u00a0<em>all<\/em>\u00a0paleontologists generally define species on the basis of morphology, rather than breeding behavior.\u00a0Morphology\u00a0refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, the more similar to one another different organisms\u00a0<em>appear<\/em>, the greater the chance that they will be classified in the same species.<\/p>\n<div>\n<h1>Classifying Living Things<\/h1>\n<\/div>\n<p>People have been trying to classify the tremendous\u00a0diversity of life\u00a0on Earth for more than two thousand years. The science of classifying organisms is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3725\">taxonomy<\/a>.<\/strong>\u00a0Classification is an important step in understanding the present diversity and past evolutionary\u00a0history of life\u00a0on Earth. It helps us make sense of the overwhelming diversity of living things.<\/p>\n<h2>Linnaean Classification<\/h2>\n<p>All modern classification systems have their\u00a0roots\u00a0in the Linnaean classification system, which was developed by Swedish botanist <a href=\"https:\/\/en.wikipedia.org\/wiki\/Carl_Linnaeus\" target=\"_blank\" rel=\"noopener noreferrer\">Carolus Linnaeus<\/a> in the 1700s. He tried to classify all living things known in his time by grouping together organisms that s<\/p>\n<figure id=\"attachment_187\" aria-describedby=\"caption-attachment-187\" style=\"width: 117px\" class=\"wp-caption alignright\"><img class=\"size-medium wp-image-187\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Biological_classification_L_Pengo_vflip.svg_-1.png\" alt=\"A diagram of the levels of classification of living things. In order: Life, Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species\" width=\"117\" height=\"300\"><figcaption id=\"caption-attachment-187\" class=\"wp-caption-text\">Figure 2.4.2 Classification of life.<\/figcaption><\/figure>\n<p>hared obvious morphological traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the \u201cfather of taxonomy.\u201d<\/p>\n<p>The Linnaean system of classification consists of a hierarchy of groupings, called\u00a0taxa\u00a0(singular, taxon). \u00a0In the original system, taxa ranged from the\u00a0kingdom\u00a0to the species. The<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3677\">\u00a0<strong>kingdom<\/strong><\/a>\u00a0(ex. plant kingdom, animal kingdom) is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. The species is the smallest and most exclusive grouping. Ideally, it consists of organisms that are similar enough to interbreed, as discussed above. Similar species are classified together in the same genus (plural, genera), then similar genera are classified together in the same family, and so on, all the way up to the kingdom.<\/p>\n<p>A phrase to help you remember the order of the groupings is shown below.\u00a0\u00a0The first letter of each word is the first letter of the level of classification.<\/p>\n<p><strong>D<\/strong>ad <strong>K<\/strong>eeps\u00a0<strong>P<\/strong>ots\u00a0<strong>C<\/strong>lean <strong>O<\/strong>r <strong>F<\/strong>amily <strong>G<\/strong>ets <strong>S<\/strong>ick<\/p>\n<div>\n<p>&nbsp;<\/p>\n<p>The hierarchy of taxa in the original Linnaean system of taxonomy included taxa from the species to the kingdom. The domain was added later.<\/p>\n<\/div>\n<h2>Binomial Nomenclature<\/h2>\n<p>Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called\u00a0binomial nomenclature,\u00a0gives each species a unique, two-word Latin name consisting of the genus name\u00a0followed by\u00a0a specific species identifier. An example is\u00a0<em>Homo sapiens,<\/em>\u00a0the two-word Latin name for humans. It literally means \u201cwise human.\u201d This is a reference to our big brains.<\/p>\n<p>Why is having two names so important? It is similar to people having a first and a last name. You may know several people with the first name Michael, but adding Michael\u2019s last name usually pins down exactly\u00a0<em>which\u00a0<\/em>Michael you mean. In the same way, having two names for a species helps to uniquely identify it.<\/p>\n<h2>Revisions in the Linnaean Classification<\/h2>\n<p>Linnaeus published his classification system in the 1700s. Since then, many new species have been discovered. Scientists can also now classify organisms on the basis of their biochemical and genetic similarities and differences, and not just their outward morphology. These changes have led to revisions in the original Linnaean system of classification.<\/p>\n<figure id=\"attachment_190\" aria-describedby=\"caption-attachment-190\" style=\"width: 423px\" class=\"wp-caption alignright\"><img class=\"wp-image-190\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/3-domains-1.png\" alt=\"A diagram showing the three domains of life and major groups within each of the domains.\" width=\"423\" height=\"423\"><figcaption id=\"caption-attachment-190\" class=\"wp-caption-text\"><em>Figure 2.4.3 The three domains of life and major groups within.<\/em><\/figcaption><\/figure>\n<p>A major change to the Linnaean system is the addition of a new taxon called the\u00a0domain. The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3647\">domain<\/a><\/strong>\u00a0is a taxon that is larger and more inclusive than the kingdom, as shown in the figure\u00a0above. Most biologists agree that there are three domains of life on Earth:\u00a0Bacteria,\u00a0Archaea, and Eukarya . Both the\u00a0Bacteria\u00a0and the\u00a0Archaea domains consist of single-celled organisms that lack a\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3705\"><strong>nucleus<\/strong><\/a>. This means that their genetic material is not enclosed within a membrane inside the cell. The Eukarya\u00a0domain, in contrast, consists of all organisms whose\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3573\"><strong>cells<\/strong>\u00a0<\/a><em>do\u00a0<\/em>have a\u00a0nucleus, so that their genetic material is enclosed within a membrane inside the cell. The Eukarya\u00a0domain is made up of both single-celled and multicellular organisms. This domain includes several kingdoms, including the animal, plant, fungus, and protist kingdoms.<\/p>\n<div>\n<p>The three domains of life, as well as how they are related to each other and to a common ancestor.\u00a0 There are several theories about how the three domains are related and which arose first, or from another.<\/p>\n<h2><span style=\"font-size: 1.424em\">Phylogenetic Classification<\/span><\/h2>\n<\/div>\n<p>Linnaeus classified organisms based on morphology. Basically, organisms were grouped together if they looked alike. After\u00a0Darwin\u00a0published his\u00a0theory of evolution\u00a0in the 1800s, scientists looked for a way to classify organisms that\u00a0accounted for\u00a0phylogeny.\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3503\">Phylogeny<\/a><\/strong>\u00a0is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, or some other tree-like diagram, like the one shown\u00a0above\u00a0to illustrate the three domains. A\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3711\">phylogenetic tree<\/a><\/strong>\u00a0shows how closely related different groups of organisms are to one another. \u00a0Each branching point represents a common ancestor of the branching groups.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">2.4 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity (within species), and ecosystem diversity.<\/li>\n<li>The formal biological definition of species is a group of actually or potentially interbreeding organisms. Our own species,\u00a0<em>Homo sapiens,<\/em>is an example. In reality, organisms are often classified into species on the basis of morphology.<\/li>\n<li>A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, which is called binomial nomenclature.<\/li>\n<li>The domain \u2014 a taxon higher than the kingdom \u2014 was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. The human species and other animal species are placed in the Eukarya\u00a0domain.<\/li>\n<li>Modern systems of classification\u00a0take into account\u00a0phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams<\/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\">2.4 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What is biodiversity? Identify three ways that biodiversity may be measured.<\/li>\n<li>Define biological species. Why is this definition often difficult to apply?<\/li>\n<li>Explain why it is important to classify living things, and outline the Linnaean system of classification.<\/li>\n<li>What is binomial nomenclature? Give an example.<\/li>\n<li>\n<div id=\"h5p-49\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-49\" class=\"h5p-iframe\" data-content-id=\"49\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.13 Review Questions\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>Contrast the Linnaean and phylogenetic systems of classification.<\/li>\n<li>Describe the taxon called the domain, and compare the three widely recognized domains of living things.<\/li>\n<li>Based on the phylogenetic tree for the three domains of life above, explain whether you think Bacteria are more closely related to Archaea or Eukarya.<\/li>\n<li>A scientist discovers a new single-celled organism. Answer the following questions about this discovery.\n<ol type=\"a\">\n<li>If this is all you know, can you place the organism into a particular domain? If so, what is the domain? If not, why not?<\/li>\n<li>What is one type of information that could help the scientist classify the organism?<\/li>\n<\/ol>\n<\/li>\n<li>Define morphology. Give an example of a morphological trait in humans.<\/li>\n<li>Which type of biodiversity is represented in the differences between humans?<\/li>\n<li>Why do you think it is important to the definition of a species that members of a species can produce\u00a0<em>fertile\u00a0<\/em>offspring?<\/li>\n<li>Go to the A-Z Animals\u00a0<a href=\"https:\/\/a-z-animals.com\/reference\/animal-classification\/\" target=\"blank\" rel=\"noopener noreferrer\">Animal Classification Page<\/a>. In the search box, put in your favorite animal and write out it's classification.<\/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\">2.4 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/DVouQRAKxYo<\/p>\n<p style=\"text-align: center\">Classification, Amoeba Sisters, 2013.<\/p>\n<\/div>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 2.4.1 (6 Kingdoms collage)<\/strong><\/p>\n<ul>\n<li><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:SalmonellaNIAID.jpg\">Salmonella<\/a>, by unknown\/ <a class=\"external text\" href=\"http:\/\/www3.niaid.nih.gov\/topics\/BiodefenseRelated\/Biodefense\/PublicMedia\/image_library.htm\" rel=\"nofollow\">NIAID<\/a> on Wikimedia Commons is in the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/li>\n<li><a href=\"https:\/\/pxhere.com\/en\/photo\/1407537\">Fern<\/a> from\u00a0<a href=\"https:\/\/pxhere.com\/\">pxhere<\/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).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/oJW6cBoCHfo\">Photo [<\/a><a href=\"https:\/\/unsplash.com\/photos\/oJW6cBoCHfo\">squirrel] <\/a>, by <a href=\"https:\/\/unsplash.com\/@erprekurat\">Radoslaw Prekurat<\/a> on <a href=\"http:\/\/unsplash.com\">Unsplash<\/a> is used under the\u00a0<a href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li><a href=\"https:\/\/pixabay.com\/photos\/blood-milk-mushroom-mushrooms-231816\/\">Blood Milk Mushroom<\/a> by <a href=\"https:\/\/pixabay.com\/users\/hans-2\/\">Hans<\/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\/de\/service\/license\/).<\/li>\n<li><a href=\"https:\/\/unsplash.com\/photos\/sT_vNkvZQWM\">Fungi<\/a> by <a href=\"https:\/\/unsplash.com\/@sjwright\">Ste Wright<\/a>\u00a0on <a href=\"http:\/\/unsplash.com\">Unsplash<\/a> is used under the\u00a0<a href=\"https:\/\/unsplash.com\/license\">Unsplash License<\/a> (https:\/\/unsplash.com\/license).<\/li>\n<li id=\"firstHeading\" class=\"firstHeading\" lang=\"en\"><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:EscherichiaColi_NIAID.jpg\">EscherichiaColi NIAID<\/a> [adapted], by <a href=\"https:\/\/www.niaid.nih.gov\/\">Rocky Mountain Laboratories,\u00a0<\/a><a class=\"extiw\" title=\"ca:NIAID\" href=\"https:\/\/ca.wikipedia.org\/wiki\/NIAID\">ca:NIAID<\/a><a href=\"https:\/\/www.niaid.nih.gov\/\">,\u00a0<\/a><a class=\"extiw\" title=\"ca:NIH\" href=\"https:\/\/ca.wikipedia.org\/wiki\/NIH\">ca:NIH<\/a> on Wikimedia Commons is in the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/li>\n<\/ul>\n<p><strong>Figure 2.4.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Biological_classification_L_Pengo_vflip.svg\">Biological classification<\/a>, by <a href=\"https:\/\/en.wikipedia.org\/wiki\/User:Pengo\">Pengo [Peter Halasz]<\/a> on Wikimedia Commons is in the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 2.4.3<\/strong><\/p>\n<p>The three domains of life and major groups within, by C. Miller, 2019, is in 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\">Amoeba Sisters. (2017, March 8). Classification. YouTube. https:\/\/www.youtube.com\/watch?v=DVouQRAKxYo&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">A-Z Animals. (2008, December 1). Animal classification. https:\/\/a-z-animals.com\/reference\/animal-classification\/<\/p>\n<p class=\"hanging-indent\"><span style=\"font-size: 1em\">TED-Ed. (2015, April 20). <\/span><span style=\"text-align: initial;font-size: 1em\">Why is biodiversity so important? - Kim Preshoff. YouTube. https:\/\/www.youtube.com\/watch?v=GK_vRtHJZu4<\/span><\/p>\n<p class=\"hanging-indent\"><span style=\"text-align: initial;text-indent: -1em;font-size: 1em\">Wikipedia contributors. (2020, June 21). Carl Linnaeus. <\/span><i style=\"text-align: initial;text-indent: -1em;font-size: 1em\">Wikipedia. <\/i><span style=\"text-align: initial;text-indent: -1em;font-size: 1em\">https:\/\/en.wikipedia.org\/w\/index.php?title=Carl_Linnaeus&amp;oldid=963767022<\/span><\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4943_4039\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3524\">keratinocytes<\/a>,<\/strong>\u00a0which produce the tough, fibrous\u00a0protein\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3525\">keratin<\/a>.<\/strong>\u00a0The innermost\u00a0cells\u00a0of the epidermis are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3524\">keratinocytes<\/a>. Each hair grows out of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3923\">follicle<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3524\">keratinocytes<\/a> filled with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_2905\">integumentary system<\/a> work with other organ systems to maintain\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_2905\">integumentary system<\/a> consists of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3521\">skin<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3531\">hair<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3899\">epidermis<\/a>, and a thicker\u00a0inner layer called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3524\">keratinocytes<\/a>, which produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_3531\">Hair<\/a> consists mainly of dead keratinocytes and grows out of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_4040\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3521\">skin<\/a>. The inner layer\u00a0is\u00a0the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_3525\">keratin<\/a>-producing epithelial cells called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3545\">Melanocytes<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_3525\">keratin<\/a>, giving their cytoplasm a granular appearance.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3705\">nucleus<\/a>\u00a0and\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_277\">DNA<\/a>\u00a0that lead to the\u00a0development\u00a0of\u00a0skin <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3549\">stratum corneum<\/a>.\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3524\">keratinocytes<\/a> that produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3525\">keratin<\/a> and make up 90 per cent of epidermal cells, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3526\">melanocytes<\/a> that produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3753\">melanin<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3540\">Langerhans cells<\/a> that fight <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3707\">pathogens<\/a> in the skin, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_4041\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_3899\">epidermis<\/a><\/strong>. The dermis consists mainly of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2815\">connective tissues<\/a>. It also contains most skin structures, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_2936\">glands<\/a> and hair <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_2937\">hypothalamus<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3006\">sensory receptors<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_4042\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_3572\">hair follicle<\/a><\/strong>\u00a0in the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3572\">hair follicle<\/a> in the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_3739\">cuticle<\/a>; the middle zone called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3737\">cortex<\/a>; and the innermost zone called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3753\">Melanin<\/a> (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3894\">eumelanin<\/a> and\/or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3900\">eyelashes<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4043\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_3521\">skin<\/a>. They are made of sheets of dead <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3403\">Nails<\/a>\u00a0are accessory organs of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3521\">skin<\/a>, consisting of sheets of dead, keratin-filled <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3912\">nail root<\/a> (which is under the epidermis), the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_3914\">nail bed<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3917\">cuticle<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_5761\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_4049\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3961\">hematopoiesis<\/a>.<\/li>\n<li><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3821\">Bones<\/a>\u00a0are organs that consist mainly of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_4030\">compact bone<\/a> tissue and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3011\">nerves<\/a>,\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3743\">blood vessels<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4050\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3665\">enzymes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_4077\">ossification<\/a>. It begins at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_4042\">osteoclasts<\/a>\u00a0resorb bone, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_4052\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_4052\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_4120\" aria-describedby=\"caption-attachment-4120\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4120\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Blausen_0686_Osteoporosis_01-1.png\" alt=\"11.7.1 Dowager's Hump\" width=\"450\" height=\"600\"><figcaption id=\"caption-attachment-4120\" class=\"wp-caption-text\"><em>Figure 11.7.1 Poor posture, or bone degeneration?<\/em><\/figcaption><\/figure>\n<div>\n<h1>Dowager\u2019s Hump<\/h1>\n<\/div>\n<p>The woman on the right in Figure 11.7.1 has a deformity in her back commonly called dowager\u2019s (widow\u2019s) hump, because it occurs most often in elderly women. Its medical name is <a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/kyphosis\/symptoms-causes\/syc-20374205\">kyphosis<\/a>, and it is defined as excessive curvature of the spinal column in the thoracic region. The curvature generally results from fractures of thoracic vertebrae. As the inset drawings suggest, these fractures may occur due to a significant decrease in bone mass, which is called osteoporosis. Osteoporosis is one of the most prevalent\u00a0disorders of the skeletal system.<\/p>\n<div>\n<h1>Common\u00a0Skeletal System\u00a0Disorders<\/h1>\n<\/div>\n<p>A number of disorders affect the skeletal system, including bone fractures and bone cancers. However, the two most common disorders of the skeletal system are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4122\">osteoporosis<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4102\">osteoarthritis<\/a>. At least ten million people in the United States have osteoporosis, and more than eight million of them are women. Osteoarthritis is even more common, affecting almost 1.4 million people in Canada, and 1 in 4 women over the age of 50. Because osteoporosis and osteoarthritis are so common, they are the focus of this section. These two disorders are also good examples to illustrate the structure and function of the skeletal system.<\/p>\n<div>\n<h1>Osteoporosis<\/h1>\n<\/div>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4122\">Osteoporosis<\/a><\/strong> is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. Bones may weaken so much that a fracture can occur with minor stress \u2014 or even spontaneously, without any stress at all. Osteoporosis is the most common cause of broken bones in the elderly, but until a bone fracture occurs, it typically causes no symptoms. The bones that break most often include those in the wrist, hip, shoulder, and spine. When the thoracic vertebrae are affected, there can be a gradual collapse of the vertebrae due to compression fractures, as shown in Figure 11.7.2. This is what causes kyphosis, as pictured above in Figure 11.7.1.<\/p>\n<figure id=\"attachment_4123\" aria-describedby=\"caption-attachment-4123\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4123\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Feature_Osteoprosis_of_Spine-1.jpg\" alt=\"11.7 Kyphosis in the spine\" width=\"400\" height=\"323\"><figcaption id=\"caption-attachment-4123\" class=\"wp-caption-text\"><em>Figure 11.7.2 Compression fractures of thoracic vertebrae are relatively common in people with osteoporosis.<\/em><\/figcaption><\/figure>\n<h2>Changes in Bone Mass with Age<\/h2>\n<p>As shown in the Figure 11.7.3, bone mass in both males and females generally peaks when people are in their thirties, with males typically attaining a higher peak mass than females. In both sexes, bone mass usually decreases after that, and this tends to occur more rapidly in females, especially after menopause. The greater decrease in females is generally attributable to low levels of estrogen in the post-menopausal years.<\/p>\n<figure id=\"attachment_4124\" aria-describedby=\"caption-attachment-4124\" style=\"width: 860px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4124 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Age_and_Bone_Mass-1.jpg\" alt=\"11.7.3 Bone density and age\" width=\"860\" height=\"553\"><figcaption id=\"caption-attachment-4124\" class=\"wp-caption-text\"><em>Figure 11.7.3 Bone mass is a measure of the total mass of calcium in the bones of the skeleton. As bone mass decreases, the risk of fractures increases.<\/em><\/figcaption><\/figure>\n<h2>What Causes Osteoporosis?<\/h2>\n<p>The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4040\">osteoblasts<\/a> and bone resorption by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4042\">osteoclasts<\/a>. Normally, bones are constantly being remodeled by these two processes, with up to ten per cent of all bone mass undergoing remodeling at any point in time. As long as these two processes are in balance, no <em>net loss<\/em> of bone occurs. There are three main ways that an imbalance between bone formation and bone resorption can occur and lead to a net loss of bone. All three ways may occur in the same individual. The three ways are described below:<\/p>\n<ol>\n<li><strong>\u00a0An individual never develops normal peak bone mass during the young adult years:<\/strong>\u00a0If the peak level is lower than normal, then there is less bone mass to begin with, making osteoporosis more likely to develop.<\/li>\n<li><strong>There is greater than normal bone resorption:<\/strong>\u00a0Bone resorption normally increases after peak bone mass is reached, but age-related bone resorption may be greater than normal for a variety of reasons. One possible reason is calcium or vitamin D deficiency, which causes the parathyroid gland to release PTH, the\u00a0hormone\u00a0that promotes resorption by osteoclasts.<\/li>\n<li><strong>There is inadequate formation of new bone by osteoblasts during remodeling:<\/strong>\u00a0Lack of estrogen may decrease the normal\u00a0deposition\u00a0of new bone. Inadequate levels of calcium and vitamin D also lead to impaired bone formation by osteoblasts.<\/li>\n<\/ol>\n<p>An imbalance between bone building and bone destruction leading to bone loss may also occur as a side effect of other disorders. For example, people with alcoholism, anorexia nervosa, or hyperthyroidism have an increased rate of bone loss. Some medications \u2014\u00a0including anti-seizure medications, chemotherapy drugs, steroid medications, and some antidepressants \u2014\u00a0also increase the rate of bone loss.<\/p>\n<h2>Diagnosing Osteoporosis<\/h2>\n<p>Osteoporosis is diagnosed by measuring a patient\u2019s bone density and comparing it with the normal level of peak bone density in a young adult reference population of the same sex as the patient. If the patient\u2019s bone density is too far below the normal peak level (as measured by a statistic called a T-score), then osteoporosis is diagnosed. Bone density is usually measured by a type of X-ray called dual-energy X-ray absorptiometry (or DEXA), an example of which is shown in Figure 11.7.4. Typically, the density is measured at the hip. Sometimes, other areas are also measured, because there may be variation in bone density in different parts of the skeleton. Osteoporosis Canada\u00a0 recommends that all women 65 years of age and older be screened with DEXA for bone density. Screening may be recommended at younger ages in people with risk factors for osteoporosis (see Risk Factors for Osteoporosis below).<\/p>\n<figure id=\"attachment_4126\" aria-describedby=\"caption-attachment-4126\" style=\"width: 530px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4126\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/DEXA_scan_screen_ALSPAC-1.jpg\" alt=\"11.7.4 DEXA Scan\" width=\"530\" height=\"354\"><figcaption id=\"caption-attachment-4126\" class=\"wp-caption-text\"><em>Figure 11.7.4 Dual-energy X-ray absorptiometry is a means of measuring bone mineral density using spectral imaging. Two X-ray beams, with different energy levels, are aimed at the patient's bones. When soft tissue absorption is subtracted out, the bone mineral density can be determined from the absorption of each beam by bone.<\/em><\/figcaption><\/figure>\n<h2>Osteoporotic Fractures<\/h2>\n<p>Fractures are the most dangerous aspect of osteoporosis, and osteoporosis is responsible for millions of fractures annually. Debilitating pain among the elderly is often caused by fractures from osteoporosis, and it can lead to further disability and early mortality. Fractures of the long\u00a0bones\u00a0(such as the femur) can impair mobility and may require surgery. Hip fracture usually requires immediate surgery, as well. The immobility associated with fractures \u2014 especially of the hip \u2014 increases the risk of deep vein thrombosis, pulmonary embolism, and pneumonia. Osteoporosis is rarely fatal, but these complications of fractures often are. Older people tend to have more falls than younger people, due to such factors as poor eyesight and balance problems, increasing their risk of fractures even more. The likelihood of falls can be reduced by removing obstacles and loose carpets or rugs in the living environment.<\/p>\n<h2>Risk Factors for Osteoporosis<\/h2>\n<p>There are a number of factors that increase the risk of osteoporosis. Eleven of them are listed below. The first five factors cannot be controlled, but the remaining factors generally can be controlled by changing behaviors.<\/p>\n<ol>\n<li>Older age<\/li>\n<li>Female sex<\/li>\n<li>European or Asian ancestry<\/li>\n<li>Family history of osteoporosis<\/li>\n<li>Short stature and small bones<\/li>\n<li>Smoking<\/li>\n<li>Alcohol consumption<\/li>\n<li>Lack of exercise<\/li>\n<li>Vitamin D deficiency<\/li>\n<li>Poor nutrition<\/li>\n<li>Consumption of soft drinks<\/li>\n<\/ol>\n<h2>Treatment and Prevention of Osteoporosis<\/h2>\n<p>Osteoporosis is often treated with medications that may slow or even reverse bone loss. Medications called bisphosphonates, for example, are commonly prescribed.\u00a0Bisphosphonates slow down the breakdown of bone,\u00a0allowing\u00a0bone rebuilding during remodeling\u00a0to\u00a0keep pace. This helps maintain bone density and decreases the risk of fractures.\u00a0The medications may be more effective in patients who have already broken bones than in those who have not, significantly reducing their risk of another fracture. Generally, patients are not recommended to stay on bisphosphonates for more than three or four years. There is no evidence for continued benefit after this time \u2014 in fact, there is a potential for adverse side effects.<\/p>\n<figure id=\"attachment_4128\" aria-describedby=\"caption-attachment-4128\" style=\"width: 491px\" class=\"wp-caption alignright\"><img class=\"wp-image-4128\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Hiking-by-jake-melara-Yh6K2eTr_FY-unsplash-scaled-2.jpg\" alt=\"11.7.5 Hiking\" width=\"491\" height=\"327\"><figcaption id=\"caption-attachment-4128\" class=\"wp-caption-text\"><em>Figure 11.7.5 Hiking is an enjoyable way to help keep bones strong and reduce the risk of osteoporosis.<\/em><\/figcaption><\/figure>\n<p>Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior. If you smoke, stop. If you drink, reduce your alcohol consumption \u2014 or cut it out altogether. Eat a nutritious diet and make sure you are getting adequate amounts of vitamin D. You should also avoid drinking carbonated beverages.<\/p>\n<p>&nbsp;<\/p>\n<p>If you\u2019re a couch potato, get involved in regular exercise. Aerobic, weight-bearing, and resistance exercises can all help maintain or increase bone mineral density (for example hiking as in Figure 11.7.5). Exercise puts stress on bones, which stimulates bone building. Good weight-bearing exercises for bone building include weight training, dancing, stair climbing, running, and hiking (see Figure 11.7.5). Biking and swimming are less beneficial, because they don\u2019t stress the bones. Ideally, you should exercise for at least 30 minutes a day most days of the week.<\/p>\n<p>&nbsp;<\/p>\n<div><span style=\"font-size: 1.602em;font-weight: bold\">Osteoarthritis<\/span><\/div>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_4129\" aria-describedby=\"caption-attachment-4129\" style=\"width: 161px\" class=\"wp-caption alignleft\"><img class=\"wp-image-4129 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Areas_affected_by_osteoarthritis-1.gif\" alt=\"11.7.6 Areas affected by Osteoarthritis\" width=\"161\" height=\"390\"><figcaption id=\"caption-attachment-4129\" class=\"wp-caption-text\"><em>Figure 11.7.6 The areas shaded in blue indicate the joints most commonly affected by OA.<\/em><\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4102\">Osteoarthritis<\/a> (<a href=\"https:\/\/www.mayoclinic.org\/diseases-conditions\/osteoarthritis\/symptoms-causes\/syc-20351925\">OA<\/a>)<\/strong> is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. Other symptoms may include joint swelling and decreased range of motion. Initially, symptoms may occur only after exercise or prolonged activity, but over time, they may become constant, negatively affecting work and normal daily activities. As shown in Figure 11.7.6, the most commonly involved joints are those near the ends of the fingers, at the bases of the thumbs, and in the neck, lower back, hips, and knees. Often, joints on one side of the body are affected more than those on the other side.<\/p>\n<p>&nbsp;<\/p>\n<h2>What Causes Osteoarthritis?<\/h2>\n<p>OA is thought to be caused by mechanical stress on the\u00a0joints\u00a0with insufficient self-repair of cartilage. The stress may be exacerbated by low-grade inflammation of the joints, as\u00a0cells\u00a0lining the joint attempt to remove breakdown products from cartilage in the synovial space. OA develops over decades as stress and inflammation cause increasing loss of articular cartilage. Eventually, bones may have no cartilage to separate them, so bones rub against one another at joints. This damages the articular surfaces of the bones and contributes to the pain and other symptoms of OA. Because of the pain, movement may be curtailed, leading to loss of muscle, as well.<\/p>\n<h2>Diagnosing Osteoarthritis<\/h2>\n<figure id=\"attachment_4131\" aria-describedby=\"caption-attachment-4131\" style=\"width: 300px\" class=\"wp-caption alignright\"><img class=\"wp-image-4131 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Hallux_valgus-1.jpg\" alt=\"11.7.7 Bunions\" width=\"300\" height=\"300\"><figcaption id=\"caption-attachment-4131\" class=\"wp-caption-text\"><em>Figure 11.7.7 A bunion is a common sign of osteoarthritis. It is typically located at the base of the big toe.<\/em><\/figcaption><\/figure>\n<p>Diagnosis of OA is typically made on the basis of signs and symptoms. Signs include joint deformities, such as bony nodules on the finger joints or bunions on the feet (as illustrated in Figure 11.7.7). Symptoms include joint pain and stiffness. The pain is usually described as a sharp ache or burning sensation, which may be in the muscles and tendons around the affected joints, as well as in the joints themselves. The pain is usually made worse by prolonged activity, and it typically improves with rest. Stiffness is most common when first arising in the morning, and it usually improves quickly as daily activities are undertaken.<\/p>\n<p>X-rays\u00a0or other tests are sometimes used to either support the diagnosis of OA or to rule out other disorders.\u00a0Blood tests might be done, for example, to look for factors that indicate rheumatoid arthritis (RA), an\u00a0autoimmune disease\u00a0in which the immune system attacks the body\u2019s joints. If these factors are not present in the\u00a0blood, then RA is unlikely, and a diagnosis of OA is more likely to be correct.<\/p>\n<h2>Risk Factors for Osteoarthritis<\/h2>\n<p>Age is the chief risk factor for osteoarthritis. By age 65, as many as 80 per cent of all people have evidence of osteoarthritis. However, people are more likely to develop OA \u2014 especially at younger ages \u2014 if they have had a joint injury. A high school football player might have a bad knee injury that damages the joint, leading to OA in the knee by the time he is in his thirties. If people have joints that are misaligned due to congenital malformations or disease, they are also more likely to develop OA. Excess body weight is another factor that increases the risk of OA, because of the added stress it places on weight-bearing joints.<\/p>\n<p>Researchers have found that people with a family history of OA have a heightened risk of developing the disorder, which suggests that genetic factors are also involved in OA. It is likely that many different genes are needed for normal cartilage and cartilage repair. If such genes are defective and cartilage is abnormal or not normally repaired, OA is more likely to result.<\/p>\n<h2>Treatment and Prevention of Osteoarthritis<\/h2>\n<p>OA cannot be cured, but the symptoms \u2014 especially the pain \u2014 can often be treated successfully to maintain good quality of life for people with OA. Treatments include exercise, efforts to decrease stress on joints, pain medications, and surgery.<\/p>\n<h3>Exercise<\/h3>\n<p>Exercise helps maintain joint mobility and also increases muscle strength. Stronger muscles may help keep the bones in joints correctly aligned, and this can reduce joint stress. Good exercises for OA include swimming, water aerobics (see Figure 11.7.8 below), and biking. These activities are recommended for OA, because they put relatively little stress on the joints.<\/p>\n<figure id=\"attachment_4133\" aria-describedby=\"caption-attachment-4133\" style=\"width: 1000px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-4133 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/07-06_WtrAerob1a-1.jpg\" alt=\"11.7.8 Water Aerobics\" width=\"1000\" height=\"350\"><figcaption id=\"caption-attachment-4133\" class=\"wp-caption-text\"><em>Figure 11.7.8 Exercising in water provides buoyancy that places less stress on joints than the same exercises would on the ground or other hard surface.<\/em><\/figcaption><\/figure>\n<div>\n<p>Exercising in water provides buoyancy that places less stress on joints than the same exercises would on the ground or other hard surface.<\/p>\n<\/div>\n<h3>De-stressing Joints<\/h3>\n<p>Efforts to decrease stress on joints include resting and using mobility devices such as canes, which reduce the\u00a0weight\u00a0placed on weight-bearing joints and also improve stability. In people who are overweight, losing weight may also reduce joint stress.<\/p>\n<h3>Pain Medications<\/h3>\n<p>The first type of pain medication likely to be prescribed for OA is acetaminophen (e.g., Tylenol). When taken as prescribed, it has a relatively low risk of serious side effects. If this medication is inadequate to relieve the pain, non-steroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen) may be prescribed. NSAIDs, however, are more likely to cause serious side effects, such as gastrointestinal bleeding, elevated\u00a0blood pressure, and increased risk of stroke. Opioids usually are reserved for patients who have suffered serious side effects or for whom other medications have failed to relieve pain. Due to the risk of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3184\">addiction<\/a>, only short-term use of opioids is generally recommended.<\/p>\n<h3>Surgery<\/h3>\n<p>Joint-replacement surgery is the most common treatment for serious OA in the knee or hip. In fact, knee and hip replacement surgeries are among the most common of all surgeries. Although they require a long period of healing and physical rehabilitation, the results are usually worth it. The replacement \u201cparts\u201d are usually pain-free and fully functional for at least a couple of decades. Quality, durability, and customization of artificial joints are constantly improving.<\/p>\n<p>Try out this neat <a href=\"https:\/\/edheads.org\/page\/hip_resurfacing\">Virtual Hip Resurfacing<\/a> activity by Edheads (you will need to enable Flash).<\/p>\n<p><span style=\"font-size: 1.602em;font-weight: bold\">Feature: Myth vs. Reality<\/span><\/p>\n<p>About one out of every 5 adults in Canada suffer from\u00a0 osteoarthritis. The more you know about this disease, the more you can do to avoid it or slow its progression. That means knowing the facts, rather than believing the myths about osteoarthritis.<\/p>\n<table class=\"grid\" style=\"border-collapse: collapse;width: 100%;height: 492px\" border=\"0\">\n<tbody>\n<tr style=\"height: 98px\">\n<td style=\"width: 35.2067%;height: 98px\">\n<h2><span style=\"color: #ff0000\">Myth<\/span><\/h2>\n<\/td>\n<td style=\"width: 64.7933%;height: 98px\">\n<h2><span style=\"color: #339966\">Reality<\/span><\/h2>\n<\/td>\n<\/tr>\n<tr style=\"height: 31px\">\n<td style=\"width: 35.2067%;height: 31px\"><em><span style=\"font-size: 16px\">\"Cracking my knuckles will cause osteoarthritis.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 31px\"><strong><span style=\"font-size: 16px\">Cracking your knuckles may lead to inflammation of your tendons, but it will not cause osteoarthritis.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 63px\">\n<td style=\"width: 35.2067%;height: 63px\"><em><span style=\"font-size: 16px\">\"My diet has no effect on my joints.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 63px\"><strong><span style=\"font-size: 16px\">What and how much you eat\u00a0<\/span><em style=\"font-size: 16px\">does<\/em><span style=\"font-size: 16px\">\u00a0affect\u00a0your body weight, and every pound you gain translates into an additional four pounds (or more!) of stress on your knees.\u00a0Being overweight, therefore, increases the chances of developing osteoarthritis \u2014 and also the rate at which it progresses.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 79px\">\n<td style=\"width: 35.2067%;height: 79px\"><em><span style=\"font-size: 16px\">\"Exercise causes osteoarthritis or makes it worse, so I should avoid it.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 79px\"><strong><span style=\"font-size: 16px\">This is one of the biggest myths about osteoarthritis. Low-impact exercise can actually lessen the pain and improve other symptoms of osteoarthritis. If you don\u2019t have osteoarthritis, exercise can reduce your risk of developing it. Low-impact exercise helps keep the\u00a0muscles\u00a0around joints strong and flexible, so they can help stabilize and protect the joints.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 47px\">\n<td style=\"width: 35.2067%;height: 47px\"><em><span style=\"font-size: 16px\">\"If my mom or dad has osteoarthritis, I will also develop it.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 47px\"><strong><span style=\"font-size: 16px\">It is true that you are more likely to develop osteoarthritis if a parent has it, but it isn\u2019t a sure thing. There are several things you can do to decrease your risk, such as getting regular exercise and maintaining a healthy weight.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 95px\">\n<td style=\"width: 35.2067%;height: 95px\"><em><span style=\"font-size: 16px\">\"Bad weather causes osteoarthritis.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 95px\"><strong><span style=\"font-size: 16px\">Weather\u00a0conditions do not cause osteoarthritis, although in some people who already have osteoarthritis, bad weather seems to make the symptoms worse. It is primarily low barometric pressure that increases osteoarthritis pain, probably because it leads to greater pressure inside the joints relative to the outside air pressure. Some people think their osteoarthritis pain is worse in cold weather, but systematic studies have not found convincing evidence for this.<\/span><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 79px\">\n<td style=\"width: 35.2067%;height: 79px\"><em><span style=\"font-size: 16px\">\"Joint pain is unavoidable as you get older, so there is no need to see a doctor for it.\"<\/span><\/em><\/td>\n<td style=\"width: 64.7933%;height: 79px\"><strong><span style=\"font-size: 16px\">Many people with osteoarthritis think there is nothing that can be done for the pain of osteoarthritis, or that surgery is the only treatment option. In reality, osteoarthritis symptoms often can be improved with a combination of exercise, weight loss, pain management techniques, and pain medications. If osteoarthritis pain interferes with daily life and lasts more than a few days, you should see your doctor.<\/span><\/strong><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 35.2067%\"><em>\"Osteoarthritis is inevitable in seniors.\"<span style=\"font-size: 16px\"><br \/>\n<\/span><\/em><\/td>\n<td style=\"width: 64.7933%\"><strong>Although many people over 65 develop osteoarthritis, there are many people who never develop it, no matter how old they live to be. You can reduce your risk of developing osteoarthritis in later life by protecting your joints throughout life.<span style=\"font-size: 16px\"><br \/>\n<\/span><\/strong><\/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\">11.7 Summary<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ul>\n<li>A number of disorders affect the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2906\">skeletal system<\/a>, including bone fractures and bone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3513\">cancers<\/a>. The two most common\u00a0disorders of the skeletal system\u00a0are osteoporosis and osteoarthritis.<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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<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 \u2014 such as bisphosphonates \u2014 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<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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<li>Diagnosis of OA is typically made on the basis of signs and symptoms, such as joint deformities, pain, and stiffness.\u00a0X-rays\u00a0or 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<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">11.7 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>Create a brochure or poster about osteoporosis to educate others about this disease.\u00a0 Include information about:\n<ol type=\"a\">\n<li>A definition of osteoporosis<\/li>\n<li>Causes<\/li>\n<li>Dangers of living with the disease<\/li>\n<li>Canadian osteoporosis statistics<\/li>\n<li>Risk factors<\/li>\n<li>Diagnosis<\/li>\n<li>Treatment<\/li>\n<\/ol>\n<\/li>\n<li>\n<div id=\"h5p-161\">\n<div class=\"h5p-content\" data-content-id=\"161\"><\/div>\n<\/div>\n<\/li>\n<li>Why is it important to build sufficient bone mass in your young adult years?<\/li>\n<li>Explain the difference in cause between rheumatoid arthritis and osteoarthritis.<\/li>\n<li>Debunk the myth: Osteoarthritis is caused by physical activity, so people who are equally active are equally susceptible to it.<\/li>\n<li>Explain how we know that estrogen generally promotes production of new bone.<\/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\">11.7 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/www.youtube.com\/watch?v=DL0_gcP15Ts<\/p>\n<p style=\"text-align: center\">Kevin Stone: The bio-future of joint replacement, TED, 2010.<\/p>\n<p>https:\/\/www.youtube.com\/watch?v=OyK0oE5rwFY<\/p>\n<p style=\"text-align: center\">The benefits of good posture - Murat Dalkilin\u00e7, TED-Ed, 2015.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h2>Attributions<\/h2>\n<p><strong>Figure 11.7.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0686_Osteoporosis_01.png\" rel=\"cc:attributionURL\">Blausen_0686_Osteoporosis_01<\/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 11.7.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:722_Feature_Osteoprosis_of_Spine.jpg\" rel=\"cc:attributionURL\">Feature_Osteoprosis_of_Spine<\/a> by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/7-3-the-vertebral-column\">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 style=\"text-align: initial;font-size: 1em\"><br \/>\nFigure 11.7.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:615_Age_and_Bone_Mass.jpg\" rel=\"cc:attributionURL\">Age_and_Bone_Mass<\/a> by\u00a0<a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-6-exercise-nutrition-hormones-and-bone-tissue\">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 11.7.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:DEXA_scan_screen_ALSPAC.jpg\" rel=\"cc:attributionURL\">DEXA_scan_screen_ALSPAC<\/a> by <a class=\"external text\" href=\"http:\/\/www.nicksmithphotography.com\/\" rel=\"nofollow\">Nick Smith photography<\/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> license.<\/p>\n<p><strong>Figure 11.7.5<\/strong><\/p>\n<p><a href=\"https:\/\/unsplash.com\/photos\/Yh6K2eTr_FY\" rel=\"cc:attributionURL\">Hiking by jake-melara-Yh6K2eTr_FY<\/a>\u00a0[photo] by <span class=\"_1JARO\"><a class=\"_3XzpS _1ByhS _4kjHg _1O9Y0 _3l__V _1CBrG xLon9\" href=\"https:\/\/unsplash.com\/@jakemelara\">Jake Melara<\/a> <\/span>on <a href=\"http:\/\/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 11.7.6<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Areas_affected_by_osteoarthritis.gif\" rel=\"cc:attributionURL\">Areas_affected_by_osteoarthritis<\/a> by National Institute of Arthritis and Musculoskeletal and Skin Diseases<a href=\"https:\/\/www.niams.nih.gov\/health-topics\/osteoarthritis\"> (NIAMS)\/ NIH<\/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\/Public_domain).<\/p>\n<p><strong>Figure 11.7.7<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Hallux_valgus.jpg\" rel=\"cc:attributionURL\">Hallux_valgus<\/a> by <a class=\"new\" title=\"User:Malmstajn (page does not exist)\" href=\"https:\/\/commons.wikimedia.org\/w\/index.php?title=User:Malmstajn&amp;action=edit&amp;redlink=1\">Malmstajn<\/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 11.7.8<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:07-06_WtrAerob1a.jpg\" rel=\"cc:attributionURL\">07-06_WtrAerob1a<\/a> by <a title=\"User:Tim Ross\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Tim_Ross\">Tim Ross<\/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\/Public_domain).<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\"><span class=\"os-title-label\"><span class=\"search-highlight text focus\" data-timestamp=\"1596584672995\" data-highlight-id=\"8a0978ab-16ef-45aa-8ae1-affcbd59684b\" 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 focus\" data-timestamp=\"1596584672995\" data-highlight-id=\"8a0978ab-16ef-45aa-8ae1-affcbd59684b\" data-highlighted=\"true\">6.23<\/span><\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"88578\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight text focus\" data-timestamp=\"1596584672995\" data-highlight-id=\"8a0978ab-16ef-45aa-8ae1-affcbd59684b\" data-highlighted=\"true\">Graph showing relationship between age and bone mass digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 6.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/6-6-exercise-nutrition-hormones-and-bone-tissue<\/span><\/span><\/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). <span class=\"os-title-label\">Figure\u00a0<\/span><span class=\"os-number\">7.22<\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"85123\" class=\"os-title\" data-type=\"title\">Osteoporosis <\/span>[digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 7.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/7-3-the-vertebral-column<\/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\">Mayo Clinic Staff. (n.d.). Kyphosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/kyphosis\/symptoms-causes\/syc-20374205<\/p>\n<p class=\"hanging-indent\">Mayo Clinic Staff. (n.d.). Osteoarthritis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/osteoarthritis\/symptoms-causes\/syc-20351925<\/p>\n<p class=\"hanging-indent\">TED. (2010, July 23). Kevin Stone: The bio-future of joint replacement. YouTube. https:\/\/youtu.be\/DL0_gcP15Ts<\/p>\n<p class=\"hanging-indent\">TED-Ed. (2015, July 30). The benefits of good posture - Murat Dalkilin\u00e7. YouTube. https:\/\/www.youtube.com\/watch?v=OyK0oE5rwFY&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">Wheatley, G., Smail, S., Bort, E. (2007). Virtual hip resurfacing [online game]. EdHeads.org. https:\/\/edheads.org\/page\/hip_resurfacing<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_4943_4053\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_4030\">compact bone tissue<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4021\">bone marrow<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4040\">osteoblasts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4041\">osteocytes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4042\">osteoclasts<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_4049\">long bones<\/a>\u00a0(such as the limb bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4050\">short bones<\/a> (such as the wrist bones), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4053\">sesamoid bones<\/a> (such as the patella), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4054\">sutural bones<\/a> in the skull, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_4094\">pivot<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4096\">hinge<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4098\">saddle<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4100\">plane<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3391\">condyloid<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_4054\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_3004\">voluntary<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_4055\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_3513\">cancer<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_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_4943_2981\">skeletal muscle<\/a> (which is attached to bones and enables <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3004\">voluntary<\/a> body movements), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_4156\">muscle fibres<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_3457\">ATP<\/a>.<\/li>\n<li>Muscles can grow larger, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_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_4943_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_4943_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_4943_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_4943_3011\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_3011\"><div tabindex=\"-1\"><p>Created by CK-12 Foundation\/Adapted by Christine Miller<\/p>\n<figure id=\"attachment_1570\" aria-describedby=\"caption-attachment-1570\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img class=\"wp-image-1564\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Cliche.jpg\" alt=\"16.5.1 Dog peeing on fire hydrant\" width=\"400\" height=\"300\"><figcaption id=\"caption-attachment-1570\" class=\"wp-caption-text\"><em>Figure 16.5.1 Just leaving a message.....<\/em><\/figcaption><\/figure>\n<div>\n<h1>Communicating with Urine<\/h1>\n<\/div>\n<p>Why do dogs pee on fire hydrants? Besides \u201chaving to go,\u201d they are marking their territory with chemicals in their urine called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4768\">pheromone<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\">. It\u2019s a form of communication, in which they are \u201csaying\u201d with odors that the yard is <\/span><em style=\"text-align: initial;font-size: 1em\">theirs<\/em><span style=\"text-align: initial;font-size: 1em\">\u00a0and other dogs should stay away. In addition to fire hydrants, dogs may urinate on fence posts, trees, car tires, and many other objects. Urination in dogs, as in people, is usually a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3004\">voluntary<\/a> process controlled by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2554\">brain<\/a>. The process of forming urine \u2014 which occurs in the kidneys \u2014 occurs constantly, and is not under voluntary control. What happens to all the urine that forms in the kidneys? It passes from the kidneys through the other organs of the urinary system, starting with the ureters.<\/span><\/p>\n<div>\n<h1>Ureters<\/h1>\n<\/div>\n<p>As shown in Figure 16.5.2, <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4725\">ureter<\/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\"> are tube-like structures that connect the kidneys with the urinary bladder. They are paired structures, with one ureter for each kidney. In adults, ureters are between 25 and 30 cm (about 10\u201312 in) long and about 3 to 4 mm in diameter.<\/span><\/p>\n<figure id=\"attachment_1570\" aria-describedby=\"caption-attachment-1570\" style=\"width: 446px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-1565\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Urinary-System-Male.jpg\" alt=\"16.5.2 Urinary System - Ureters\" width=\"446\" height=\"449\"><figcaption id=\"caption-attachment-1570\" class=\"wp-caption-text\"><em>16.5.2 Besides the kidneys, the urinary system includes two ureters, the urinary bladder, and the urethra.<\/em><\/figcaption><\/figure>\n<p>Each ureter arises in the pelvis of a kidney (the renal pelvis in Figure 16.5.3). It then passes down the side of the kidney, and finally enters the back of the bladder. At the entrance to the bladder, the ureters have sphincters that prevent the backflow of urine.<\/p>\n<figure id=\"attachment_1570\" aria-describedby=\"caption-attachment-1570\" style=\"width: 388px\" class=\"wp-caption aligncenter\"><img class=\" wp-image-1567\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Adrenal-glands-on-Kidney-by-NCI-Public-Domain-1.jpg\" alt=\"16.5.3 Renal Pelvis and Ureter\" width=\"388\" height=\"357\"><figcaption id=\"caption-attachment-1570\" class=\"wp-caption-text\"><em>16.5.3 Urine collects in the renal pelvis, which is continuous with the ureter. The ureter then carries the urine from the kidney to the urinary bladder.<\/em><\/figcaption><\/figure>\n<p>The walls of the ureters are composed of multiple layers of different types of tissues.\u00a0 The innermost layer is a special type of epithelium, called transitional epithelium. Unlike the epithelium lining most organs, transitional epithelium is capable of stretching and does not produce mucus. It lines much of the urinary system, including the renal pelvis, bladder, and much of the urethra, in addition to the ureters. Transitional epithelium allows these organs to stretch and expand as they fill with urine or allow urine to pass through. The next layer of the ureter walls is made up of loose connective tissue containing elastic fibres, nerves, and blood and lymphatic vessels. After this layer are two layers of smooth muscles, an inner circular layer, and an outer longitudinal layer. The smooth muscle layers can contract in waves of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2890\">peristalsis<\/a> to propel urine down the ureters from the kidneys to the urinary bladder. The outermost layer of the ureter walls consists of fibrous tissue.<\/p>\n<div>\n<h1>Urinary Bladder<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4731\">urinary bladder<\/a><\/strong>\u00a0is a hollow, muscular, and stretchy organ that rests on the pelvic floor. It collects and stores <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4717\">urine<\/a> from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2988\">kidney<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\">\u00a0before the urine is eliminated through <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4726\">urination<\/a>. As shown in Figure 16.5.4, urine enters the urinary bladder from the ureters through two ureteral openings on either side of the back wall of the bladder. Urine leaves the bladder through a sphincter called the internal urethral sphincter. When the sphincter relaxes and opens, it allows urine to flow out of the bladder and into the urethra.<\/span><\/p>\n<figure id=\"attachment_1570\" aria-describedby=\"caption-attachment-1570\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-1568\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/2605_The_Bladder.jpg\" alt=\"16.5.4 Urinary Bladder\" width=\"1024\" height=\"627\"><figcaption id=\"caption-attachment-1570\" class=\"wp-caption-text\"><em>Figure 16.5.4 This diagram of the urinary bladder shows (a) a cross-sectional drawing of the entire bladder and (b) a microscopic cross-section of the tissues in the wall of the bladder.<\/em><\/figcaption><\/figure>\n<p>Like the ureters, the bladder is lined with transitional epithelium, which can flatten out and stretch as needed as the bladder fills with urine. The next layer (lamina propria) is a layer of loose connective tissue, nerves, and blood and lymphatic vessels. This is followed by a submucosa layer, which connects the lining of the bladder with the detrusor muscle in the walls of the bladder. The outer covering of the bladder is peritoneum, which is a smooth layer of epithelial cells that lines the abdominal cavity and covers most abdominal organs.<\/p>\n<p>The detrusor muscle in the wall of the bladder is made of smooth muscle fibres controlled by both the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2533\">autonomic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3014\">somatic<\/a> nervous systems. As the bladder fills, the detrusor muscle automatically relaxes to allow it to hold more urine. When the bladder is about half full, the stretching of the walls triggers the sensation of needing to urinate. When the individual is ready to void, conscious nervous signals cause the detrusor muscle to contract, and the internal urethral sphincter to relax and open. As a result, urine is forcefully expelled out of the bladder and into the urethra.<\/p>\n<div>\n<h1>Urethra<\/h1>\n<\/div>\n<p>The\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4732\">urethra<\/a><\/strong> is a tube that connects the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4731\">urinary bladder<\/a> to the external urethral orifice, which is the opening of the urethra on the surface of the body. As shown in Figure 16.5.5, the urethra in males travels through the penis, so it is much longer than the urethra in females. In males, the urethra averages about 20 cm (about 7.8 in) long, whereas in females, it averages only about 4.8 cm (about 1.9 in) long. In males, the urethra carries semen (as well as urine), but in females, it carries only urine.\u00a0 In addition, in males, the urethra passes through the prostate gland (part of the reproductive system) which is absent in women.<\/p>\n<figure id=\"attachment_1570\" aria-describedby=\"caption-attachment-1570\" style=\"width: 512px\" class=\"wp-caption aligncenter\"><img class=\"size-full wp-image-1570\" src=\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-Male_and_female_urethral_openings.svg_.png\" alt=\"16.5.5\" width=\"512\" height=\"355\"><figcaption id=\"caption-attachment-1570\" class=\"wp-caption-text\"><em>Figure 16.5.5 The male pelvis on the left and the female pelvis on the right. Notice how much longer the male urethra is because it travels through the length of the penis to reach the external urethral orifice.<\/em><\/figcaption><\/figure>\n<p>Like the ureters and bladder, the proximal (closer to the bladder) two-thirds of the urethra are lined with transitional epithelium. The distal (farther from the bladder) third of the urethra is lined with mucus-secreting epithelium. The mucus helps protect the epithelium from urine, which is corrosive. Below the epithelium is loose connective tissue, and below that are layers of smooth muscle that are continuous with the muscle layers of the urinary bladder. When the bladder contracts to forcefully expel urine, the smooth muscle of the urethra relaxes to allow the urine to pass through.<\/p>\n<p>In order for urine to leave the body through the external urethral orifice, the external urethral sphincter must relax and open. This sphincter is a striated muscle that is controlled by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3014\">somatic nervous system<\/a>, so it is under conscious, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3004\">voluntary<\/a> control in most people (exceptions are infants, some elderly people, and patients with certain injuries or disorders). The muscle can be held in a contracted state and hold in the urine until the person is ready to urinate. Following urination, the smooth muscle lining the urethra automatically contracts to re-establish muscle tone, and the individual consciously contracts the external urethral sphincter to close the external urethral opening.<\/p>\n<div>\n<div class=\"textbox textbox--key-takeaways\">\n<header class=\"textbox__header\">\n<h1 class=\"textbox__title\"><span style=\"color: #ffffff\">16.5 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_4943_4725\">Ureters<\/a>\u00a0are tube-like structures that connect the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2988\">kidneys<\/a>\u00a0with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4731\">urinary bladder<\/a>. Each ureter arises at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4724\">renal pelvis<\/a> of a kidney and travels down through the abdomen to the urinary bladder. The walls of the ureter contain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2982\">smooth muscle<\/a> that can contract to push <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4717\">urine<\/a> through the ureter by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_2890\">peristalsis<\/a>. The walls are lined with transitional epithelium that can expand and stretch.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4731\">urinary bladder<\/a> is a hollow, muscular organ that rests on the pelvic floor. It is also lined with transitional epithelium. The function of the bladder is to collect and store urine from the kidneys before the urine is eliminated through urination. Filling of the bladder triggers the sensation of needing to urinate. When a conscious decision to urinate is made, the detrusor muscle in the bladder wall contracts and forces urine out of the bladder and into the urethra.<\/li>\n<li>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4732\">urethra<\/a> is a tube that connects the urinary bladder to the external urethral orifice. Somatic nerves control the sphincter at the distal end of the urethra. This allows the opening of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_4627\">sphincter<\/a> for urination to be under <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_4943_3004\">voluntary<\/a> control.<\/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\">16.5 Review Questions<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<ol>\n<li>What are ureters?\u00a0 Describe the location of the ureters relative to other urinary tract organs.<\/li>\n<li>Identify layers in the walls of a ureter. How do they contribute to the ureter\u2019s function?<\/li>\n<li>Describe the urinary bladder. What is the function of the urinary bladder?<\/li>\n<li>\n<div id=\"h5p-318\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-318\" class=\"h5p-iframe\" data-content-id=\"318\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"16.5 Quiz\"><\/iframe><\/div>\n<\/div>\n<\/li>\n<li>How does the nervous system control the urinary bladder?<\/li>\n<li>What is the urethra?<\/li>\n<li>How does the nervous system control urination?<\/li>\n<li>Identify the sphincters that are located along the pathway from the ureters to the external urethral orifice.<\/li>\n<li>What are two differences between the male and female urethra?<\/li>\n<li>When the bladder muscle contracts, the smooth muscle in the walls of the urethra _________ .<\/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\">16.5 Explore More<\/span><\/h1>\n<\/header>\n<div class=\"textbox__content\">\n<p>https:\/\/youtu.be\/2Brajdazp1o<\/p>\n<p style=\"text-align: center\">The taboo secret to better health | Molly Winter, TED. 2016.<\/p>\n<p>https:\/\/youtu.be\/dg4_deyHLvQ<\/p>\n<p style=\"text-align: center\">What Happens When You Hold Your Pee? SciShow, 2016.<\/p>\n<p>&nbsp;<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<h2>Attributions<\/h2>\n<p><strong>Figure 16.5.1<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Cliche.jpg\" rel=\"cc:attributionURL\">Cliche<\/a> by <a class=\"external text\" href=\"https:\/\/www.flickr.com\/people\/27735730@N00\" rel=\"nofollow\">Jackie<\/a> on Wikimedia Common s 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 16.5.2<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:UrinarySystemMale.jpg\" rel=\"cc:attributionURL\">Urinary System Male<\/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) license.<\/p>\n<p><strong>Figure 16.5.3<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Kidney_and_adrenal_gland.jpg\" rel=\"cc:attributionURL\">Adrenal glands on Kidney by NCI Public Domain<\/a>\u00a0by Alan Hoofring (Illustrator) \/<a class=\"extiw\" title=\"en:National Cancer Institute\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Cancer_Institute\">National Cancer Institute<\/a> (<a href=\"https:\/\/visualsonline.cancer.gov\/details.cfm?imageid=4355\">photo ID 4355<\/a>) on Wikimedia Commons is in the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Public_domain\">public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).<\/p>\n<p><strong>Figure 16.5.4<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:2605_The_Bladder.jpg\" rel=\"cc:attributionURL\">2605_The_Bladder<\/a> by <a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-2-gross-anatomy-of-urine-transport\">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. <span class=\"os-caption\"><span class=\"search-highlight text last\" data-timestamp=\"1597636975236\" data-highlight-id=\"b46f272f-e324-486a-9b46-6607817bb903\" data-highlighted=\"true\">(Micrograph originally provided by <span class=\"search-highlight first text last\" data-timestamp=\"1597636975280\" data-highlight-id=\"fda2d69c-8a26-4beb-b12b-daf80f2c011d\" data-highlighted=\"true\">the<\/span>\u00a0Regents of\u00a0<span class=\"search-highlight first text last\" data-timestamp=\"1597636975286\" data-highlight-id=\"855d8b77-088b-43c9-81b4-0e0ff4f489e1\" data-highlighted=\"true\">the<\/span> University of Michigan Medical School \u00a9 2012.)<\/span><\/span><\/p>\n<p><strong>Figure 16.5.5<\/strong><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Male_and_female_urethral_openings.svg\" rel=\"cc:attributionURL\">512px-Male_and_female_urethral_openings.svg<\/a>\u00a0by <a title=\"User:Andrybak\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Andrybak\">andrybak<\/a> (derivative work) on 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. (Original: <a title=\"File:Male anatomy blank.svg\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Male_anatomy_blank.svg\">Male anatomy blank.svg<\/a>:\u00a0<a class=\"external text\" href=\"http:\/\/www.luckymojo.com\/faqs\/altsex\/penis.html\" rel=\"nofollow\">alt.sex FAQ<\/a>, derivative work:\u00a0<a title=\"User:Tsaitgaist\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Tsaitgaist\">Tsaitgaist<\/a>\u00a0<a title=\"File:Female anatomy with g-spot.svg\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Female_anatomy_with_g-spot.svg\">Female anatomy with g-spot.svg<\/a>:\u00a0<a title=\"User:Tsaitgaist\" href=\"https:\/\/commons.wikimedia.org\/wiki\/User:Tsaitgaist\">Tsaitgaist<\/a>.)<\/p>\n<h2>References<\/h2>\n<p class=\"hanging-indent\"><span class=\"os-title-label\"><span class=\"search-highlight text\" data-timestamp=\"1597636975236\" data-highlight-id=\"b46f272f-e324-486a-9b46-6607817bb903\" 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=\"1597636975236\" data-highlight-id=\"b46f272f-e324-486a-9b46-6607817bb903\" data-highlighted=\"true\">25.4<\/span><\/span><span class=\"os-divider\">\u00a0<\/span><span id=\"3366\" class=\"os-title\" data-type=\"title\"><span class=\"search-highlight text\" data-timestamp=\"1597636975236\" data-highlight-id=\"b46f272f-e324-486a-9b46-6607817bb903\" data-highlighted=\"true\">Bladder\u00a0<\/span><\/span><span class=\"os-caption\"><span class=\"search-highlight text last\" data-timestamp=\"1597636975236\" data-highlight-id=\"b46f272f-e324-486a-9b46-6607817bb903\" data-highlighted=\"true\">(a) Anterior cross section of the bladder. (b)\u00a0<span class=\"search-highlight first text last\" data-timestamp=\"1597636975256\" data-highlight-id=\"b7b616c9-437f-4937-a7f5-c1b47387a77e\" data-highlighted=\"true\">The<\/span>\u00a0detrusor muscle of\u00a0<span class=\"search-highlight first text last\" data-timestamp=\"1597636975257\" data-highlight-id=\"31417372-bff6-4d54-941d-580ae31d2bcf\" data-highlighted=\"true\">the bladder<\/span> (source: monkey tissue) LM \u00d7 448 [digital image].\u00a0 In <em>Anatomy and Physiology<\/em> (Section 7.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-2-gross-anatomy-of-urine-transport\u00a0<\/span><\/span><\/p>\n<p class=\"hanging-indent\">SciShow. (2016, January 22). What happens when you hold your pee? YouTube. https:\/\/www.youtube.com\/watch?v=dg4_deyHLvQ&amp;feature=youtu.be<\/p>\n<p class=\"hanging-indent\">TED. (2016, September 2). The taboo secret to better health | Molly Winter. YouTube. https:\/\/www.youtube.com\/watch?v=2Brajdazp1o&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_4943_5835\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_4943_5835\"><div tabindex=\"-1\"><p>A hollow, tube-like structure through which blood flows in the cardiovascular system; vein, artery, or capillary.<\/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":4,"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-4943","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\/4943","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\/4943\/revisions"}],"predecessor-version":[{"id":6461,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapters\/4943\/revisions\/6461"}],"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\/4943\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/media?parent=4943"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/pressbooks\/v2\/chapter-type?post=4943"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/contributor?post=4943"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-json\/wp\/v2\/license?post=4943"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}