11.4 Structure of Bone
Roasted Bone Marrow
Do you recognize the food item in the top left of Figure 11.4.1? It’s roasted , 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’s a soft tissue that, in adults, may be mostly fat. You’ll learn more about bone marrow and other tissues that make up bones when you read this section.
What Are Bones?
are organs that consist primarily of bone tissue, also called osseous tissue. is a type of connective tissue consisting mainly of 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.
Bone Anatomy
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 cells.
Types of Osseous Tissue
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.
Flat bones are typically enveloped by compact bone, with a center of spongy bone.
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. 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.
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.
Other Tissues in Bones
Besides compact and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum.
- is a soft connective tissue found inside a cavity, called the marrow cavity. There are two types of marrow in adults — 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 .
- 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.
Bone Cells
As shown in Figure 11.4.6, bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.
- are bone cells with a single nucleus that make and mineralize bone matrix. They make a protein mixture that 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.
- are 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 communication with other bone cells.
- are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the minerals in bone and release them into the blood.
- are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.
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 of mineral ions in the blood.
Types of Bones
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.
- 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.
- 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.
- are 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).
- are embedded in tendons, the connective tissues that bind muscles to bones. Sesamoid bones hold tendons farther away from joints, 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.
- are very small bones located between the major bones of the skull, within the joints (sutures) between the larger bones. They are not always present.
- 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.
Feature: Reliable Sources
Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often cure many 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’s condition.
Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:
- How does one become a potential bone marrow donor?
- Who can and who cannot donate bone marrow?
- How is a bone marrow donation made?
- What risks are there in donating bone marrow?
11.4 Summary
- are organs that consist mainly of (or osseous tissue). Osseous tissue is a type of connective tissue consisting of a matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard, without making it brittle.
- There are two types of osseous tissues: tissue and 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.
- Besides osseous tissues, bones also contain , , , and .
- Bone tissue is composed of four different types of bone cells: , , , and . 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.
- There are six types of bones in the human body: (such as the limb bones), (such as the wrist bones), (such as the patella), in the skull, and (such as the vertebrae).
11.4 Review Questions
- Describe osseous tissue.
- Why are bones hard, but not brittle?
- Compare and contrast the compact and spongy bone.
- What non-osseous tissues are found in bones?
- List four types of bone cells and their functions.
- Identify six types of bones. Give an example of each type.
- Compare and contrast yellow bone marrow and red bone marrow.
- Which type of bone cell divides to produce new bone cells? Where is this cell type located?
- Where do osteoblasts and osteocytes come from? How are they related to each other?
- Which type of bone is embedded in tendons?
11.4 Explore More
The Skeletal System: Crash Course A&P #19, CrashCourse, 2015.
Bone Remodeling and Modeling, Amgen, 2012.
How bones make blood – Melody Smith, TED-Ed, 2020.
Attributions
Figure 11.4.1
Bone_marrow_grilled_on_the_barbecue,_sliced_young_raw_garlic,_salted_leek_flowers_from_last_year,_lovage,_and_kale_(19098148350) by City Foodsters on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 11.4.2
Bone_cross-section.svg by Pbroks13 on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.3
Anatomy_of_a_Flat_Bone by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.4
the-detail-of-the-bones-the-structure-of-the-bones-spongy-bone-tramčina-close-up-structure on pxfuel are used according to the pxfuel Terms of Use.
Figure 11.4.5
Spongy_bone_-_Trabecular_bone_2_–_Smart-Servier by Laboratoires Servier/ Smart Servier website on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 11.4.6
Bone_cells by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.7
Blausen_0229_ClassificationofBones by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
References
Amgen. (2012, January 19). Bone remodeling and modeling. YouTube. https://www.youtube.com/watch?v=0dV1Bwe2v6c
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 6.9 Anatomy of a flat bone [digital image]. In Anatomy and Physiology (Section 6.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/6-3-bone-structure
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 6.11 Bone cells [digital image]. In Anatomy and Physiology (Section 6.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/6-3-bone-structure
CK-12 Foundation. (n.d.). Communication: Identifies means of communication between animals. ck12.org. https://www.ck12.org/c/life-science/communication/
CrashCourse. (2015, May 18). The skeletal system: Crash Course A&P #19. YouTube. https://www.youtube.com/watch?v=rDGqkMHPDqE
TED-Ed. (2020, January 27). How bones make blood – Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8
Created by CK-12 Foundation/Adapted by Christine Miller
Steady as She Goes
This device (Figure 7.8.1) looks simple, but it controls a complex system that keeps a home at a steady temperature — 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 — including the human body— maintain a steady state called homeostasis.
What Is Homeostasis?
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 , , , and throughout 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.
Set Point and Normal Range
For any given variable, such as body temperature or blood glucose level, there is a particular that is the physiological optimum value. The set point for human body temperature, for example, is about 37 degrees C (98.6 degrees F). As the body works to maintain 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 . 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).
A good analogy for set point, normal range, and maintenance of homeostasis is driving. When you are driving a vehicle on the road, you are supposed to drive in the centre of your lane — this is analogous to the . Sometimes, you are not driving in the exact centre of the lane, but you are still within your lines, so you are in the equivalent of the . 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. You'd move left if you were too close to the shoulder, or right if too close to the centre line — which is analogous to our next concept, to maintain .
Maintaining Homeostasis
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.
- The is 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.
- The monitors the values of the variable and sends data on it to the control centre.
- The 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.
- The 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.
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 . Biological examples of positive feedback are much less common.
Negative Feedback
In a , feedback serves to reduce an excessive response and keep a variable within the . Two processes controlled by negative feedback are body temperature regulation and control of blood glucose.
Body Temperature
Body temperature regulation involves , whether it lowers the temperature or raises it, as shown in Figure 7.8.3 and explained in the text that follows.
Cooling Down
The human body’s temperature regulatory centre is the in the brain. When the hypothalamus receives data from sensors in the skin and brain that body temperature is higher than the , it sets into motion the following responses:
- Blood vessels in the skin dilate (vasodilation) to allow more blood from the warm body core to flow close to the surface of the body, so heat can be radiated into the environment.
- 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 heat with it.
- Breathing becomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increases heat loss from the lungs.
Heating Up
When the brain’s temperature regulatory centre receives data that body temperature is lower than the set point, it sets into motion the following responses:
- Blood vessels in the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body, which reduces heat loss from the surface.
- As temperature falls lower, random signals to skeletal muscles are triggered, causing them to contract. This causes shivering, which generates a small amount of heat.
- The may be stimulated by the brain (via the pituitary gland) to secrete more thyroid hormone. This hormone increases metabolic activity and heat production in cells throughout the body.
- The may also be stimulated to secrete the hormone . This hormone causes the breakdown of glycogen (the carbohydrate used for energy storage in animals) to , which can be used as an energy source. This catabolic chemical process is , or heat producing.
Blood Glucose
In controlling the blood glucose level, certain endocrine cells in the pancreas (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.
- If the blood glucose level rises above the normal range, pancreatic beta cells release the hormone insulin 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.
- If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormone glucagon into 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.
https://www.youtube.com/watch?v=Iz0Q9nTZCw4
Homeostasis and Negative/Positive Feedback, Amoeba Sisters, 2017.
Positive Feedback
In a , 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.
Blood Clotting
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.
Childbirth
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 to the pituitary gland, which 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 nerve impulses to the brain, and the entire process stops.
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.
When Homeostasis Fails
Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, may 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 — or even death. is 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.
Normal aging may bring about a reduction in the efficiency of the body’s control systems, which makes the body more susceptible to disease. Older 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.
Feature: My Human Body
is 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.
Of the two types of diabetes, is the most common, accounting for about 90 per cent of all cases of diabetes in the United States. Type 2 diabetes 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.
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.
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’s how:
- Lose weight. Any weight loss is beneficial. Losing as little as seven per cent of your weight may be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.
- Exercise regularly. You should try to exercise for 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 blood pressure and improve your heart health. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.
- Adopt a healthy diet. 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.
- Control stress. Stress can increase your blood glucose and also raise your blood pressure and risk of heart disease. When you feel stressed out, do breathing exercises or take a brisk walk or jog. Try to replace stressful thoughts with more calming ones.
- Establish a support system. Enlist 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.
7.8 Summary
- 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.
- For any given variable, such as body temperature, there is a particular that is the physiological optimum value. The spread of values around the set point that is considered insignificant is called the .
- Homeostasis is generally maintained by a that includes a , , , and . 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.
- 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.
- Sometimes homeostatic mechanisms fail, resulting in . Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, which makes the elderly more susceptible to disease.
7.8 Review Questions
- Compare and contrast negative and positive feedback loops.
- Explain how negative feedback controls body temperature.
- Give two examples of physiological processes controlled by positive feedback loops.
- 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. What do you think might be the evolutionary benefit of the milk production regulation mechanism you described?
- Explain why homeostasis is regulated by negative feedback loops, rather than positive feedback loops.
- 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 pituitary 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.
- What is the stimulus in this system? Explain your answer.
- What is the control centre in this system? Explain your answer.
- In this system, is the pituitary considered the stimulus, sensor, control centre, or effector? Explain your answer.
7.8 Explore More
https://www.youtube.com/watch?v=LSgEJSlk6W4
Homeostasis - What Is Homeostasis - What Is Set Point For Homeostasis - Homeostasis In The Human Body, Whats Up Dude, 2017.
https://www.youtube.com/watch?v=XMsJ-3qRVJM
Attributions
Figure 7.8.1
Nest_Thermostat by Amanitamano on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
Figure 7.8.2
Negative_Feedback_Loops by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
Figure 7.8.3
Body Temperature Homeostasis by OpenStax College, Biology is used under a CC BY 4.0 license.
Figure 7.8.4
Homeostasis_of_blood_sugar by Christinelmiller on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 7.8.5
Positive_Feedback_Diagram_Blood_Clotting by Elliottuttle on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 7.8.6
Pregnancy-Positive_Feedback by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
References
Amoeba Sisters. (2017, September 7). Homeostasis and negative/positive feedback. YouTube. https://www.youtube.com/watch?v=Iz0Q9nTZCw4&feature=youtu.be
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 1.10 Negative feedback loop [digital image/ diagram]. In Anatomy and Physiology (Section 1.5). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-5-homeostasis
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 1.11 Positive feedback loop
Cognito. (2018, December 18). GCSE Biology - Homeostasis #38. YouTube. https://www.youtube.com/watch?v=XMsJ-3qRVJM&feature=youtu.be
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
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 OpenStax, Biology (Section 33.3). https://cnx.org/contents/GFy_h8cu@10.8:BP24ZReh@7/Homeostasis
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&feature=youtu.be
A rigid organ that constitutes part of the vertebrate skeleton in animals.
Created by CK-12 Foundation/Adapted by Christine Miller
As you learned in this chapter, the human body consists of many complex systems that normally work together efficiently — like a well-oiled machine — to carry out life’s 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 , 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 (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.
Recall that Jayson’s 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’s 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’s 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.
To try to find the exact cause of Jayson’s 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’s 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.
To treat Jayson’s 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’s 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.
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 – both in the causes of hydrocephalus and in its treatment.
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.
Thanks to Jayson’s 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 “machine."
To learn more about hydrocephalus and its treatment, watch this video from Boston Children's Hospital:
https://www.youtube.com/watch?v=bHD8zYImKqA
Hydrocephalus and its treatment | Boston Children’s Hospital, 2011.
Chapter 7 Summary
This chapter provided an overview of the organization and functioning of the human body. You learned that:
- The human body consists of multiple parts that function together to maintain life. The biology of the human body incorporates the body’s structure — or — and the body’s functioning, or .
- The organization of the human body is a hierarchy of increasing size and complexity, starting at the level of and and ending at the level of the entire .
- 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.
-
- Variations in cell function are generally reflected in variations in cell structure.
- 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.
- 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.
- 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 , osteoblasts, osteogenic cells, and osteoclasts.
- 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.
-
- , 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.
- , such as skin and mucous membranes, protect the body and its internal organs and secrete or absorb substances.
- are made up of cells that have the unique ability to contract. They include , , and tissues.
- are made up of , which transmit messages, and of various types, which play supporting roles.
- An is a structure that consists of two or more types of tissues that work together to do the same job. The and the are two examples.
-
- Many organs are composed of a major tissue that performs the organ’s main function, as well as other tissues that play supporting roles.
- The human body contains five organs that are considered vital for survival: the heart, brain, , , and . If any of these five organs stops functioning, death of the organism is imminent without medical intervention.
- An is a group of organs that work together to carry out a complex overall function. For example, the provides structure to the body and protects internal organs.
-
- There are 11 major organ systems in the human organism. They are the , , , , , , , , , , and . Only the reproductive system varies significantly between males and females.
- The human body is divided into a number of body cavities. 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.
-
- The is at the anterior (or front) of the trunk. It is subdivided into the, and the .
- The is at the posterior (or back) of the body, and includes the head and the back of the trunk. It is subdivided into the and .
- 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 and . The autonomic nervous system controls involuntary body functions, such as heart rate and digestion. The endocrine system secretes into the blood that travel to body cells and influence their activities.
-
- 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 with oxygen to produce carbon dioxide and energy. Cellular respiration requires the interaction of the digestive, cardiovascular, and respiratory systems.
- The 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.
- Playing softball or doing other voluntary physical activities may involve the interaction of nervous, muscular, skeletal, respiratory, and cardiovascular systems.
- 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.
-
- For any given variable (such as body temperature), there is a particular that is the physiological optimum value. The spread of values around the set point that is considered insignificant is called the .
- Homeostasis is generally maintained by a that includes a , , , and . 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.
- Sometimes homeostatic mechanisms fail, resulting in . Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, making the elderly more susceptible to disease.
- 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.
The severe and broad impact of hydrocephalus on the body’s 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.
Chapter 7 Review
-
- Compare and contrast tissues and organs.
- Which type of tissue lines the inner and outer surfaces of the body?
- What is a vital organ? What happens if a vital organ stops working?
- Name three organ systems that transport or remove wastes from the body.
- Name two types of tissue in the digestive system.
- Describe one way in which the integumentary and cardiovascular systems work together to regulate homeostasis in the human body.
- True or False: Body cavities are filled with air.
- In which organ system is the pituitary gland? Describe how the pituitary gland increases metabolism.
- 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?
- 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.
- What is the stimulus in this feedback loop?
- If the level of B1 falls significantly below the set point, what do you think happens to the production of A1? Why?
- What is the effector in this feedback loop?
- 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?
- What are the two main systems that allow various organ systems to communicate with each other?
- What are two functions of the hypothalamus?
Attributions
Figure 7.9.1
3D Medical Illustration Meninges Details by Scientific Animations on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 7.9.2
Hydrocephalus with Shunt from CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
References
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 1.3 Levels of structural organization of the human body [digital image]. In Anatomy and Physiology (Section 1.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-2-structural-organization-of-the-human-body
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 1.4 Organ systems of the human body [digital image]. In Anatomy and Physiology (Section 1.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-2-structural-organization-of-the-human-body
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 1.15 Dorsal and ventral body cavities [digital image]. In Anatomy and Physiology (Section 1.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-6-anatomical-terminology
Boston Children's Hospital. (2011, ). Hydrocephalus and its treatment | Boston Children’s Hospital. YouTube. https://www.youtube.com/watch?v=bHD8zYImKqA&feature=youtu.be
Brainard, J/ CK-12 Foundation. (2016). Figure 2 An illustration of a brain shunt [digital image]. In CK-12 College Human Biology (Section 9.8) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/9.8/
File:Body cavities lateral view labeled.jpg. (2018, January 4). Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Body_Cavities_Lateral_view_labeled.jpg&oldid=276851269. (Original image: Figure 1.15 Dorsal and ventral body cavities, from OpenStax, Anatomy and Physiology.)
File:Body cavities lateral view labeled.jpg. (2018, January 4). Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Body_Cavities_Lateral_view_labeled.jpg&oldid=276851269. (Original image: OpenStax [Version 8.25 from the textbook OpenStax Anatomy and Physiology] adapted for Review questions by Christine Miller].
Created by: CK-12/Adapted by Christine Miller
Is It Magic?
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.
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 compound we know as table salt? It isn't magic — it's chemistry, and it happens in a chemical reaction.
What Is a Chemical Reaction?
A is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a , and a substance that forms as a result of a chemical reaction is called a . During the reaction, the reactants are used up to create the products.
The burning of methane gas, as shown in the picture below, is a chemical reaction. In this reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of , which are 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 water.
Chemical Equations
Chemical reactions can be represented by chemical equations. A is a symbolic way of showing what happens during a chemical reaction. The burning of methane, for example, can be represented by the chemical equation:
CH4 + 2O2 → CO2 + 2H2O
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 2 in front of O2 and H2O, called the coefficient, shows that two oxygen molecules and two water molecules are involved in the reaction. If just one molecule is involved, no number is placed in front of the chemical symbol. Note the subscript of 2 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 two oxygen atoms. If there is no subscript, then there is a single atom. Thus, one water molecule is made up of two hydrogen atoms and one oxygen atom. In order for this chemical reaction to take place, one methane molecule reacts with two oxygen molecules to form one carbon dioxide molecule and two water molecules.
Conservation of Mass
In a chemical reaction, the quantity of each element does 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 — which was first demonstrated convincingly by French chemist Antoine Lavoisier in 1785 — 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. The conservation of mass is reflected in a reaction's chemical equation. The same number of atoms of each element appears 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?
Chemical vs. Physical Changes
Many processes that happen all around us on a daily basis involve . 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. 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.
Other common examples of chemical changes include a cake baking, metal rusting, and a candle burning. More practice is below.
Figure 3.8.5 Chemical changes often involve chemical reactions as well.
3.8 Summary
- A is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a , and a substance that forms during a chemical reaction is called a . During the chemical reaction, bonds break in reactants and new bonds form in products.
- Chemical reactions can be represented by . According to the , 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.
- Many chemical reactions — such as iron rusting and organic matter rotting — occur all around us each day, but not all changes are chemical processes. Some changes — like ice melting or paper being torn into smaller pieces — are physical processes that do not involve chemical reactions and the formation of new substances.
3.8 Review Questions
- What is a chemical reaction?
- Define the reactants and products in a chemical reaction.
- List three examples of common changes that involve chemical reactions.
- Define a chemical bond.
- What is a chemical equation? Give an example.
- What does it mean for a chemical equation to be balanced? Why must a chemical equation be balanced?
- Our cells use glucose (C6H12O6) to obtain energy in a chemical reaction called cellular respiration. In this reaction, six oxygen molecules (O2) react with one glucose molecule. Answer the following questions about this reaction:
- How many oxygen atoms are in one molecule of glucose?
- Write out what the reactant side of this equation would look like.
- In total, how many oxygen atoms are in the reactants? Explain how you calculated your answer.
- 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?
- Answer the following questions about the following equation: CH4+ 2O2 → CO2 + 2H2O
- Can carbon dioxide (CO2)transform into methane (CH4) and oxygen (O2) in this reaction? Why or why not?
- How many molecules of carbon dioxide (CO2) are produced in this reaction?
- Is the evaporation of liquid water into water vapor a chemical reaction? Why or why not?
- Why do bonds break in the reactants during a chemical reaction?
3.8 Explore More
https://www.youtube.com/watch?v=2S6e11NBwiw&feature=emb_logo
The law of conservation of mass - Todd Ramsey, TED-Ed, 2015.
https://www.youtube.com/watch?v=37pir0ej_SE
Chemical Changes: Crash Course Kids #19.2, by Crash Course Kids, 2015.
Attributions
Figure 3.8.1
Chlorine_gas_in_high_concentration by Larenmclane on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 3.8.2
Tags: Salt Salt Shaker Spices Kitchen Spice Component; salt-4160306_1280 by katie175 from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
Figure 3.8.3
Tags: Gas Flame Gas Stove Italy Gas Cook Kitchen by moerschy from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
Figure 3.8.4
Antoine_lavoisier 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 public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 3.8.5
- Ice cream melting by Aron Visuals on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Kombucha [photo] by Klara Avsenik on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Grated cheese by Steve Buissinne on PublicDomainPictures is used under the CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).
References
Crash Course Kids. (2015, July 16). Chemical changes: Crash Course Kids #19.2. YouTube. https://www.youtube.com/watch?v=37pir0ej_SE
TED-Ed. (2015, February 26 ). The law of conservation of mass - Todd Ramsey. YouTube. https://www.youtube.com/watch?v=2S6e11NBwiw&feature=emb_logo
Wikipedia contributors. (2020, June 15). Antoine Lavoisier. Wikipedia. https://en.wikipedia.org/w/index.php?title=Antoine_Lavoisier&oldid=962631283
Created by CK-12 Foundation/Adapted by Christine Miller
Art in a Cup
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’s 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.
What Are Psychoactive Drugs?
are 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 , 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).
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.
Figure 8.8.2 These psychoactive drugs are legal and accessible almost anywhere.
Classes of Psychoactive Drugs
Psychoactive drugs are divided into different classes based on their pharmacological effects. Several classes are listed below, along with examples of commonly used drugs in each class.
- are drugs that stimulate the brain and increase alertness and wakefulness. Examples of stimulants include caffeine, nicotine, cocaine, and amphetamines (such as Adderall).
- are 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.
- are 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).
- are 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).
- are 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.
- are drugs that produce feelings of empathy, or sympathy with other people. Examples of empathogens include amphetamines and MDMA.
Many psychoactive drugs have multiple effects, so they may be placed in more than one class. One example is MDMA, pictured below, 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.
Mechanisms of Action
Psychoactive drugs generally produce their effects by affecting brain chemistry, which in turn may cause changes in a person’s 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.
- are drugs that increase the activity of particular . 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.
- are 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.
Consider the example of the neurotransmitter . This is one of the most common neurotransmitters in the brain, and it normally has an inhibitory effect on cells. GABA agonists — which increase its activity — include ethanol, barbiturates, and benzodiazepines, among other psychoactive drugs. All of these drugs work by promoting the activity of GABA receptors in the brain.
Uses of Psychoactive Drugs
You may have been prescribed psychoactive drugs by your doctor. For example, your doctor may have prescribed you an 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.
Medical Uses
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.
Many psychiatric disorders are also managed with psychoactive drugs. Antidepressants like sertraline, for example, are used to treat depression, anxiety, and eating 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) — as well as mood stabilizers, such as lithium — are used to treat schizophrenia and bipolar disorder.
Ritual Uses
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 Amanita muscaria, which contains a hallucinogenic drug called muscimol, was used for similar purposes. Various other psychoactive drugs — including jimsonweed, psilocybin mushrooms, and cannabis — have also been used for millennia, by various peoples, for ritual purposes.
Recreational Uses
The recreational use of psychoactive drugs generally has the purpose of altering one’s consciousness and creating a feeling of euphoria commonly called a “high.” 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.
Some investigators have suggested that the urge to alter one’s 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.
Addiction, Dependence, and Rehabilitation
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. refers 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. may 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.
Rehabilitation for drug dependence and addiction typically involves psychotherapy, which may include both individual and group therapy. Organizations such as Alcoholics Anonymous (AA) and Narcotics Anonymous (NA) may also be helpful for people trying to recover from addiction. These groups are self-described as international mutual aid fellowships, and their 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. The drug methadone, for example, is commonly used to treat heroin addiction.
Feature: Human Biology in the News
In North America, a lot of media attention is currently given to a rising tide of opioid addiction and overdose deaths. are drugs derived from the opium poppy or synthetic versions of such drugs. They include the illegal drug heroin, as well as prescription painkillers such as 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.
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:
- In 2016, there were almost 2,500 opioid-related deaths in Canada — almost 7 per day.
- The number of prescriptions written for opioids quadrupled between 1999 and 2010. If you have been prescribed codeine, fentanyl, morphine, oxycodone, hydromorphone or medical heroin, then you have been prescribed an opiate.
- There are many long-term health effects of using opioids, which include:
- Increased tolerance to the drug.
- Liver damage.
- Substance use disorder or addiction.
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, more treatment programs, and more widespread distribution and use of the opioid-overdose antidote naloxone (Narcan). Opioids can slow or stop a person's breathing, which is what usually causes overdose deaths. Naloxone helps the person wake up and keeps them breathing until emergency medical treatment can be provided.
What, if anything, will work to stop the opioid epidemic in Canada and the United States? Keep watching the news to find out.
8.8 Summary
- 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).
- 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.
- Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as agonists — which enhance the activity of particular — or as antagonists, which decrease the activity of particular neurotransmitters.
- Psychoactive drugs are used for various purposes, including medical, ritual, and recreational purposes.
- Misuse of psychoactive drugs may lead to , 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 on the drug. Rehabilitation typically involves psychotherapy, and sometimes the temporary use of other psychoactive drugs.
8.8 Review Questions
- What are psychoactive drugs?
- Identify six classes of psychoactive drugs, along with an example of a drug in each class.
- Compare and contrast psychoactive drugs that are agonists and psychoactive drugs that are antagonists.
- Describe two medical uses of psychoactive drugs.
- Give an example of a ritual use of a psychoactive drug.
- Generally speaking, why do people use psychoactive drugs recreationally?
- Define addiction.
- Identify possible withdrawal symptoms associated with physical dependence on a psychoactive drug.
- Why might a person with a heroin addiction be prescribed the psychoactive drug methadone?
- The prescription drug Prozac inhibits the reuptake of the neurotransmitter serotonin, causing more serotonin to be present in the synapse. Prozac can elevate mood, which is why it is sometimes used to treat depression. Answer the following questions about Prozac:
- Is Prozac an agonist or an antagonist for serotonin? Explain your answer.
- Is Prozac a psychoactive drug? Explain your answer.
- Name three classes of psychoactive drugs that include opioids.
- True or False: All psychoactive drugs are either illegal or available by prescription only.
- True or False: Anxiolytics might be prescribed by a physician.
- Name two drugs that activate receptors for the neurotransmitter GABA. Why do you think these drugs generally have a depressant effect?
8.8 Explore More
https://www.youtube.com/watch?v=foLf5Bi9qXs
How does caffeine keep us awake? - Hanan Qasim, TED-Ed, 2017.
https://www.youtube.com/watch?v=8qK0hxuXOC8
How do drugs affect the brain? - Sara Garofalo, TED-Ed, 2017.
https://www.youtube.com/watch?v=Nlcr1jd_Tok
Is marijuana bad for your brain? - Anees Bahji, TED-Ed, 2019.
Attributions
Figure 8.8.1
Cappucino Art by drew-coffman-tZKwLRO904E [photo] by Drew Coffman on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 8.8.2
- 3804, Saint-Laurent, Montreal - Cannabis Culture shop by Exile on Ontario St (Montreal, Canada) on Wikimedia Commons is used under a CC BY SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/deed.en) license.
- Drive Through Cigarette Store by Cosmo Spacely on Flickr is used under CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
- Franklin-Nicollet Liquors by Max Sparber on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Figure 8.8.3
Ecstasy_monogram by Drug Enforcement Administration on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 8.8.4
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) by U.S. Navy photo by Photographer’s Mate Airman Timothy F. Sosais on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 8.8.5
Peyote Lophophora_williamsii_pm by Peter A. Mansfeld on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 8.8.6
Cat under effects of catnip/Self Indulgence by Katieb50 on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
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
Belzak, L., & Halverson, J. (2018). The opioid crisis in Canada: a national perspective. La crise des opioïdes au Canada : une perspective nationale. Health promotion and chronic disease prevention in Canada : research, policy and practice, 38(6), 224–233. https://doi.org/10.24095/hpcdp.38.6.02
British Columbia Regional Service Committee of Narcotics Anonymous. (n.d.). Welcome to the B.C. region of N.A. [website]. https://www.bcrna.ca/
Centers for Disease Control and Prevention (CDC). (2011 November 4). Vital signs: overdoses of prescription opioid pain relievers—United States, 1999–2008. Morbidity and Mortality Weekly Report (MMWR),60(43):1487-1492. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6043a4.htm
TED-Ed. (2017, June 29). How do drugs affect the brain? - Sara Garofalo. YouTube. https://www.youtube.com/watch?v=8qK0hxuXOC8&feature=youtu.be
TED-Ed. (2017, July 17). How does caffeine keep us awake? - Hanan Qasim. YouTube. https://www.youtube.com/watch?v=foLf5Bi9qXs&feature=youtu.be
TED-Ed. (2019, December 2). Is marijuana bad for your brain? - Anees Bahji. YouTube. https://www.youtube.com/watch?v=Nlcr1jd_Tok&feature=youtu.be
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study Conclusion: Fading Memory
The illustration above (Figure 8.9.1) shows some of the molecular and cellular changes that occur in Alzheimer’s 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.
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’s 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 can perform 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.
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.
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’s symptoms of mild AD involve deficits in memory, such as trouble remembering where she placed objects, recent conversations, and appointments.
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 like reasoning, language, and interpretation of sensory stimuli. You can also see how the frontal lobe — which controls executive functions such as planning, self-control, and abstract thought — becomes increasingly damaged.
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.
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’s brain. This increase in neuronal communication can help restore some of the functions lost in early Alzheimer’s disease and may slow the progression of symptoms.
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. Scientists, however, are studying the ways in which neurogenesis might be increased in cases of disease or injury to the brain. They 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.
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. A radiolabeled 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.
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 as 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.
Chapter 8 Summary
In this chapter, you learned about the human nervous system. Specifically, you learned that:
- The is the organ system that coordinates all of the body’s and actions by transmitting signals to and from different parts of the body. It has two major divisions: the (CNS) and the (PNS).
- The CNS includes the brain and spinal cord.
- The PNS consists mainly of nerves that connect the CNS with the rest of the body. It has two major divisions: the and the . 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.
- The autonomic nervous system is further divided into the (which controls the fight-or-flight response), the (which controls most routine involuntary responses), and the (which provides local control for digestive processes).
- Signals sent by the nervous system are electrical signals called . They are transmitted by special, electrically excitable cells called , which are one of two major types of cells in the nervous system.
- 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.
- The main parts of a neuron include the , , and . 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.
- Axons are often wrapped in an electrically-insulating , which is produced by oligodendrocytes or schwann cells, both of which are types of neuroglia. Electrical impulses called occur at gaps in the myelin sheath, called , which speeds the conduction of nerve impulses down the axon.
- , or the formation of new neurons by cell division, may occur in a mature human brain — but only to a limited extent.
- The nervous tissue in the and consists of gray matter — which contains mainly unmyelinated cell bodies and dendrites of neurons — 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.
- 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. carry nerve impulses away from the body and toward the central nervous system, carry them away from the central nervous system and toward the body, and often carry them between sensory and motor neurons.
- A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.
- The 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.
- 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’s axon as an electric current.
- A nerve impulse is transmitted to another cell at either an electrical or a chemical . At a chemical synapse, chemicals are released from the into the synaptic cleft between cells. The chemicals travel across the cleft to the and bind to receptors embedded in its membrane.
- 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.
- The CNS includes the brain and spinal cord. It is physically protected by , , and . It is chemically protected by the blood-brain barrier.
- 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’s energy, primarily in the form of .
-
- The brain is divided into three major parts, each with different functions: the forebrain, the midbrain and the hindbrain.
- The forebrain includes the , the , the , the and the . 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. 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.
- The thalamus, hypothalamus, hippocampus and amygdala are all part of the limbic system which helps regulate memories, coordination and attention
- The brain is divided into three major parts, each with different functions: the forebrain, the midbrain and the hindbrain.
- The 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.
- 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.
- 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.
- The tissues that make up the PNS are and . Nerves are bundles of axons and ganglia are groups of cell bodies. Nerves are classified as sensory, motor, or a mix of the two.
- 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.
- 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.
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include (mechanical forces), (temperature), (pain), (light), and (chemicals).
- includes the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.
- 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 “tells” us what we are seeing.
- Common vision problems include (nearsightedness), (farsightedness), and (age-related decline in close vision).
- 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 “tells” us what we are hearing.
- The ear is also the organ responsible for the sense of , 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.
- and smellno post 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.
- 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).
- Psychoactive drugs are divided into different classes according to their pharmacological effects. They include , , , , , and . Many psychoactive drugs have multiple effects, so they may be placed in more than one class.
- Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as , which enhance the activity of particular neurotransmitters, or as , which decrease the activity of particular neurotransmitters.
- Psychoactive drugs are used for medical, ritual, and recreational purposes.
- Misuse of psychoactive drugs may lead to , which is the compulsive use of a drug, despite its negative consequences. Sustained use of an addictive drug may produce physical or psychological on the drug. Rehabilitation typically involves psychotherapy, and sometimes the temporary use of other psychoactive drugs.
In addition to the nervous system, there is another system of the body that is important for coordinating and regulating many different functions – the endocrine system. You will learn about the endocrine system in the next chapter.
Chapter 8 Review
- 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.
- Which part of the brain is neuron A located in — the cerebellum, cerebrum, or brain stem? Explain how you know.
- 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?
- 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?
- Neuron A forms a chemical synapse with neuron B in the spinal cord. How is the signal from neuron A transmitted to neuron B?
- Is neuron A in the central nervous system (CNS) or peripheral nervous system (PNS)?
- 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?
- What part of the PNS is involved in this pathway — the autonomic nervous system or the somatic nervous system? Explain your answer.
- What are the differences between a neurotransmitter receptor and a sensory receptor?
- 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.
- Electrical gradients are responsible for the resting potential and action potential in neurons. Answer the following questions about the electrical characteristics of neurons.
- Define an electrical gradient, in the context of a cell.
- What is responsible for maintaining the electrical gradient that results in the resting potential?
- Compare and contrast the resting potential and the action potential.
- Where along a myelinated axon does the action potential occur? Why does it happen here?
- What does it mean that the action potential is “all-or-none?”
- Compare and contrast Schwann cells and oligodendrocytes.
- 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.
- 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.
Attributions
Figure 8.9.1
Alzheimers_Disease by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 8.9.2
Alzheimer’s Disease stagess by NIH Image Gallery on Flickr is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Created by CK-12/Adapted by Christine Miller
So Many Species!
The collage shows a single in each of the six kingdoms into which all of Earth's living things are commonly classified. How many species are there in each ? In a word: millions. A total of almost two million living species have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million unique species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.
What Is Biodiversity?
Biological diversity, or , refers 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.
- Species diversity refers to the number of different species in an ecosystem or on Earth as a whole. This is the most common way to measure biodiversity. Current estimates for Earth's total number of living species range from 5 to 30 million species.
- Genetic diversity refers to the variation in genes within all of these species.
- Ecosystem diversity refers to the variety of ecosystems on Earth. An is a system formed by populations of many different species interacting with each other and their environment.
https://www.youtube.com/watch?v=GK_vRtHJZu4
Why is Biodiversity So Important? - Kim Preshoff, TEDEd, 2015
Defining a Species
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. Formally, a is 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, Homo sapiens. All humans can potentially interbreed with each other, but not with members of any other species.
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 — 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 all paleontologists generally define species on the basis of morphology, rather than breeding behavior. Morphology refers 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 appear, the greater the chance that they will be classified in the same species.
Classifying Living Things
People have been trying to classify the tremendous diversity of life on Earth for more than two thousand years. The science of classifying organisms is called . Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. It helps us make sense of the overwhelming diversity of living things.
Linnaean Classification
All modern classification systems have their roots in the Linnaean classification system, which was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things known in his time by grouping together organisms that s
hared obvious morphological traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the “father of taxonomy.”
The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon). In the original system, taxa ranged from the kingdom to the species. The (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.
A phrase to help you remember the order of the groupings is shown below. The first letter of each word is the first letter of the level of classification.
Dad Keeps Pots Clean Or Family Gets Sick
The hierarchy of taxa in the original Linnaean system of taxonomy included taxa from the species to the kingdom. The domain was added later.
Binomial Nomenclature
Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called binomial nomenclature, gives each species a unique, two-word Latin name consisting of the genus name followed by a specific species identifier. An example is Homo sapiens, the two-word Latin name for humans. It literally means “wise human.” This is a reference to our big brains.
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’s last name usually pins down exactly which Michael you mean. In the same way, having two names for a species helps to uniquely identify it.
Revisions in the Linnaean Classification
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.
A major change to the Linnaean system is the addition of a new taxon called the domain. The is a taxon that is larger and more inclusive than the kingdom, as shown in the figure above. Most biologists agree that there are three domains of life on Earth: Bacteria, Archaea, and Eukarya . Both the Bacteria and the Archaea domains consist of single-celled organisms that lack a . This means that their genetic material is not enclosed within a membrane inside the cell. The Eukarya domain, in contrast, consists of all organisms whose do have a nucleus, so that their genetic material is enclosed within a membrane inside the cell. The Eukarya domain is made up of both single-celled and multicellular organisms. This domain includes several kingdoms, including the animal, plant, fungus, and protist kingdoms.
The three domains of life, as well as how they are related to each other and to a common ancestor. There are several theories about how the three domains are related and which arose first, or from another.
Phylogenetic Classification
Linnaeus classified organisms based on morphology. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s, scientists looked for a way to classify organisms that accounted for phylogeny. is 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 above to illustrate the three domains. A shows how closely related different groups of organisms are to one another. Each branching point represents a common ancestor of the branching groups.
2.4 Summary
- Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity (within species), and ecosystem diversity.
- The formal biological definition of species is a group of actually or potentially interbreeding organisms. Our own species, Homo sapiens,is an example. In reality, organisms are often classified into species on the basis of morphology.
- 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.
- The domain — a taxon higher than the kingdom — 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 domain.
- Modern systems of classification take into account phylogenies, 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
2.4 Review Questions
- What is biodiversity? Identify three ways that biodiversity may be measured.
- Define biological species. Why is this definition often difficult to apply?
- Explain why it is important to classify living things, and outline the Linnaean system of classification.
- What is binomial nomenclature? Give an example.
- Contrast the Linnaean and phylogenetic systems of classification.
- Describe the taxon called the domain, and compare the three widely recognized domains of living things.
- 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.
- A scientist discovers a new single-celled organism. Answer the following questions about this discovery.
- 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?
- What is one type of information that could help the scientist classify the organism?
- Define morphology. Give an example of a morphological trait in humans.
- Which type of biodiversity is represented in the differences between humans?
- Why do you think it is important to the definition of a species that members of a species can produce fertile offspring?
- Go to the A-Z Animals Animal Classification Page. In the search box, put in your favorite animal and write out it's classification.
2.4 Explore More
https://youtu.be/DVouQRAKxYo
Classification, Amoeba Sisters, 2013.
Attributions
Figure 2.4.1 (6 Kingdoms collage)
- Salmonella, by unknown/ NIAID on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
- Fern from pxhere, is used under a CC0 1.0 universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
- Photo [squirrel] , by Radoslaw Prekurat on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Blood Milk Mushroom by Hans on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
- Fungi by Ste Wright on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- EscherichiaColi NIAID [adapted], by Rocky Mountain Laboratories, ca:NIAID, ca:NIH on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 2.4.2
Biological classification, by Pengo [Peter Halasz] on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 2.4.3
The three domains of life and major groups within, by C. Miller, 2019, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2017, March 8). Classification. YouTube. https://www.youtube.com/watch?v=DVouQRAKxYo&feature=youtu.be
A-Z Animals. (2008, December 1). Animal classification. https://a-z-animals.com/reference/animal-classification/
TED-Ed. (2015, April 20). Why is biodiversity so important? - Kim Preshoff. YouTube. https://www.youtube.com/watch?v=GK_vRtHJZu4
Wikipedia contributors. (2020, June 21). Carl Linnaeus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Carl_Linnaeus&oldid=963767022
Created by CK-12 Foundation/Adapted by Christine Miller
Art for All Eras
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 — about 2 m2 in adults. The skin is the major organ in the .
What Is the Integumentary System?
In addition to the skin, the includes the hair and nails, 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 the environment, and help the body maintain .
The Skin
The is remarkable not only because it is the body’s 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).
Outer Layer of Skin
The outer layer of skin is the . This layer is thinner than the inner layer (the dermis). The epidermis consists mainly of epithelial cells, called , which produce the tough, fibrous protein . The innermost cells of the epidermis are that 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 , the cells that produce the brown pigment melanin, which gives skin most of its colour. Although the epidermis contains some sensory receptor cells — called — it contains no nerves, blood vessels, or other structures.
Inner Layer of Skin
The is the inner, thicker layer of skin. It consists mainly of tough , 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.
Functions of the Skin
The skin has multiple roles in the body. Many of these roles are related to . The skin’s main functions are preventing water loss 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.
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.
Hair
is a fibre found only in mammals. It consists mainly of keratin-producing . Each hair grows out of a in the . By the time the hair reaches the surface, it consists mainly of dead cells filled with . 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.
Nails
Fingernails and toenails consist of dead filled with . The keratin makes them hard but flexible, which is important for the functions they serve. prevent 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.
Interactions with Other Organ Systems
The skin and other parts of the work with other organ systems to maintain .
- The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.
- Vitamin D is needed by the digestive system to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.
- To control body temperature, the skin works with the cardiovascular system to either lose body heat, or to conserve it through vasodilation or vasoconstriction.
- To detect certain sensations from the outside world, the nervous system depends on nerve receptors in the skin.
10.2 Summary
- The consists of the , , and . Functions of the integumentary system include providing a protective covering for the body, sensing the environment, and helping the body maintain homeostasis.
- The skin consists of two distinct layers: a thinner outer layer called the , and a thicker inner layer called the .
- The epidermis consists mainly of epithelial cells called , which produce . 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.
- The dermis consists mainly of tough and many structures, including blood vessels, sensory receptors, nerves, hair follicles, and oil and sweat glands.
- The ’s main functions are preventing water loss from the body, serving as a barrier to the entry of microorganisms, synthesizing vitamin D, blocking UV light, and helping to regulate body temperature.
- consists mainly of dead keratinocytes and grows out of in 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 eyes. Hair all over the body provides tactile sensory input.
- Like hair, also 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.
10.2 Review Questions
- Name the organs of the integumentary system.
- Compare and contrast the epidermis and dermis.
- Identify functions of the skin.
- What is the composition of hair?
- Describe three physiological roles played by hair.
- What do nails consist of?
- List two functions of nails.
- In terms of composition, what do the outermost surface of the skin, the nails, and hair have in common?
- Identify two types of cells found in the epidermis of the skin. Describe their functions.
- Which structure and layer of skin does hair grow out of?
- Identify three main functions of the integumentary system. Give an example of each.
- What are two ways in which the integumentary system protects the body against UV radiation?
10.2 Explore More
https://www.youtube.com/watch?v=OxPlCkTKhzY
The science of skin - Emma Bryce, TED-Ed, 2018.
https://www.youtube.com/watch?v=ZSJITdsTze0&feature=emb_logo
Why do we have to wear sunscreen? - Kevin P. Boyd, TED-Ed, 2013.
https://www.youtube.com/watch?time_continue=1&v=Lfhot7tQcWs&feature=emb_logo
Scarification | National Geographic, 2008.
Attributions
Figure 10.2.1
Maud_Stevens_Wagner -The Plaza Gallery, Los Angeles, 1907 from the Library of Congress on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.2.2
Anatomy_The_Skin_-_NCI_Visuals_Online by Don Bliss (artist) from National Cancer Institute, on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.2.3
shashank-shekhar-Db1J_qp_ctc [photo] by Shashank Shekhar on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.2.4
Eyelashes by aryan-dhiman-93NBu0zG_H4 [photo] by Aryan Dhiman on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Reference
National Geographic. (2008). Scarification | National Geographic. YouTube. https://www.youtube.com/watch?v=Lfhot7tQcWs&t=1s
TED-Ed. (2018, March 12). The science of skin - Emma Bryce. YouTube. https://www.youtube.com/watch?v=OxPlCkTKhzY&feature=youtu.be
TED-Ed. (2013, August 6). Why do we have to wear sunscreen? - Kevin P. Boyd. YouTube. https://www.youtube.com/watch?v=ZSJITdsTze0&feature=youtu.be
Created by CK-12 Foundation/Adapted by Christine Miller
Feel the Burn
The person in Figure 10.3.1 is no doubt feeling the burn — sunburn, that is. occurs when the outer layer of the skin is damaged by from the sun or 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 skin . 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 .
What is the Epidermis?
The is the outer of the two main layers of the . The inner layer is the . 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 — but structurally distinct from — the that line the mouth, anus, urethra, and vagina.
Structure of the Epidermis
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 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.
Cells of the Epidermis
There are several different types of cells in the epidermis. All of the cells are necessary for the important functions of the epidermis.
- The epidermis consists mainly of stacks of -producing epithelial cells called . 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.
- Another eight per cent of epidermal cells are . These cells produce the pigment melanin that protects the dermis from UV light.
- About one per cent of epidermal cells are . These are immune system cells that detect and fight pathogens entering the skin.
- Less than one per cent of epidermal cells are , which respond to light touch and connect to nerve endings in the dermis.
Layers of the Epidermis
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.
Stratum Basale
The is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the . The stratum basale contains stem cells — called — which divide to form all the 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. and are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.
Stratum Spinosum
Just above the stratum basale is the . This is the thickest of 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 .
Stratum Granulosum
The next layer above the stratum spinosum is the . In this layer, keratinocytes have become nearly filled with , giving their cytoplasm a granular appearance. are released by keratinocytes in this layer to form a lipid barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from blood vessels in the dermis to receive nutrients. Each dying cell digests its own and , leaving behind only a tough, keratin-filled shell.
Stratum Lucidum
Only on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the . This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.
Stratum Corneum
The uppermost layer of the epidermis everywhere on the body is the . 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.
Functions of the Epidermis
The epidermis has several crucial functions in the body. These functions include protection, water retention, and vitamin D synthesis.
Protective Functions
The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.
Protection from Physical Damage
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.
Protection from Pathogens
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’s why it is important to clean and cover even a minor 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 of about 5.0), low amounts of water, the presence of antimicrobial substances produced by epidermal cells, and competition with non-pathogenic microorganisms that normally live on the epidermis.
Protection from UV Light
that penetrates the epidermis can damage epidermal cells. In particular, it can cause mutations in that lead to the development of skin , 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 reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body’s vitamin B9 level by 50 per cent.
s in the stratum basale of the epidermis contain small organelles called , which produce, store, and transport the dark brown pigment . As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to in 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.
Water Retention
Skin's ability to hold water and not lose it to the surrounding environment is due mainly to the . arranged 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.
Although the skin is impermeable to water, it is not impermeable to all substances. Instead, the skin is , allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.
- Selective permeability allows certain medications to enter the bloodstream through the capillaries in the . 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 (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).
- 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.
Vitamin D Synthesis
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.
What Gives Skin Its Colour?
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.
- The pigment is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.
- is a red pigment found in red blood cells. 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.
Hear what Bill Nye has to say about the subject of skin colour in the video here.
Bacteria on Skin
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.
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. If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain species, 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 hair follicles) and in adjacent areas of the skin.
Feature: Myth vs. Reality
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.
Myth |
Reality |
"Sunblock and sunscreen are just different names for the same type of product. They both work the same way and are equally effective." | 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. |
"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." | Even on cloudy days, a significant amount of UV radiation penetrates the atmosphere to strike Earth’s surface. Therefore, using sunscreens or sunblocks to protect exposed skin is important even when there are clouds in the sky. |
"People who have dark skin, such as African Americans, do not need to worry about skin damage from UV light." | 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. |
"Sunscreens with an SPF (sun protection factor) of 15 are adequate to fully protect the skin from UV light." | 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 sun exposure and reapplying sunscreens often, especially if you are sweating or spending time in the water. |
"Using tanning beds is safer than tanning outside in natural sunlight." | 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 skin cancer than 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. |
10.3 Summary
- The is the outer of the two main layers of the skin. It is very thin, but has a complex structure.
- Cell types in the epidermis include that produce and make up 90 per cent of epidermal cells, that produce , that fight in the skin, and that respond to light touch.
- 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.
- The innermost layer of the epidermis is the , which contains stem cells that divide to form new keratinocytes. The next layer is the , which is the thickest layer and contains Langerhans cells and spiny keratinocytes. This is followed by the , in which keratinocytes are filling with keratin and starting to die. The is next, but only on the palms and soles. It consists of translucent dead keratinocytes. The outermost layer is the , which consists of flat, dead, tightly packed keratinocytes that form a tough, waterproof barrier for the rest of the epidermis.
- Functions of the epidermis include protecting underlying tissues from physical damage and pathogens. Melanin in the epidermis absorbs and protects underlying tissues from . The epidermis also prevents loss of water from the body and synthesizes vitamin D.
- 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.
- The surface of healthy skin normally is covered by vast numbers of representing about one thousand species from 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.
10.3 Review Questions
- What is the epidermis?
- Identify the types of cells in the epidermis.
- Describe the layers of the epidermis.
- State one function of each of the four epidermal layers found all over the body.
- Explain three ways the epidermis protects the body.
- What makes the skin waterproof?
- Why is the selective permeability of the epidermis both a benefit and a risk?
- How is vitamin D synthesized in the epidermis?
- Identify three pigments that impart colour to skin.
- Describe bacteria that normally reside on the skin, and explain why they do not usually cause infections.
- Explain why the keratinocytes at the surface of the epidermis are dead, while keratinocytes located deeper in the epidermis are still alive.
- Which layer of the epidermis contains keratinocytes that have begun to die?
- Explain why our skin is not permanently damaged if we rub off some of the surface layer by using a rough washcloth.
10.3 Explore More
https://www.youtube.com/watch?v=27lMmdmy-b8
Jonathan Eisen: Meet your microbes, TED, 2015.
https://www.youtube.com/watch?v=9AcQXnOscQ8
Why Do We Blush?, SciShow, 2014.
https://www.youtube.com/watch?v=_r4c2NT4naQ
The science of skin colour - Angela Koine Flynn, TED-Ed, 2016.
Attributions
Figure 10.3.1
Sunburn by QuinnHK at English Wikipedia on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 10.3.2
Blausen_0353_Epidermis by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.3.3
Isaac's scraped knee close-up by Alpha on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 10.3.4
Nicoderm by RegBarc on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license. (No machine-readable author provided for original.)
Figure 10.3.5
Staphylococcus aureus bacteria, MRSA by Microbe World on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
References
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
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&feature=youtu.be
SciShow. (2014, July 15). Why do we blush? YouTube. https://www.youtube.com/watch?v=9AcQXnOscQ8
TED. (2015, July 17). Jonathan Eisen: Meet your microbes. YouTube. https://www.youtube.com/watch?v=27lMmdmy-b8
TED-Ed. (2016, February 16). The science of skin color - Angela Koine Flynn. YouTube. https://youtu.be/_r4c2NT4naQ
Created by CK-12 Foundation/Adapted by Christine Miller
Goose Bumps
No doubt you’ve 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.
What is the Dermis?
The is the inner of the two major layers that make up the skin, the outer layer being the . The dermis consists mainly of . It also contains most skin structures, such as and blood 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 (“below the skin”) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes.
Anatomy of the Dermis
The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibres — which provide toughness — 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.
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.
Papillary Layer
The is the upper layer of the dermis, just below the 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 — or papillae — that extend upward into the epidermis. The papillae contain capillaries and sensory touch receptors.
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.
Reticular Layer
The 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 and hair .
Structures in the Dermis
Both papillary and reticular layers of the dermis contain numerous , which make the skin the body’s primary sensory organ for the sense of . 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 . The circulatory components of the dermis are shown in Figure 10.4.4 below.
Glands
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.
Sweat Glands
produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles, or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
- occur in skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation.
- are 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 puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria causes body odor.
Sebaceous Glands
s are exocrine glands that produce a thick, fatty substance called sebum. is 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 — except for the palms of the hands and soles of the feet, where hair does not grow.
Hair Follicles
s are the structures where hairs originate (see the diagram above). 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 .
Functions of the Dermis
The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.
Temperature Regulation
Several structures in the reticular layer of the dermis are involved in regulating body temperature. For example, when body temperature rises, the of the 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.
When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body heat. The arrector pili muscles also 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.
Sensing the Environment
Sensory receptors in the dermis are mainly responsible for the body’s tactile senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send nerve impulses to 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. s sense mechanical forces such as pressure, roughness, vibration, and stretching. s sense variations in temperature that are above or below body temperature. s 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.
- Free nerve endings sense pain and temperature variations.
- Merkel cells sense light touch, shapes, and textures.
- Meissner’s corpuscles sense light touch.
- Pacinian corpuscles sense pressure and vibration.
- Ruffini corpuscles sense stretching and sustained pressure.
Excreting Wastes
The sweat released by is one way the body excretes waste products. Sweat contains excess water, salts (electrolytes), and other waste products that the body must get rid of to maintain . 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 blood, 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 , including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.
Feature: My Human Body
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.
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’t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.
Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of bacteria. 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.
Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic acid. 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.
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.
What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:
- Wash affected or acne-prone skin (such as the face) twice a day, and after sweating.
- Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.
- Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.
- Rinse with lukewarm water, and avoid using very hot or cold water.
- Shampoo your hair regularly.
- Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.
- Keep your hands off your face. Avoid touching your skin throughout the day.
- Stay out of the sun and tanning beds. Some acne medications make your skin very sensitive to UV light.
10.4 Summary
- The is the inner and thicker of the two major layers that make up the skin. It consists mainly of a matrix of s that provide strength and stretch. It also contains almost all skin structures, including and s.
- The dermis has two layers. The upper has papillae extending upward into the epidermis and loose connective tissues. The lower 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 are structures where hairs originate.
- 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.
10.4 Review Questions
- What is the dermis?
- Describe the basic anatomy of the dermis.
- Compare and contrast the papillary and reticular layers of the dermis.
- What causes epidermal ridges, and why can they be used to identify individuals?
- Name the two types of sweat glands in the dermis, and explain how they differ.
- What is the function of sebaceous glands?
- Describe the structures associated with hair follicles.
- Explain how the dermis helps regulate body temperature.
- Identify three specific kinds of tactile receptors in the dermis, along with the type of stimuli they sense.
- How does the dermis excrete wastes? What waste products does it excrete?
- What are subcutaneous tissues? Which layer of the dermis provides cushioning for subcutaneous tissues? Why does this layer provide most of the cushioning, instead of the other layer?
- For each of the functions listed below, describe which structure within the dermis carries it out.
- Brings nutrients to and removes wastes from dermal and lower epidermal cells
- Causes hairs to move
- Detects painful stimuli on the skin
10.4 Explore More
https://www.youtube.com/watch?v=FX-FwK0IIrE
How do you get rid of acne? SciShow, 2016.
https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
When You Can't Scratch Away An Itch, Seeker, 2013.
Attributions
Figure 10.4.1
Goose_bumps by EverJean on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 10.4.2
Layers_of_the_Dermis by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.3
Fingerprint_detail_on_male_finger_in_Třebíč,_Třebíč_District by Frettie on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.4
Blausen_0802_Skin_Dermal Circulation by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.5
Anatomy_The_Skin_-_NCI_Visuals_Online by Don Bliss (artist) / National Cancer Institute (National Institutes of Health, with the ID 4604) is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.4.6
Blausen_0809_Skin_TactileReceptors by BruceBlaus on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.4.7
Akne-jugend by Ellywa on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/public_domain). (No machine-readable author provided. Ellywa assumed, based on copyright claims).
References
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 Anatomy and Physiology (Section 5.1 Layers of the skin). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/5-1-layers-of-the-skin
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
SciShow. (2016, October 26). How do you get rid of acne? YouTube. https://www.youtube.com/watch?v=FX-FwK0IIrE
Seeker. (2013, October 26). When you can't scratch away an itch. YouTube. https://www.youtube.com/watch?v=VcHQWMAClhQ&feature=emb_logo
Created by CK-12 Foundation/Adapted by Christine Miller
Fashion Statement
This colourful hairstyle makes quite a fashion statement. Many people spend a lot of time and money on their hair, even if they don’t have an exceptional hairstyle like this one. Besides its display value, hair actually has important physiological functions.
What is Hair?
is a filament that grows from a in the of the skin. It consists mainly of tightly packed, keratin-filled cells called . The human body is 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.
Structure of Hair
The part of the hair located within the follicle is called the . The 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.
Follicle and Root
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.
Shaft
The is 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.
- The (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 water. The cuticle is also covered with a layer of lipids, 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.
- The 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 colour.
- The is 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 cells full of keratin.
Characteristics of Hair
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.
Hair Colour
All natural hair colours are the result of , 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. is the dominant pigment in brown hair and black hair, and is the dominant pigment in red hair. Blond hair results when you have only a small amount of melanin in the hair. Gray and white hair occur when melanin production slows down, and eventually stops.
Figure 10.5.3 Variation in hair colouration. Which types of melanin are present for each hair colour shown?
Hair Texture
Hair exists in a variety of textures. The main aspects of hair texture are the curl pattern, thickness, and consistency.
- The shape of the determines the shape of the hair shaft. The shape of the , 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 .
- The size of the hair follicle determines the thickness of hair. Thicker hair has greater volume than thinner hair.
- 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, which causes it to be more porous.
Functions of Hair
In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by .
The function of hair in other 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, 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.
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.
Some hairs, such as the , 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 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.
Hair in Human Evolution
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.
Loss of Body Hair
One for the loss of body hair in the human lineage is that it would have facilitated cooling of the body by the evaporation of sweat. Humans also evolved far more than 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.
These hypotheses may explain why we lost body hair, but they can’t explain why we didn’t 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 . 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 reproduction.
Evolution of Curly Hair
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.
Social and Cultural Significance of Hair
Hair has great social significance for human beings. Body hair is an indicator of biological sex, because hair distribution is . 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.
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’s hair symbolizes liberation from one’s 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 — but just a beard, and not a mustache.
Unfortunately, sometimes hairstyle, colour and characteristics are used to apply stereotypes, particularly with respect to women. "Blonde jokes" are a good example of how negative stereotypes are maintained despite having no actual truth behind them. Many stereotypes related to hair are hidden, even from persons perpetrating the stereotype. 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. 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. What are your initial reactions? Are these reactions founded in fact? Do you harbor an unfair bias?
Figure 10.5.9 What are your biases? Are they fair?
10.5 Summary
- Hair is a filament that grows from a in the of the skin. It consists mainly of tightly packed, keratin-filled cells called . The human body is almost completely covered with hair follicles.
- The part of a hair that is within the follicle is the . This is the only living part of a hair. The part of a hair that is visible above the skin surface is the . It consists of dead cells.
- 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.
- A hair shaft has three zones: the outermost zone called the ; the middle zone called the ; and the innermost zone called the .
- Genetically controlled, visible characteristics of hair include hair colour, hair texture, and the extent of balding in adult males. ( and/or ) is the pigment that gives hair its colour. Aspects of hair texture include curl pattern, thickness, and consistency.
- Functions of head hair include providing insulation and protecting skin on the head from . Hair everywhere on the body has an important sensory function. Hair in and protects the eyes from dust, dirt, sweat, and other potentially harmful substances. The eyebrows also play a role in nonverbal communication.
- 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.
- Hair has social significance for human beings, because it is an indicator of biological sex, age, and ethnic ancestry. Human hair also has cultural significance. Hairstyle may be an indicator of social group membership, for example.
10.5 Review Questions
-
- Compare and contrast the hair root and hair shaft.
- Describe hair follicles.
- Explain variation in human hair colour.
- What factors determine the texture of hair?
- Describe two functions of human hair.
- What hypotheses have been proposed for the loss of body hair during human evolution?
- Discuss the social and cultural significance of human hair.
- Describe one way in which hair can be used as a method of communication in humans.
- 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.
10.5 Explore More
https://www.youtube.com/watch?v=8diYLhl8bWU
Why do some people go bald? - Sarthak Sinha, TED-Ed, 2015.
https://www.youtube.com/watch?v=kNw8V_Fkw28
Hair Love | Oscar®-Winning Short Film (Full) | Sony Pictures Animation, 2019.
https://www.youtube.com/watch?v=hDW5e3NR1Cw
Why do we care about hair | Naomi Abigail | TEDxBaDinh, TEDx Talks, 2015.
Attributions
Figure 10.5.1
Hair by jessica-dabrowski-TETR8YLSqt4 [photo] by Jessica Dabrowski on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.5.2
Blausen_0438_HairFollicleAnatomy_02 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.5.3
- Standing tall by Ilaya Raja on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Blond-haired woman smiling by Carlos Lindner on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Smith Mountain Lake redhead by Chris Ross Harris on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Through the look of experience by Laura Margarita Cedeño Peralta on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.5.4
Curly hair by chris-benson-clvEami9RN4 [photo] by Chris Benson on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.5.5
1024px-PilioerectionAnimation by AnthonyCaccese on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 10.5.6
Pout by alexander-dummer-Em8I8Z_DwA4 [photo] by Alexander Dummer on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.5.7
Cotton_top_tamarin_monkey._(12046035746) by Bernard Spragg. NZ, from Christchurch, New Zealand on Wikimedia Commons is used under a CC0 1.0 Universal
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 10.5.8
Amish hairstyle by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 10.5.9
- Rainbow Hair Bubble Man by Behrouz Jafarnezhad on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Pink hair in Atlanta, United States by Tammie Allen on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Magdalena 2 by Valerie Elash on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Perfect Style by Daria Volkova on Unsplash is used under the Unsplash License (https://unsplash.com/license)
- Stay Classy by Fayiz Musthafa on Unsplash is used under the Unsplash License (https://unsplash.com/license)
- Take your time by Jan Tinneberg on Unsplash is used under the Unsplash License (https://unsplash.com/license)
References
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
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 CK-12 College Human Biology (Section 12.5) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/12.5/
Sony Pictures Animation. (2019, December 5). Hair love | Oscar®-winning short film (Full) | Sony Pictures Animation. YouTube. https://www.youtube.com/watch?v=kNw8V_Fkw28
TED-Ed. (2015, August 25). Why do some people go bald? – Sarthak Sinha. YouTube. https://www.youtube.com/watch?v=8diYLhl8bWU
TEDx Talks. (2015, February 4). Why do we care about hair | Naomi Abigail | TEDxBaDinh. YouTube. https://www.youtube.com/watch?v=hDW5e3NR1Cw
Created by CK-12 Foundation/Adapted by Christine Miller
Nail Art
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’s easy to forget that they also have important biological functions.
What Are Nails?
s are accessory organs of the . They are made of sheets of dead 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.
Nail Anatomy
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.
- The is 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.
- The (or body) is the portion of the nail that is external to the skin. It is the visible part of the nail.
- The is 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.
- The is the area of skin under the nail plate. It is pink in colour, due to the presence of capillaries in the dermis.
- The is a layer of dead epithelial cells that 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.
- The is a groove in the skin in which the side edges of the nail plate are embedded.
Nail Growth
Nails grow from a deep layer of living epidermal tissue, known as the , 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.
Formation of the Nail Root and Nail Plate
The produced by the nail matrix accumulate to form tough, hard, translucent sheets of dead cells filled with . 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.
The proximal end of the nail plate near the root has a whitish crescent shape called the . 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.
Rate of Nail Growth
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’s 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.
Functions of Nails
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.
Nails and Health
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.
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 — if not thousands — 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.
Nails — especially toenails — 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.
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.
Feature: Reliable Sources
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.
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:
- What training and certification are required for professional nail technicians?
- What licenses and inspections are required for nail salons?
- What hygienic practices should be followed in nail salons to reduce the risk of infections being transmitted to clients?
- Which professional nail products are potentially harmful to the human body and which are safer?
- How likely is it to have an adverse health consequence when you get a professional manicure or pedicure?
- What steps can you take to ensure that a professional manicure or pedicure is safe?
10.6 Summary
- are accessory organs of the , consisting of sheets of dead, keratin-filled . The keratin in nails makes them hard, but flexible.
- A nail has three main parts: the (which is under the epidermis), the (which is the visible part of the nail), and the (which is the distal edge of the nail). Other structures under or around a nail include the , , and .
- A nail grows from a deep layer of living epidermal tissues — called the — 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 epidermis.
- Fingernails grow faster than toenails. Actual rates of growth depend on many factors, such as age, sex, and season.
- Functions of nails include protecting the digits, enhancing sensations and precise movements of the fingertips, and acting as tools.
- 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.
- Nails — and especially toenails — are prone to fungus infections. Nails are more permeable than skin and can absorb several harmful substances, such as herbicides.
10.6 Review Questions
- What are nails?
- Explain why most of the nail plate looks pink.
- Describe a lunula.
- Explain how a nail grows.
- Identify three functions of nails.
- Give several examples of how nails are related to health.
- What is the cuticle of the nail composed of? What is the function of the cuticle? Why is it a bad idea to cut the cuticle during a manicure?
- Is the nail plate composed of living or dead cells?
10.6 Explore More
https://www.youtube.com/watch?v=G35kPhbUZdg
Longest Fingernails - Guinness World Records 60th Anniversary,
Guinness World Records, 2014.
https://www.youtube.com/watch?v=aTSVHwzkYI4&feature=emb_logo
5 Things Your Nails Can Say About Your Health, SciShow, 2015.
https://www.youtube.com/watch?v=7w2gCBL1MCg
Claws vs. Nails - Matthew Borths, TED-Ed, 2019.
Attributions
Figure 10.6.1
Nails by allison-christine-vPrqHSLdF28 [photo] by allison christine on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 10.6.2
Blausen_0406_FingerNailAnatomy by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 10.6.3
Chinese_doctor_with_long_finger_nails_(an_aristocrat),_ca.1920_(CHS-249) by Pierce, C.C. (Charles C.), 1861-1946 from the USC Digital Library on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.6.4
Toenail fungus Nagelpilz-3 by Pepsyrock on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/public_domain).
Figure 10.6.5
OLYMPUS DIGITAL CAMERA by Stoive at the English language Wikipedia, on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.
References
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
Guiness World Records. (2014, December 8). Longest fingernails - Guinness World Records 60th Anniversary. YouTube. https://www.youtube.com/watch?v=G35kPhbUZdg
SciShow. (2015, September 14). 5 things your nails can say about your health. YouTube. https://www.youtube.com/watch?v=aTSVHwzkYI4
TED-Ed. (2019, October 29). Claws vs. nails - Matthew Borths. YouTube. https://www.youtube.com/watch?v=7w2gCBL1MCg
The ability of an organism to maintain constant internal conditions despite external changes.
Created by CK-12 Foundation/Adapted by Christine Miller
Roasted Bone Marrow
Do you recognize the food item in the top left of Figure 11.4.1? It’s roasted , 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’s a soft tissue that, in adults, may be mostly fat. You’ll learn more about bone marrow and other tissues that make up bones when you read this section.
What Are Bones?
are organs that consist primarily of bone tissue, also called osseous tissue. is a type of connective tissue consisting mainly of 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.
Bone Anatomy
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 cells.
Types of Osseous Tissue
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.
Flat bones are typically enveloped by compact bone, with a center of spongy bone.
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. 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.
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.
Other Tissues in Bones
Besides compact and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum.
- is a soft connective tissue found inside a cavity, called the marrow cavity. There are two types of marrow in adults — 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 .
- 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.
Bone Cells
As shown in Figure 11.4.6, bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.
- are bone cells with a single nucleus that make and mineralize bone matrix. They make a protein mixture that 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.
- are 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 communication with other bone cells.
- are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the minerals in bone and release them into the blood.
- are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.
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 of mineral ions in the blood.
Types of Bones
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.
- 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.
- 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.
- are 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).
- are embedded in tendons, the connective tissues that bind muscles to bones. Sesamoid bones hold tendons farther away from joints, 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.
- are very small bones located between the major bones of the skull, within the joints (sutures) between the larger bones. They are not always present.
- 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.
Feature: Reliable Sources
Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often cure many 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’s condition.
Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:
- How does one become a potential bone marrow donor?
- Who can and who cannot donate bone marrow?
- How is a bone marrow donation made?
- What risks are there in donating bone marrow?
11.4 Summary
- are organs that consist mainly of (or osseous tissue). Osseous tissue is a type of connective tissue consisting of a matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard, without making it brittle.
- There are two types of osseous tissues: tissue and 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.
- Besides osseous tissues, bones also contain , , , and .
- Bone tissue is composed of four different types of bone cells: , , , and . 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.
- There are six types of bones in the human body: (such as the limb bones), (such as the wrist bones), (such as the patella), in the skull, and (such as the vertebrae).
11.4 Review Questions
- Describe osseous tissue.
- Why are bones hard, but not brittle?
- Compare and contrast the compact and spongy bone.
- What non-osseous tissues are found in bones?
- List four types of bone cells and their functions.
- Identify six types of bones. Give an example of each type.
- Compare and contrast yellow bone marrow and red bone marrow.
- Which type of bone cell divides to produce new bone cells? Where is this cell type located?
- Where do osteoblasts and osteocytes come from? How are they related to each other?
- Which type of bone is embedded in tendons?
11.4 Explore More
https://www.youtube.com/watch?v=rDGqkMHPDqE&feature=emb_logo
The Skeletal System: Crash Course A&P #19, CrashCourse, 2015.
https://www.youtube.com/watch?v=0dV1Bwe2v6c&feature=emb_logo
Bone Remodeling and Modeling, Amgen, 2012.
https://www.youtube.com/watch?v=1Qfmkd6C8u8
How bones make blood - Melody Smith, TED-Ed, 2020.
Attributions
Figure 11.4.1
Bone_marrow_grilled_on_the_barbecue,_sliced_young_raw_garlic,_salted_leek_flowers_from_last_year,_lovage,_and_kale_(19098148350) by City Foodsters on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 11.4.2
Bone_cross-section.svg by Pbroks13 on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.3
Anatomy_of_a_Flat_Bone by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.4
the-detail-of-the-bones-the-structure-of-the-bones-spongy-bone-tramčina-close-up-structure on pxfuel are used according to the pxfuel Terms of Use.
Figure 11.4.5
Spongy_bone_-_Trabecular_bone_2_–_Smart-Servier by Laboratoires Servier/ Smart Servier website on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 11.4.6
Bone_cells by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.4.7
Blausen_0229_ClassificationofBones by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
References
Amgen. (2012, January 19). Bone remodeling and modeling. YouTube. https://www.youtube.com/watch?v=0dV1Bwe2v6c
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 6.9 Anatomy of a flat bone [digital image]. In Anatomy and Physiology (Section 6.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/6-3-bone-structure
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 6.11 Bone cells [digital image]. In Anatomy and Physiology (Section 6.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/6-3-bone-structure
CK-12 Foundation. (n.d.). Communication: Identifies means of communication between animals. ck12.org. https://www.ck12.org/c/life-science/communication/
CrashCourse. (2015, May 18). The skeletal system: Crash Course A&P #19. YouTube. https://www.youtube.com/watch?v=rDGqkMHPDqE
TED-Ed. (2020, January 27). How bones make blood - Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8
Created by CK-12 Foundation/Adapted by Christine Miller
Break A Leg
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’ll learn how bones can do all of these things in this section.
Bone Growth
Early in the development of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. 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:
- Cartilage “model” of bone forms. This model continues to grow as ossification takes place.
- Ossification begins at a primary ossification center in the middle of bone.
- Ossification then starts to occur at secondary ossification centers at the ends of bone.
- The medullary cavity forms. This cavity will contain red bone marrow.
- Areas of ossification meet at epiphyseal plates, and articular cartilage forms. Bone growth ends.
The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.
Primary and Secondary Ossification Centers
When bone forms from cartilage, ossification begins with a point in the cartilage called the . This generally appears during fetal development, although a few short bones begin their primary ossification after birth. Ossification happens toward both ends of the bone from the primary ossification center, and — in the case of long bones — it eventually forms the shaft of the bone.
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.
Skeletal Maturity
Throughout childhood, the 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 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.
The use of by teens can speed up 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.
Bone Remodeling
Even after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process called . 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 — which are bone cells that resorb bone and dissolve its minerals — and , which are bone cells that make new bone matrix.
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 , 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.
The action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of , , 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 (PTH) and inhibited by calcitonin, which is produced by the (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.
Bone Repair
(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 — with a cast and/or pins surgically inserted into the bone, for example (as shown in Figure 11.5.5) — until the bone’s natural healing process is complete. This process may take several weeks.
Although bone repair is a natural physiological process, it may be promoted or inhibited by several factors. Fracture 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.
Feature: Myth vs. Reality
Bone fractures are fairly common, and there are many myths about them. Knowing the facts is important, because fractures generally require emergency medical treatment.
Myth | Reality |
---|---|
"A bone fracture is a milder injury than a broken bone." | A bone fracture is the same thing as a broken bone. |
"If you still have full range of motion in a limb, then it must not be fractured." | Even if a bone is fractured, the muscles and tendons attached to it may still be able to move the bone normally. This is especially likely if the bone is cracked — but not broken — 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. |
"A fracture always produces a bruise." | Many — but not all — 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. |
"Fractures are so painful that you will immediately know if you break a bone." | 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. |
"You can tell when a bone is fractured because there will be very localized pain over the break." | A broken bone is often accompanied by injuries to surrounding muscles or ligaments. As a result, the pain may extend far beyond the location of 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. |
11.5 Summary
- is very active tissue. Its cells are constantly forming and resorbing bone matrix.
- Early in the development of a human fetus, the skeleton is made almost entirely of . The relatively soft cartilage gradually turns into hard bone. This is called . It begins at a in the middle of bone, and later also occurs at 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.
- Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, resorb bone, and make new bone to replace it. Bone remodeling shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral in the blood.
- 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, 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.
11.5 Review Questions
- Outline how bone develops starting early in the fetal stage, and through the age of skeletal maturity.
- Describe the process of bone remodeling. When does it occur?
- What purposes does bone remodeling serve?
- Define bone repair. How long does this process take?
- Explain how bone repair occurs.
- Identify factors that may affect bone repair.
- 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.
- If bones can repair themselves, why are casts and pins sometimes necessary in the process?
- When calcium levels are low, which type of bone cell causes the release of calcium to the bloodstream?
- Which tissue and bone cell type are primarily involved in bone repair after a fracture?
- Describe one way in which hormones are involved in bone remodeling.
11.5 Explore More
https://www.youtube.com/watch?v=yJoQj5-TIvE
How to grow a bone - Nina Tandon, TED-Ed, 2015.
https://www.youtube.com/watch?v=-P6LsendHxU&feature=emb_logo
Healing Process of Bone Fracture, Aldo Fransiskus Marsetio, 2015.
https://www.youtube.com/watch?v=RC2w_9DcY38
The Skeleton From Fetal to Adult, Samantha Espolt, 2012.
Attributions
Figure 11.5.1
First_plaster_long_leg_cast…._-_9383569051 by 4x4king10 on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 11.5.2
Bone_growth by Chaldor (derivative work) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain). (Original, Illu_bone_growth.jpg is by Fuelbottle)
Figure 11.5.3
Calcium_Homeostasis by OpenStax on Wikimedia commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 11.5.4
Broken Arm by Ashley Chung is used with permission.
Figure 11.5.5
Broken Arm with plate and pins by Ashley Chung is used with permission.
References
Aldo Fransiskus Marsetio. (2015, ). Healing process of bone fracture. YouTube. https://www.youtube.com/watch?v=-P6LsendHxU
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]. In Anatomy and Physiology (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
Samantha Espolt. (2012, ). The skeleton from fetal to adult. YouTube. https://www.youtube.com/watch?v=RC2w_9DcY38&t=3s
TED-Ed. (2015, ). How to grow a bone - Nina Tandon. YouTube. https://www.youtube.com/watch?v=yJoQj5-TIvE
Created by CK-12 Foundation/Adapted by Christine Miller
Dowager’s Hump
The woman on the right in Figure 11.7.1 has a deformity in her back commonly called dowager’s (widow’s) hump, because it occurs most often in elderly women. Its medical name is kyphosis, 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 disorders of the skeletal system.
Common Skeletal System Disorders
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 and . 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.
Osteoporosis
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 — 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.
Changes in Bone Mass with Age
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.
What Causes Osteoporosis?
The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation by and bone resorption by . 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 net loss 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:
- An individual never develops normal peak bone mass during the young adult years: If the peak level is lower than normal, then there is less bone mass to begin with, making osteoporosis more likely to develop.
- There is greater than normal bone resorption: Bone 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 hormone that promotes resorption by osteoclasts.
- There is inadequate formation of new bone by osteoblasts during remodeling: Lack of estrogen may decrease the normal deposition of new bone. Inadequate levels of calcium and vitamin D also lead to impaired bone formation by osteoblasts.
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 — including anti-seizure medications, chemotherapy drugs, steroid medications, and some antidepressants — also increase the rate of bone loss.
Diagnosing Osteoporosis
Osteoporosis is diagnosed by measuring a patient’s 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’s 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 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).
Osteoporotic Fractures
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 bones (such as the femur) can impair mobility and may require surgery. Hip fracture usually requires immediate surgery, as well. The immobility associated with fractures — especially of the hip — 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.
Risk Factors for Osteoporosis
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.
- 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
- Consumption of soft drinks
Treatment and Prevention of Osteoporosis
Osteoporosis is often treated with medications that may slow or even reverse bone loss. Medications called bisphosphonates, for example, are commonly prescribed. Bisphosphonates slow down the breakdown of bone, allowing bone rebuilding during remodeling to keep pace. This helps maintain bone density and decreases the risk of fractures. The 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 — in fact, there is a potential for adverse side effects.
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 — 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.
If you’re 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’t stress the bones. Ideally, you should exercise for at least 30 minutes a day most days of the week.
(OA) 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.
What Causes Osteoarthritis?
OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage. The stress may be exacerbated by low-grade inflammation of the joints, as cells lining 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.
Diagnosing Osteoarthritis
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.
X-rays or other tests are sometimes used to either support the diagnosis of OA or to rule out other disorders. Blood tests might be done, for example, to look for factors that indicate rheumatoid arthritis (RA), an autoimmune disease in which the immune system attacks the body’s joints. If these factors are not present in the blood, then RA is unlikely, and a diagnosis of OA is more likely to be correct.
Risk Factors for Osteoarthritis
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 — especially at younger ages — 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.
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.
Treatment and Prevention of Osteoarthritis
OA cannot be cured, but the symptoms — especially the pain — 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.
Exercise
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.
Exercising in water provides buoyancy that places less stress on joints than the same exercises would on the ground or other hard surface.
De-stressing Joints
Efforts to decrease stress on joints include resting and using mobility devices such as canes, which reduce the weight placed on weight-bearing joints and also improve stability. In people who are overweight, losing weight may also reduce joint stress.
Pain Medications
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 blood 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 , only short-term use of opioids is generally recommended.
Surgery
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 “parts” are usually pain-free and fully functional for at least a couple of decades. Quality, durability, and customization of artificial joints are constantly improving.
Try out this neat Virtual Hip Resurfacing activity by Edheads (you will need to enable Flash).
Feature: Myth vs. Reality
About one out of every 5 adults in Canada suffer from 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.
Myth |
Reality |
"Cracking my knuckles will cause osteoarthritis." | Cracking your knuckles may lead to inflammation of your tendons, but it will not cause osteoarthritis. |
"My diet has no effect on my joints." | What and how much you eat does affect your body weight, and every pound you gain translates into an additional four pounds (or more!) of stress on your knees. Being overweight, therefore, increases the chances of developing osteoarthritis — and also the rate at which it progresses. |
"Exercise causes osteoarthritis or makes it worse, so I should avoid it." | 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’t have osteoarthritis, exercise can reduce your risk of developing it. Low-impact exercise helps keep the muscles around joints strong and flexible, so they can help stabilize and protect the joints. |
"If my mom or dad has osteoarthritis, I will also develop it." | It is true that you are more likely to develop osteoarthritis if a parent has it, but it isn’t a sure thing. There are several things you can do to decrease your risk, such as getting regular exercise and maintaining a healthy weight. |
"Bad weather causes osteoarthritis." | Weather conditions 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. |
"Joint pain is unavoidable as you get older, so there is no need to see a doctor for it." | 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. |
"Osteoarthritis is inevitable in seniors." |
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. |
11.7 Summary
- A number of disorders affect the , including bone fractures and bone . The two most common disorders of the skeletal system are osteoporosis and osteoarthritis.
- 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.
- Osteoporosis is diagnosed by measuring a patient’s 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.
- 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.
- 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.
- (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.
- 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.
- 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.
11.7 Review Questions
- Create a brochure or poster about osteoporosis to educate others about this disease. Include information about:
- A definition of osteoporosis
- Causes
- Dangers of living with the disease
- Canadian osteoporosis statistics
- Risk factors
- Diagnosis
- Treatment
- Why is it important to build sufficient bone mass in your young adult years?
- Explain the difference in cause between rheumatoid arthritis and osteoarthritis.
- Debunk the myth: Osteoarthritis is caused by physical activity, so people who are equally active are equally susceptible to it.
- Explain how we know that estrogen generally promotes production of new bone.
11.7 Explore More
https://www.youtube.com/watch?v=DL0_gcP15Ts
Kevin Stone: The bio-future of joint replacement, TED, 2010.
https://www.youtube.com/watch?v=OyK0oE5rwFY
The benefits of good posture - Murat Dalkilinç, TED-Ed, 2015.
Attributions
Figure 11.7.1
Blausen_0686_Osteoporosis_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.7.2
Feature_Osteoprosis_of_Spine by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.7.3
Age_and_Bone_Mass by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.7.4
DEXA_scan_screen_ALSPAC by Nick Smith photography on Wikimedia Commons is used under a CC BY-SA 3.0 license.
Figure 11.7.5
Hiking by jake-melara-Yh6K2eTr_FY [photo] by Jake Melara on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 11.7.6
Areas_affected_by_osteoarthritis by National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)/ NIH on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 11.7.7
Hallux_valgus by Malmstajn on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 11.7.8
07-06_WtrAerob1a by Tim Ross on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
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 6.23 Graph showing relationship between age and bone mass digital image]. In Anatomy and Physiology (Section 6.6). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/6-6-exercise-nutrition-hormones-and-bone-tissue
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 7.22 Osteoporosis [digital image]. In Anatomy and Physiology (Section 7.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/7-3-the-vertebral-column
Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
Mayo Clinic Staff. (n.d.). Kyphosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/kyphosis/symptoms-causes/syc-20374205
Mayo Clinic Staff. (n.d.). Osteoarthritis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925
TED. (2010, July 23). Kevin Stone: The bio-future of joint replacement. YouTube. https://youtu.be/DL0_gcP15Ts
TED-Ed. (2015, July 30). The benefits of good posture - Murat Dalkilinç. YouTube. https://www.youtube.com/watch?v=OyK0oE5rwFY&feature=youtu.be
Wheatley, G., Smail, S., Bort, E. (2007). Virtual hip resurfacing [online game]. EdHeads.org. https://edheads.org/page/hip_resurfacing
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study Conclusion: A Pain in the Foot
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 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 — which is what Sophia was experiencing. There may also be pain, numbness, or tingling in the toes associated with metatarsalgia.
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 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’s hump (or ), which is often seen in women with osteoporosis.
Don’t think you are immune to stress fractures just because you don’t 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. 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.
Sophia learned through her online research that wearing high heels can also lead to foot deformities, such as bunions and hammertoes. 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 — 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.
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.
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’s 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 — and not for a cure — OA could be an unfortunate long-term consequence of wearing high heels.
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.
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:
- Wearing shoes that fit well, have plenty of room in the toes, are supportive, and are comfortable right away. There should be no “break-in” period needed for shoes.
- 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.
- If high heels must be worn, it should only be for a limited period of time.
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 — 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.
Chapter 11 Summary
In this chapter, you learned about the skeletal system. Specifically, you learned that:
- The is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.
- are organs made of supportive connective tissues, mainly the tough protein . 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.
- 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 , and producing endocrine hormones.
- 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.
- The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton.
- The 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.
-
- The provides a bony framework for the head. It consists of 22 different bones: eight in the , which encloses the brain, and 14 in the face, which includes the upper and lower jaw.
- The vertebral column is a flexible, S-shaped column of 33 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.
- The 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.
- The 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.
-
- 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 carpal bones, the hand contains five metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.
- 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 seven tarsal bones, and the foot contains five metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.
- 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.
- 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.
- 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.
-
- There are two types of osseous tissues: and . 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.
- Besides osseous tissues, bones also contain nerves, blood vessels, , and .
- Bone tissue is composed of four different types of bone cells: , , , and . 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.
- There are six types of bones in the human body: (such as the limb bones), (such as the wrist bones), (such as the patella), in the skull, and (such as the vertebrae).
- Early in the development of a human fetus, the skeleton is made almost entirely of . The relatively soft cartilage gradually turns into hard bone — a process that is called . 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.
- 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. shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral homeostasis in the blood.
- 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.
- are locations at which bones of the skeleton connect with one another.
- Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.
- 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.
- 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.
-
- Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: , , , , , and joints.
- 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.
- 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.
-
- Osteoporosis is diagnosed by measuring a patient’s 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.
- 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.
- 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.
- (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.
-
- 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.
- 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.
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’t 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.
Chapter 11 Review
-
- Why does the rib cage need to be flexible? Why can it be flexible?
- In general, what do “girdles” in the skeletal system do?
- Would swimming be more effective as an exercise for preventing osteoporosis or as a treatment for osteoarthritis? Explain your answer.
- Explain why some of the vertebrae become misshapen in the condition called dowager’s hump (or kyphosis).
- Explain why osteoarthritis often involves inflammation in the joints.
- 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?
- Describe two roles that calcium in bones play in the body.
Attributions
Figure 11.8.1
Running Shoes by bruno-nascimento-PHIgYUGQPvU [photo] by Bruno Nascimento on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 11.8.2
Metatarsalgia/ Best Shoes for Metatarsalgia by Esther Max on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 11.8.3
Gray290_-_Mratatarsus (1) by Henry Vandyke Carter (1831-1897) (Revised by Warren H. Lewis, coloured by Was a bee) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain. (Bartleby.com: Gray’s Anatomy, Plate 290)
Figure 11.8.4
Heels by gavin-allanwood-ndpX28miBtE-unsplash by Photo by Gavin Allanwood on Unsplash is used under the Unsplash License (https://unsplash.com/license).
References
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
VanDyke Carter, H. (1858). Illustration plate 290. In H. Gray, Anatomy of the Human Body. Lea & Febiger. Bartleby.com, 2000. www.bartleby.com/107/.
Created by CK-12 Foundation/Adapted by Christine Miller
Case Study: Needing to Relax
This dog (Figure 12.1.1) is expressing his interest in something — perhaps a piece of food — 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.
Edward has a rare condition called cervical dystonia, which is also called spasmodic torticollis. In this condition, the muscles in the neck contract involuntarily, often causing the person’s 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.
Cervical dystonia is painful and can significantly interfere with a person’s ability to carry out their usual daily activities. In Edward’s 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.
Although it can be caused by an injury, there is no known cause of cervical dystonia — and there is also no cure. Fortunately for Edward, and others who suffer from cervical dystonia, 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!
The substance is botulinum toxin, one preparation of which may be familiar to you by its brand name — . It is a neurotoxin produced by the bacterium Clostridium botulinum, and can cause a life-threatening illness called botulism. 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 like cerebral palsy; and even overactive bladder.
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.
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’s cervical dystonia, and how this toxin works to inhibit muscle contraction.
Chapter Overview: Muscular System
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:
- The different types of — skeletal, cardiac, and smooth muscle — and their different characteristics and functions.
- How muscle cells are specialized to contract and cause and movements.
- The ways in which muscle contraction is controlled.
- How can grow or shrink, causing changes in strength.
- The structure and organization of skeletal muscles, including the different types of muscle fibres, and how actin and myosin filaments move across each other — according to the sliding filament theory — to cause muscle contraction.
- tissue in the heart that contracts to pump blood through the body.
- tissue that makes up internal organs and structures, such as the digestive system, blood vessels, and uterus.
- The physical and mental health benefits of aerobic and anaerobic exercise, such as running and weight lifting.
- How individuals vary in their response to exercise.
- 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’s disease).
As you read the chapter, think about the following questions:
- How is the contraction of skeletal muscles controlled?
- 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?
- What is one disorder involving a lack of sufficient muscle contraction? Why does it occur?
Attributions
Figure 12.1.1
Whiskey’s 2nd Birthday by Kelly Hunter on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 12.1.2
1024px-Dystonia2010 by James Heilman, MD on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
References
Botulism [online article]. (2018, January 10). World Health Organization (WHO). https://www.who.int/news-room/fact-sheets/detail/botulism
Mayo Clinic Staff. (n.d.) Cervical dystonia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/cervical-dystonia/symptoms-causes/syc-20354123
Created by CK-12 Foundation/Adapted by Christine Miller
Marvelous Muscles
Does the word muscle make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya, and she’s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren’t 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’t control internal muscles like these. It’s 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.
What Is the Muscular System?
The consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of , 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 , which makes up the walls of the heart, and , which control movement in other internal organs and structures.
Muscle Structure and Function
Muscles are organs composed mainly of muscle cells, which are also called (mainly in skeletal and cardiac muscle) or (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 . The sliding filaments increase the tension in — or shorten the length of — muscle cells, causing a contraction. Muscle contractions are responsible for virtually all the movements of the body, both inside and out.
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.
Smooth muscles in the walls of blood vessels contract to cause , which may help conserve body heat. Relaxation of these muscles causes , 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 . Think 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.
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.
Muscle Hypertrophy and Atrophy
Muscles can grow larger, or . This generally occurs through increased use, although hormonal or other influences can also play a role. The increase in 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.
Muscles can also grow smaller, or , which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time — for example, because of a broken bone or surgery — 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 “skin and bones.” Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).
Many diseases, including and , are often associated with muscle atrophy. Atrophy of muscles also happens with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as . The 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.
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.
Interactions with Other Body Systems
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 . Let’s 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.
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 (smooth muscle) or specialized cells in the heart (cardiac muscle). 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.
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 . Skeletal muscles are attached to the skeleton by tough connective tissues called . Many skeletal muscles are attached to the ends of bones that meet at 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.
12.2 Summary
- The consists of all the muscles of the body. There are three types of muscle: (which is attached to bones and enables body movements), (which makes up the walls of the heart and makes it beat), and (which is found in the walls of internal organs and other internal structures and controls their movements).
- Muscles are organs composed mainly of muscle cells, which may also be called or . Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in .
- Muscles can grow larger, or . This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or . This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size — but not the number — of muscle fibres changes. The size of muscles is the main determinant of muscle strength.
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the , hormones, or other factors.
12.2 Review Questions
- What is the muscular system?
- Describe muscle cells and their function.
- Identify three types of muscle tissue and where each type is found.
- Define muscle hypertrophy and muscle atrophy.
- What are some possible causes of muscle hypertrophy?
- Give three reasons that muscle atrophy may occur.
- How do muscles change when they increase or decrease in size?
- How do changes in muscle size affect strength?
- Explain why astronauts can easily lose muscle mass in space.
- Describe how the terms muscle cells, muscle fibres, and myocytes relate to each other.
- Name two systems in the body that work together with the muscular system to carry out movements.
- Describe one way in which the muscular system is involved in regulating body temperature.
12.2 Explore More
https://www.youtube.com/watch?v=VVL-8zr2hk4
How your muscular system works - Emma Bryce, TED-Ed, 2017.
https://www.youtube.com/watch?v=Ujr0UAbyPS4&feature=emb_logo
3D Medical Animation - Peristalsis in Large Intestine/Bowel || ABP ©, AnimatedBiomedical, 2013.
https://www.youtube.com/watch?v=LkXwfTsqQgQ&feature=emb_logo
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.
Attributions
Figure 12.2.1
Natalia_Zabolotnaya_2012b by Simon Q on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Figure 12.2.2
Bougle_whole2_retouched by Bouglé, Julien from the National LIbrary of Medicine (NLM) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 12.2.3
Daniel_Tani_iss016e027910 by NASA/ International Space Station Imagery on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine/bowel || ABP ©. YouTube. https://www.youtube.com/watch?v=Ujr0UAbyPS4&feature=youtu.be
Bouglé, J. (1899). Le corps humain en grandeur naturelle : planches coloriées et superposées, avec texte explicatif. J. B. Baillière et fils. In Historical Anatomies on the Web. http://www.nlm.nih.gov/exhibition/historicalanatomies/bougle_home.html
TED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https://www.youtube.com/watch?v=VVL-8zr2hk4&feature=youtu.be
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https://www.youtube.com/watch?v=LkXwfTsqQgQ&feature=youtu.be
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Natalya_Zabolotnaya&oldid=962630409
Created by CK-12 Foundation/Adapted by Christine Miller
Communicating with Urine
Why do dogs pee on fire hydrants? Besides “having to go,” they are marking their territory with chemicals in their urine called . It’s a form of communication, in which they are “saying” with odors that the yard is theirs and 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 process controlled by the . The process of forming urine — which occurs in the kidneys — 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.
Ureters
As shown in Figure 16.5.2, s 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 10–12 in) long and about 3 to 4 mm in diameter.
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.
The walls of the ureters are composed of multiple layers of different types of tissues. 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 to propel urine down the ureters from the kidneys to the urinary bladder. The outermost layer of the ureter walls consists of fibrous tissue.
Urinary Bladder
The is a hollow, muscular, and stretchy organ that rests on the pelvic floor. It collects and stores from the before the urine is eliminated through . 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.
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.
The detrusor muscle in the wall of the bladder is made of smooth muscle fibres controlled by both the and 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.
Urethra
The is a tube that connects the 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. In addition, in males, the urethra passes through the prostate gland (part of the reproductive system) which is absent in women.
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.
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 , so it is under conscious, 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.
16.5 Summary
- are tube-like structures that connect the with the . Each ureter arises at the of a kidney and travels down through the abdomen to the urinary bladder. The walls of the ureter contain that can contract to push through the ureter by . The walls are lined with transitional epithelium that can expand and stretch.
- The 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.
- The 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 for urination to be under control.
16.5 Review Questions
- What are ureters? Describe the location of the ureters relative to other urinary tract organs.
- Identify layers in the walls of a ureter. How do they contribute to the ureter’s function?
- Describe the urinary bladder. What is the function of the urinary bladder?
- How does the nervous system control the urinary bladder?
- What is the urethra?
- How does the nervous system control urination?
- Identify the sphincters that are located along the pathway from the ureters to the external urethral orifice.
- What are two differences between the male and female urethra?
- When the bladder muscle contracts, the smooth muscle in the walls of the urethra _________ .
16.5 Explore More
https://youtu.be/2Brajdazp1o
The taboo secret to better health | Molly Winter, TED. 2016.
https://youtu.be/dg4_deyHLvQ
What Happens When You Hold Your Pee? SciShow, 2016.
Attributions
Figure 16.5.1
Cliche by Jackie on Wikimedia Common s is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 16.5.2
Urinary System Male by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 16.5.3
Adrenal glands on Kidney by NCI Public Domain by Alan Hoofring (Illustrator) /National Cancer Institute (photo ID 4355) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 16.5.4
2605_The_Bladder by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 16.5.5
512px-Male_and_female_urethral_openings.svg by andrybak (derivative work) on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license. (Original: Male anatomy blank.svg: alt.sex FAQ, derivative work: Tsaitgaist Female anatomy with g-spot.svg: Tsaitgaist.)
References
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 25.4 Bladder
SciShow. (2016, January 22). What happens when you hold your pee? YouTube. https://www.youtube.com/watch?v=dg4_deyHLvQ&feature=youtu.be
TED. (2016, September 2). The taboo secret to better health | Molly Winter. YouTube. https://www.youtube.com/watch?v=2Brajdazp1o&feature=youtu.be
A hollow, tube-like structure through which blood flows in the cardiovascular system; vein, artery, or capillary.