10.4 Dermis
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
How do you get rid of acne? SciShow, 2016.
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
The inner layer of skin that is made of tough connective tissue and contains blood vessels, nerve endings, hair follicles, and glands.
The outer layer of skin that consists mainly of epithelial cells and lacks nerve endings, blood vessels, and other structures.
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/Adapted by Christine Miller
Figure 6.3.1 How would you classify these people?
Why Classify?
What do you see when you look at Figure 6.3.1? Did you sort individuals into categories based in gender, age, body type, facial features, skin colour or other characteristics? As humans, we seem to have a penchant for classifying and labeling people and things. It helps us establish a sense of order in the world around us. The 18th century taxonomist Carl Linnaeus, for example, classified virtually all known living things into different species, genera, families, and other taxonomic categories. His classifications were based on observable phenotypic characteristics, such as skin colour. Modern biological classifications of living things are usually based on phylogenetic relationships. Phylogenies reflect evolutionary history and group together living things that are related by descent from a common ancestor.
Starting with Linnaeus and continuing to the present, scientists and others have attempted to classify human variation. There are three basic approaches to classification: typological, populational, and clinal.
Typological Approach
The typological approach involves creating a , which is a system of discrete types, or categories. This approach was widely used by scientists up through the early 20th century. Racial classifications are typological classifications. They place people into a small number of discrete categories, or races, based on a few readily observable traits, such as skin colour, hair texture, facial features, and body build.
Racial Classifications and Racism
Racial classifications of humans probably go back as long as people distinguished “us” from “them.” An early “scientific” classification of humans into races is Linnaeus’ 1735 classification. He divided Homo sapiens into continental races, which he named europaeus, asiaticus, americanus, and afer. Linnaeus described these races in terms of observable physical traits. He also associated, inaccurately, each race with different personality qualities and behaviors. For example, he described Homo sapiens europaeus as active and adventurous and Homo sapiens afer as lazy and careless. In 1795, the German naturalist Johann Friedrich Blumenbach proposed five major races of Homo sapiens, which he named the caucasoid, mongoloid, negroid, American Indian, and Malayan races. Blumenbach thought that the caucasoid race was the original race, and that the other races arose in a process of “degeneration” from the caucasoids.
In 1870, the English biologist Thomas Huxley classified Homo sapiens into nine races which were distributed geographically. The map in Figure 6.3.2 shows how Huxley thought the races were distributed worldwide. Each colour represents one of Huxley’s proposed races. These categories included “Australoid,” “Xanthochroi,” “Melanochori,” “Negroes,” and “Mongoloids,” and they are not used today. It should be noted that Huxley did not hold such strong negative stereotypes about non-European (or non-caucasoid) races as did his intellectual forebears. Huxley, however, still attributed different behaviors to racial groups that had nothing to do with the colour of their skin or continent of origin.
By the early 20th century, so-called was a popular ideology. This was the idea that race is a biological concept and that human behavior is partly determined by race. At around 1950, in a series of groundbreaking studies of skeletal anatomy, anthropologist Franz Boas showed that cranial (skull) shape and size were highly malleable, depending on environmental factors (such as health and nutrition). He contrasted this with racial anthropologists' claims that head shape is a stable racial trait. In this way, Boas demonstrated that this commonly used racial trait was determined by the environment, and not just genes. Boas also worked to demonstrate that differences in human behavior are not determined primarily by innate biological dispositions, but are largely the result of cultural differences acquired through social learning.
Unfortunately, still persists today — in society at large, if not in science. This is the association of racial traits (such as skin colour) with unrelated traits (such as intelligence), often leading to prejudice and discrimination against people based only on how they look. The concept of human race is real, not in a biological sense, but in a social sense. Racial stereotypes and racism are deeply ingrained in our history and culture, and they have real material effects on human lives.
Additional Problems with Typological Classification
Besides the problem of racism, there are other problems with typological approaches to the biological classification of Homo sapiens. One problem is that most human biological traits are not either present or absent, but instead vary on a continuum. This type of distribution cannot be adequately represented by discrete categories, such as races. The typological approach also results in groupings of people that may be similar in terms of some traits, but not others. How people are grouped together depends on which traits are chosen. In addition, the number of groups that are needed to classify people depends on the number of traits that are used. The greater the number of traits, the greater the number of racial categories there must be. If racial categories depend on the traits chosen to define them, it is clear that the racial classifications are arbitrary and do not reflect biological reality.
Another problem with typological classifications is that they lead to the mistaken belief that people within typological categories are more similar to each other than they are to people in other categories. There is actually more variation within than between typological groups. An estimated 90 per cent of human genetic variation occurs between people within races, and only 10 per cent occurs between races. Clearly, races are far from homogenous in terms of their genetic composition. In short, we are all more alike than we are different.
Populational Approach
By the middle of the 20th century, scientists started advocating a populational approach to classifying Homo sapiens. This approach is based on the idea that the breeding population is the only biologically meaningful group. The is the unit of evolution, and it includes people who have mated and produced offspring together for many generations. As a result, members of the same breeding population should share many genetic traits. You would also expect them to have many of the same phenotypic traits, because of their similar genetic makeup.
While the populational approach makes sense in theory, in reality, it can rarely be applied, because most human populations are not closed breeding populations. Some people have always selected mates from outside their local population (even mating with archaic humans such as Neanderthals). This tendency has increased dramatically in recent centuries with the advent of efficient means of traveling long distances. As a consequence, there are very few remaining distinct breeding populations within the human species.
An example of one such population is the Sentinelese, a small population of hunter-gatherers who live alone on a small island in the Andaman Islands (see the map). The Sentinelese are thought to be direct descendants of the first modern humans to leave Africa, and they may have lived in the Andaman Islands for as long as 60 thousand years. The Sentinelese are also one of the most isolated human populations on Earth. The fact that their language is distinctly different from other Andaman Islands languages is evidence that they have had little contact with other people for thousands of years. Although closed breeding populations (such as the Sentinelese) may be useful for investigating questions about evolutionary processes, they are not useful for classifying most of humanity.
Clinal Approach
By the 1960s, scientists began to use a clinal approach to classify human variation. This approach maps variation in traits over geographic regions (such as continents) or even worldwide. Clinal models are a useful way of describing human variation that does not lead to discrete races or other categories of people.
In Figure 6.3.4 you can see a worldwide clinal map for type O blood in the human ABO blood group system. The frequency of this trait is shown for the indigenous populations of various regions. It is lowest throughout Asia and highest in Native American populations in both North and South America. This geographic distribution results from the complex interaction of a variety of factors, including natural selection, genetic drift, and gene flow. You can read more about geographic variation in blood types in the concept Variation in Blood Types.
Clinal maps for many genetic traits show variation that changes gradually from one geographic area to another, which may happen because of the nature of gene flow. Gene flow occurs when mating takes place between people in different populations. The likelihood of mating with others depends on their distance from us. You may not marry the boy or girl next door, but your mate is more likely to be someone in the same state or country than someone on another continent.
Natural selection has a major impact on the clinal distribution of some traits, because variation in the traits tracks variation in selective pressures. For example, the environmental stressor of malaria varies throughout Africa with climate, as you can see in the left-hand map below (Figure 6.3.5). The sickle cell trait that protects from malaria has a similar distribution, as shown in the right-hand map.
Figure 6.3.5
6.3 Summary
- Humans seem to have a need to and label people based on their similarities and differences. Three approaches to classifying human variation include typological, populational, and clinal approaches.
- The typological approach involves creating a typology, which is a system of discrete categories, or races. This approach was widely used by scientists until the early 20th century. Racial categories are based on observable phenotypic traits (such as skin colour), but other traits and behaviors are often assumed to apply to racial groups, as well. The use of racial classifications often leads to .
- By the mid-20th century, scientists started advocating a population approach. This assumes that the breeding population, which is the unit of evolution, is the only biologically meaningful group. While this approach makes sense in theory, in reality, it can rarely be applied to actual human populations. With few exceptions, most human populations are not closed breeding populations.
- By the 1960s, scientists began to use a clinal approach to classify human variation. This approach maps variation in the frequency of traits or alleles over geographic regions or worldwide. Clinal maps for many genetic traits show variation that changes gradually from one geographic area to another. This type of distribution may result from gene flow and/or natural selection.
6.3 Review Questions
- Name the 18th century taxonomist that classified virtually all known living things.
- Describe the typological approach to classifying human variation.
- Discuss why typological classifications of Homo sapiens are associated with racism.
- Why is the breeding population considered to be the most meaningful biological group?
- Explain why it is generally unrealistic to apply a populational approach to classifying the human species.
- What does a clinal map show?
- Explain how gene flow and natural selection can result in a gradual change in the frequency of a trait over geographic space.
- Most human traits vary on a continuum. Explain why this presents a problem for the typological classification approach.
6.3 Explore More
https://www.youtube.com/watch?v=ntimKsWDUpA&feature=emb_logo
The Biology of Race in the Absence of Biological Races,
Centre for Genetic Medicine, 2015.
https://www.youtube.com/watch?v=QOSPNVunyFQ
Nina Jablonski breaks the illusion of skin color, TED, 2009.
https://www.youtube.com/watch?v=_r4c2NT4naQ
The science of skin color - Angela Koine Flynn, TED-Ed, 2016.
Attributions
Figure 6.3.1
- Three women sitting by flowers and laughing by Priscilla Du Preez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Two women sitting on sofa by AllGo on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Young people in conversation by Alexis Brown on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Men talking in the cold by Anna Vander Stel on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Laughing by the tracks by Priscilla Du Preez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 6.3.2
Huxley_races by Wobble on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 6.3.3
Nicobar_Islands is edited by M.Minderhoud on Wikimedia Commons, and was released into the public domain by its original author, www.demis.nl. (See also approval email on de.wp and its clarification.)
Figure 6.3.4
Map_of_Group_O/ (Percent of Native population that has the O blood type) by Ephert on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license. (Original Spanish edition by Maulucioni)
Figure 6.3.5
- Malaria distribution by Muntuwandi at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
- Sickle cell distribution by Muntuwandi at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
References
Centre for Genetic Medicine. (2015, July 14). The biology of race in the absence of biological races. YouTube. https://www.youtube.com/watch?v=ntimKsWDUpA&feature=youtu.be
TED. (2009, August 7). Nina Jablonski breaks the illusion of skin color. YouTube. https://www.youtube.com/watch?v=QOSPNVunyFQ&feature=youtu.be
TED-Ed. (2016, February 16). The science of skin color - Angela Koine Flynn. YouTube. https://www.youtube.com/watch?v=_r4c2NT4naQ&feature=youtu.be
Wikipedia contributors. (2020, June 27). Carl Linnaeus. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Carl_Linnaeus&oldid=964690855
Wikipedia contributors. (2020, May 18). Franz Boas. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Franz_Boas&oldid=957282443
Wikipedia contributors. (2020, July 5). Johann Friedrich Blumenbach. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Johann_Friedrich_Blumenbach&oldid=966196943
Wikipedia contributors. (2020, July 11). Sentinelese. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Sentinelese&oldid=967121254
Wikipedia contributors. (2020, July 14). Thomas Henry Huxley. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Thomas_Henry_Huxley&oldid=967701553
The upper layer of the dermis with papillae extending upward into the epidermis.
A thin, fibrous, extracellular matrix that separates the lining of an internal or external body surface from underlying connective tissue.
The lower layer of the dermis that gives the dermis strength and elasticity and contains many dermal structures such as glands and hair follicles.
An anatomical structure that consists of a small cluster of cells, surrounding a central cavity.
Created by CK-12 Foundation/Adapted by Christine Miller
Vampires
From Bram Stoker’s famous novel about Count Dracula, to films such as Van Helsing and the Twilight Saga, fantasies featuring vampires (like the one in Figure 14.5.1) have been popular for decades. Vampires, in fact, are found in centuries-old myths from many cultures. In such myths, vampires are generally described as creatures that drink blood — preferably of the human variety — for sustenance. Dracula, for example, is based on Eastern European folklore about a human who attains immortality (and eternal damnation) by drinking the blood of others.
What Is Blood?
The average adult body contains between 4.7 and 5.7 litres of blood. More than half of that amount is fluid. Most of the rest of that amount consists of blood cells. The relative amounts of the various components in blood are illustrated in Figure 14.5.2. The components are also described in detail below.
is a fluid connective tissue that circulates throughout the body through blood vessels of the cardiovascular system. What makes blood so special that it features in widespread myths? Although blood accounts for less than 10% of human body weight, it is quite literally the elixir of life. As blood travels through the vessels of the , it delivers vital substances (such as nutrients and oxygen) to all of the , and carries away their metabolic wastes. It is no exaggeration to say that without blood, cells could not survive. Indeed, without the oxygen carried in blood, cells of the start to die within a matter of minutes.
Functions of Blood
Blood performs many important functions in the body. Major functions of blood include:
- Supplying tissues with oxygen, which is needed by all cells for .
- Supplying cells with nutrients, including , , and fatty acids.
- Removing metabolic wastes from cells, including carbon dioxide, urea, and lactic acid.
- Helping to defend the body from and other foreign substances.
- Forming clots to seal broken blood vessels and stop bleeding.
- Transporting and other messenger molecules.
- Regulating the of the body, which must be kept within a narrow range (7.35 to 7.45).
- Helping regulate body temperature (through and ).
Blood Plasma
is the liquid component of human blood. It makes up about 55% of blood by volume. It is about 92% water, and contains many dissolved substances. Most of these substances are , but plasma also contains trace amounts of glucose, mineral ions, hormones, carbon dioxide, and other substances. In addition, plasma contains blood cells. When the cells are removed from plasma, as in Figure 14.5.2 above, the remaining liquid is clear but yellow in colour.
Blood Cells
The cells in blood include , , and . These different types of blood cells are shown in the photomicrograph (Figure 14.5.3) and described in the sections that follow.
Erythrocytes
The most numerous cells in blood are red blood cells, also called s. One microlitre of blood contains between 4.2 and 6.1 million red blood cells, and red blood cells make up about 25% of all the cells in the human body. The cytoplasm of a mature erythrocyte is almost completely filled with hemoglobin, the iron-containing protein that binds with oxygen and gives the cell its red colour. In order to provide maximum space for hemoglobin, mature erythrocytes lack a cell and most s. They are little more than sacks of hemoglobin.
Erythrocytes also carry proteins called antigens that determine blood type. is a genetic characteristic. The best known human blood type systems are the ABO and Rhesus systems.
- In the ABO system, there are two common antigens, called antigen A and antigen B. There are four ABO blood types, A (only A antigen), B (only B antigen), AB (both A and B antigens), and O (neither A nor B antigen). The ABO antigens are illustrated in Figure 14.5.4.
- In the Rhesus system, there is just one common antigen. A person may either have the antigen (Rh+) or lack the antigen (Rh-).
Blood type is important for medical reasons. A person who needs a blood transfusion must receive blood of a compatible type. Blood that is compatible lacks antigens that the patient's own blood also lacks. For example, for a person with type A blood (no B antigen), compatible types include any type of blood that lacks the B antigen. This would include type A blood or type O blood, but not type AB or type B blood. If incompatible blood is transfused, it may cause a potentially life-threatening reaction in the patient’s blood.
Leukocytes
s (also called white blood cells) are cells in blood that defend the body against invading microorganisms and other threats. There are far fewer leukocytes than red blood cells in blood. There are normally only about 1,000 to 11,000 white blood cells per microlitre of blood. Unlike erythrocytes, leukocytes have a nucleus. White blood cells are part of the body’s immune systemno post. They destroy and remove old or abnormal cells and cellular debris, as well as attack pathogens and foreign substances. There are five main types of white blood cells, which are described in Table 14.5.1: neutrophils, eosinophils, basophils, lymphocytes, and monocytes. The five types differ in their specific immune functions.
Type of Leukocyte | Per cent of All Leukocytes | Main Function(s) |
---|---|---|
Neutrophil | 62% | Phagocytize (engulf and destroy) bacteria and fungi in blood. |
Eosinophil | 2% | Attack and kill large parasites; carry out allergic responses. |
Basophil | less than 1% | Release histamines in inflammatory responses. |
Lymphocyte | 30% | Attack and destroy virus-infected and tumor cells; create lasting immunity to specific pathogens. |
Monocyte | 5% | Phagocytize pathogens and debris in tissues. |
Thrombocytes
s, also called platelets, are actually cell fragments. Like erythrocytes, they lack a nucleus and are more numerous than white blood cells. There are about 150 thousand to 400 thousand thrombocytes per microlitre of blood.
The main function of thrombocytes is blood clotting, or . This is the process by which blood changes from a liquid to a gel, forming a plug in a damaged blood vessel. If blood clotting is successful, it results in , which is the cessation of blood loss from the damaged vessel. A blood clot consists of both platelets and proteins, especially the protein fibrin. You can see a scanning electron microscope photomicrograph of a blood clot in Figure 14.5.5.
Coagulation begins almost instantly after an injury occurs to the endothelium of a blood vessel. Thrombocytes become activated and change their shape from spherical to star-shaped, as shown in Figure 14.5.6. This helps them aggregate with one another (stick together) at the site of injury to start forming a plug in the vessel wall. Activated thrombocytes also release substances into the blood that activate additional thrombocytes and start a sequence of reactions leading to fibrin formation. Strands of fibrin crisscross the platelet plug and strengthen it, much as rebar strengthens concrete.
Formation and Degradation of Blood Cells
Blood is considered a , because blood cells form inside bones. All three types of blood cells are made in red marrow within the medullary cavity of bones in a process called . Formation of blood cells occurs by the proliferation of stem cells in the marrow. These stem cells are self-renewing — when they divide, some of the daughter cells remain stem cells, so the pool of stem cells is not used up. Other daughter cells follow various pathways to differentiate into the variety of blood cell types. Once the cells have differentiated, they cannot divide to form copies of themselves.
Eventually, blood cells die and must be replaced through the formation of new blood cells from proliferating stem cells. After blood cells die, the dead cells are phagocytized (engulfed and destroyed) by white blood cells, and removed from the circulation. This process most often takes place in the and .
Blood Disorders
Many human disorders primarily affect the blood. They include cancers, genetic disorders, poisoning by toxins, infections, and nutritional deficiencies.
- is a group of cancers of the blood-forming tissues in the bone marrow. It is the most common type of cancer in children, although most cases occur in adults. Leukemia is generally characterized by large numbers of abnormal leukocytes. Symptoms may include excessive bleeding and bruising, fatigue, fever, and an increased risk of infections. Leukemia is thought to be caused by a combination of genetic and environmental factors.
- refers to any of several genetic disorders that cause dysfunction in the blood clotting process. People with hemophilia are prone to potentially uncontrollable bleeding, even with otherwise inconsequential injuries. They also commonly suffer bleeding into the spaces between joints, which can cause crippling.
- occurs when inhaled carbon monoxide (in fumes from a faulty home furnace or car exhaust, for example) binds irreversibly to the in erythrocytes. As a result, oxygen cannot bind to the red blood cells for transport throughout the body, and this can quickly lead to suffocation. Carbon monoxide is extremely dangerous, because it is colourless and odorless, so it cannot be detected in the air by human senses.
- is a virus that infects certain types of leukocytes and interferes with the body’s ability to defend itself from pathogens and other causes of illness. HIV infection may eventually lead to (acquired immunodeficiency syndrome). AIDS is characterized by rare infections and cancers that people with a healthy immune systemno post almost never acquire.
- is a disorder in which the blood has an inadequate volume of erythrocytes, reducing the amount of oxygen that the blood can carry, and potentially causing weakness and fatigue. These and other signs and symptoms of anemia are shown in Figure 14.5.8. Anemia has many possible causes, including excessive bleeding, inherited disorders (such as sickle cell hemoglobin), or nutritional deficiencies (iron, folate, or B12). Severe anemia may require transfusions of donated blood.
Feature: Myth vs. Reality
Donating blood saves lives. In fact, with each blood donation, as many as three lives may be saved. According to Government Canada, up to 52% of Canadians have reported that they or a family member have needed blood or blood products at some point in their lifetime. Many donors agree that the feeling that comes from knowing you have saved lives is well worth the short amount of time it takes to make a blood donation. Nonetheless, only a minority of potential donors actually donate blood. There are many myths about blood donation that may help explain the small percentage of donors. Knowing the facts may reaffirm your decision to donate if you are already a donor — and if you aren’t a donor already, getting the facts may help you decide to become one.
Myth | Reality |
---|---|
"Your blood might become contaminated with an infection during the donation." | There is no risk of contamination because only single-use, disposable catheters, tubing, and other equipment are used to collect blood for a donation. |
"You are too old (or too young) to donate blood." | There is no upper age limit on donating blood, as long as you are healthy. The minimum age is 16 years. |
"You can’t donate blood if you have high blood pressure." | As long as your blood pressure is below 180/100 at the time of donation, you can give blood. Even if you take blood pressure medication to keep your blood pressure below this level, you can donate. |
"You can’t give blood if you have high cholesterol." | Having high cholesterol does not affect your ability to donate blood. Taking cholesterol-lowering medication also does not disqualify you. |
"You can’t donate blood if you have had a flu shot." | Having a flu shot has no effect on your ability to donate blood. You can even donate on the same day that you receive a flu shot. |
"You can’t donate blood if you take medication." | As long as you are healthy, in most cases, taking medication does not preclude you from donating blood. |
"Your blood isn’t needed if it’s a common blood type." | All types of blood are in constant demand. |
14.5 Summary
- is a that circulates throughout the body in the . Blood supplies tissues with oxygen and nutrients and removes their metabolic wastes. Blood helps defend the body from and other threats, transports and other substances, and helps keep the body’s and temperature in homeostasis.
- is the liquid component of blood, and it makes up more than half of blood by volume. It consists of water and many dissolved substances. It also contains blood cells, including erythrocytes, leukocytes and thrombocytes.
- , (also known as red blood cells) are the most numerous cells in blood. They consist mostly of , which carries oxygen. Erythrocytes also carry that determine .
- Leukocytes (also referred to as white blood cells) are less numerous than erythrocytes and are part of the body’s immune systemno post. There are several different types of leukocytes that differ in their specific immune functions. They protect the body from abnormal cells, microorganisms, and other harmful substances.
- Thrombocytes (also called platelets) are cell fragments that play important roles in blood clotting, or coagulation. They stick together at breaks in blood vessels to form a clot and stimulate the production of fibrin, which strengthens the clot.
- All blood cells form by proliferation of stem cells in red bone marrow in a process called . When blood cells die, they are phagocytized by leukocytes and removed from the circulation.
- Disorders of the blood include , which is cancer of the bone-forming cells; , which is any of several genetic blood-clotting disorders; , which prevents erythrocytes from binding with oxygen and causes suffocation; infection, which destroys certain types of leukocytes and can cause ; and , in which there are not enough erythrocytes to carry adequate oxygen to body tissues.
14.5 Review Questions
- What is blood? Why is blood considered a connective tissue?
- Identify four physiological roles of blood in the body.
- Describe plasma and its components.
14.5 Explore More
https://youtu.be/e-5wqwp64MM
Joe Landolina: This gel can make you stop bleeding instantly, TED, 2014.
https://youtu.be/hgp8LtwFSBA
Can Synthetic Blood Help The World's Blood Shortage? Science Plus, 2016.
https://youtu.be/1Qfmkd6C8u8
How bones make blood - Melody Smith, TED-Ed, 2020.
Attributions
Figure 14.5.1
vampire_PNG32 from pngimg.com is used under a CC BY-NC 4.0 (https://creativecommons.org/licenses/by-nc/4.0/) license.
Figure 14.5.2
Blood-centrifugation-scheme by KnuteKnudsen at English Wikipedia on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 14.5.3
SEM_blood_cells by Bruce Wetzel and Harry Schaefer (Photographers)/ NCI AV-8202-3656 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).
Figure 14.5.4
ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).
Figure 14.5.5
Blood_clot_in_scanning_electron_microscopy by Janice Carr from CDC/ Public Health Image LIbrary (PHIL) ID #7308 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).
Figure 14.5.6
Blausen_0740_Platelets by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 14.5.7
Platelet_Party_900x by Awkward Yeti (used with permission of the author) © All Rights Reserved
Figure 14.5.8
Symptoms_of_anemia.svg by Mikael Häggström on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:public_domain).
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.
Blood, organ and tissue donation. (2020, April 28). Government of Canada. https://www.canada.ca/en/public-health/services/healthy-living/blood-organ-tissue-donation.html#a3
Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising. https://www.blood.ca
Science Plus. (2016, March 2). Can synthetic blood help the world's blood shortage? https://www.youtube.com/watch?v=hgp8LtwFSBA&feature=youtu.be
TED. (2014, November 20). Joe Landolina: This gel can make you stop bleeding instantly. YouTube. https://www.youtube.com/watch?v=e-5wqwp64MM&feature=youtu.be
TED-Ed. (2020, January 27). How bones make blood - Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8&feature=youtu.be
Image shows a labelled diagram of the posterior (from the back) view of the kidneys. The aorta and renal arteries are clearly visible bringing blood to each kidney. The left kidney sits a bit higher than the right kidney.
The process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle.
Long chains of hydrocarbons with a carboxyl group and a methyl group at opposite ends. Can be either saturated, containing mostly single bonds between adjacent carbons, or unsaturated, containing many double bonds between adjacent carbons.
An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses.
A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.
A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.
The movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration.
The movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration.
Small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps.
Created by: CK-12/Adapted by Christine Miller
Oh, the Agony!
Wearing braces can be very uncomfortable, but it is usually worth it. Braces and other orthodontic treatments can re-align the teeth and jaws to improve bite and appearance. Braces can change the position of the teeth and the shape of the jaws because the human body is malleable. Many phenotypic traits — even those that have a strong genetic basis — can be molded by the environment. Changing the in response to the environment is just one of several ways we respond to environmental stress.
Types of Responses to Environmental Stress
There are four different types of responses that humans may make to cope with :
- Adaptation
- Developmental adjustment
- Acclimatization
- Cultural responses
The first three types of responses are biological in nature, and the fourth type is cultural. Only adaptation involves genetic change and occurs at the level of the population or species. The other three responses do not require genetic change, and they occur at the individual level.
Adaptation
An is a genetically-based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations generally evolve in a population over many generations in response to stresses that last for a long period of time. Adaptations come about through . Those individuals who inherit a trait that confers an advantage in coping with an environmental stress are likely to live longer and reproduce more. As a result, more of their genes pass on to the next generation. Over many generations, the genes and the trait they control become more frequent in the population.
A Classic Example: Hemoglobin S and Malaria
Probably the most frequently-cited example of a genetic adaptation to an environmental stress is sickle cell trait. As you read in the previous section, people with sickle cell trait have one abnormal allele (S) and one normal allele (A) for hemoglobin, the red blood cell protein that carries oxygen in the blood. Sickle cell trait is an adaptation to the environmental stress of malaria, because people with the trait have resistance to this parasitic disease. In areas where malaria is endemic (present year-round), the sickle cell trait and its have evolved to relatively high frequencies. It is a classic example of natural selection favoring for a with two . This type of selection keeps both alleles at relatively high frequencies in a population.
To Taste or Not to Taste
Another example of an adaptation in humans is the ability to taste bitter compounds. Plants produce a variety of toxic compounds in order to protect themselves from being eaten, and these toxic compounds often have a bitter taste. The ability to taste bitter compounds is thought to have evolved as an adaptation, because it prevented people from eating poisonous plants. Humans have many different genes that code for bitter taste receptors, allowing us to taste a wide variety of bitter compounds.
A harmless bitter compound called phenylthiocarbamide (PTC) is not found naturally in plants, but it is similar to toxic bitter compounds that are found in plants. Humans' ability to taste this harmless substance has been tested in many different populations. In virtually every population studied, there are some people who can taste PTC (called tasters), and some people who cannot taste PTC, (called nontasters). The ratio of tasters to non-tasters varies among populations, but on average, 75 per cent of people can taste PTC and 25 per cent cannot.
Like many scientific discoveries, human variation in PTC-taster status was discovered by chance. Around 1930, a chemist named Arthur Fox was working with powdered PTC in his lab. Some of the powder accidentally blew into the air. Another lab worker noticed that the powdered PTC tasted bitter, but Fox couldn't detect any taste at all. Fox wondered how to explain this difference in PTC-tasting ability. Geneticists soon determined that PTC-taster status is controlled by a single with two common alleles, usually represented by the letters T and t. The T allele encodes a chemical receptor protein (found in taste buds on the tongue, as illustrated in Figure 6.4.2) that can strongly bind to PTC. The other allele, t, encodes a version of the receptor protein that cannot bind as strongly to PTC. The particular combination of these two alleles that a person inherits determines whether the person finds PTC to taste very bitter (TT), somewhat bitter (Tt), or not bitter at all (tt).
If the ability to taste bitter compounds is advantageous, why does every human population studied contain a significant percentage of people who are nontasters? Why has the nontasting allele been preserved in human populations at all? Some scientists hypothesize that the nontaster allele actually confers the ability to taste some other, yet-to-be identified, bitter compound in plants. People who inherit both alleles would presumably be able to taste a wider range of bitter compounds, so they would have the greatest ability to avoid plant toxins. In other words, the for the taster gene would be the most fit and favored by .
Most people no longer have to worry whether the plants they eat contain toxins. The produce you grow in your garden or buy at the supermarket consists of known varieties that are safe to eat. However, natural selection may still be at work in human populations for the PTC-taster gene, because PTC tasters may be more sensitive than nontasters to bitter compounds in tobacco and vegetables in the cabbage family (that is, cruciferous vegetables, such as the broccoli, cauliflower, and cabbage pictured in Figure 6.4.3).
- People who find PTC to taste very bitter are less likely to smoke tobacco, presumably because tobacco smoke has a stronger bitter taste to these individuals. In this case, selection would favor taster genotypes, because tasters would be more likely to avoid smoking and its serious health risks.
- Strong tasters find cruciferous vegetables to taste bitter. As a result, they may avoid eating these vegetables (and perhaps other foods, as well), presumably resulting in a diet that is less varied and nutritious. In this scenario, natural selection might work against taster genotypes.
Figure 6.4.3 Cruciferous vegetables.
Developmental Adjustment
It takes a relatively long time for genetic change in response to environmental stress to produce a population with adaptations. Fortunately, we can adjust to some environmental stresses more quickly by changing in nongenetic ways. One type of nongenetic response to stress is developmental adjustment. This refers to phenotypic change that occurs during development in infancy or childhood, and that may persist into adulthood. This type of change may be irreversible by adulthood.
Phenotypic Plasticity
Developmental adjustment is possible because humans have a high degree of phenotypic plasticity, which is the ability to alter the in response to changes in the environment. Phenotypic plasticity allows us to respond to changes that occur within our lifetime, and it is particularly important for species (like our own) that have a long generation time. With long generations, evolution of genetic adaptations may occur too slowly to keep up with changing environmental stresses.
Developmental Adjustment and Cultural Practices
Developmental adjustment may be the result of naturally occurring environmental stresses or cultural practices, including medical or dental treatments. Like our example at the beginning of this section, using braces to change the shape of the jaw and the position of the teeth is an example of a dental practice that brings about a developmental adjustment. Another example of developmental adjustment is the use of a back brace to treat scoliosis (see images in Figure 6.4.4). Scoliosis is an abnormal curvature from side to side in the spine. If the problem is not too severe, a brace, if worn correctly, should prevent the curvature from worsening as a child grows, although it cannot straighten a curve that is already present. Surgery may be required to do that.
Developmental Adjustment and Nutritional Stress
An important example of developmental adjustment that results from a naturally occurring environmental stress is the cessation of physical growth that occurs in children who are under nutritional stress. Children who lack adequate food to fuel both growth and basic metabolic processes are likely to slow down in their growth rate — or even to stop growing entirely. Shunting all available calories and nutrients into essential life functions may keep the child alive at the expense of increasing body size.
Table 6.4.1 shows the effects of inadequate diet on children's' growth in several countries worldwide. For each country, the table gives the prevalence of stunting in children under the age of five. Children are considered stunted if their height is at least two standard deviations below the median height for their age in an international reference population.
Table 6.4.1
Percentage of Stunting in Young Children in Selected Countries (2011-2015)
Percentage of Stunting in Young Children in Selected Countries (2011-2015) | |
Country | Per cent of Children Under Age 5 with Stunting |
United States | 2.1 |
Turkey | 9.5 |
Mexico | 13.6 |
Thailand | 16.3 |
Iraq | 22.6 |
Philippines | 33.6 |
Pakistan | 45.0 |
Papua New Guinea | 49.5 |
After a growth slow-down occurs and if adequate food becomes available, a child may be able to make up the loss of growth. If food is plentiful, the child may grow more rapidly than normal until the original, genetically-determined growth trajectory is reached. If the inadequate diet persists, however, the failure of growth may become chronic, and the child may never reach his or her full potential adult size.
Phenotypic plasticity of body size in response to dietary change has been observed in successive generations within populations. For example, children in Japan were taller, on average, in each successive generation after the end of World War II. Boys aged 14-15 years old in 1986 were an average of about 18 cm (7 in.) taller than boys of the same age in 1959, a generation earlier. This is a highly significant difference, and it occurred too quickly to be accounted for by genetic change. Instead, the increase in height is a developmental adjustment, thought to be largely attributable to changes in the Japanese diet since World War II. During this period, there was an increase in the amount of animal protein and fat, as well as in the total calories consumed.
Acclimatization
Other responses to environmental stress are reversible and not permanent, whether they occur in childhood or adulthood. The development of reversible changes to environmental stress is called . Acclimatization generally develops over a relatively short period of time. It may take just a few days or weeks to attain a maximum response to a stress. When the stress is no longer present, the acclimatized state declines, and the body returns to its normal baseline state. Generally, the shorter the time for acclimatization to occur, the more quickly the condition is reversed when the environmental stress is removed.
Acclimatization to UV Light
A common example of acclimatization is tanning of the skin (see Figure 6.4.5). This occurs in many people in response to exposure to ultraviolet radiation from the sun. Special pigment cells in the skin, called melanocytes, produce more of the brown pigment melanin when exposed to sunlight. The melanin collects near the surface of the skin where it absorbs UV radiation so it cannot penetrate and potentially damage deeper skin structures. Tanning is a reversible change in the phenotype that helps the body deal temporarily with the environmental stress of high levels of UV radiation. When the skin is no longer exposed to the sun’s rays, the tan fades, generally over a period of a few weeks or months.
Figure 6.4.5 Tanning of the skin occurs in many people in response to exposure to ultraviolet radiation from the sun.
Acclimatization to Heat
Another common example of acclimatization occurs in response to heat. Changes that occur with heat acclimatization include increased sweat output and earlier onset of sweat production, which helps the body stay cool because evaporation of sweat takes heat from the body’s surface in a process called evaporative cooling. It generally takes a couple of weeks for maximum heat acclimatization to come about by gradually working out harder and longer at high air temperatures. The changes that occur with acclimatization just as quickly subside when the body is no longer exposed to excessive heat.
Acclimatization to High Altitude
Short term acclimatization to high altitude occurs as a response to low levels of oxygen in the blood. This reduced level of oxygen is detected by carotid bodies, which will trigger in increase in breathing and heart rate. Over a period of weeks the body will compensate by increasing red blood cell production, thereby improving the oxygen-carrying capacity of the blood. This is why mountaineers wishing to climb to the peak of Mount Everest must complete the full climb in portions; it is recommended that climbers spend 2-3 days acclimatizing for every 600 metres of elevation increase. In addition, the higher to altitude, the longer it make take to acclimatize; climbers are advised to spend 4-5 days acclimatizing at base camp (whether the base camp in Nepal or China) before completing the final leg of the climb to the peak. The concentration of red blood cells gradually decreases to normal levels once a climber returns to their normal elevation.
Cultural Responses
More than any other species, humans respond to environmental stresses with learned behaviors and technology. These cultural responses allow us to change our environments to control stresses, rather than changing our bodies genetically or physiologically to cope with the stresses. Even archaic humans responded to some environmental stresses in this way. For example, Neanderthals used shelters, fires, and animal hides as clothing to stay warm in the cold climate in Europe during the last ice age. Today, we use more sophisticated technologies to stay warm in cold climates while retaining our essentially tropical-animal anatomy and physiology. We also use technology (such as furnaces and air conditioners) to avoid temperature stress and stay comfortable in hot or cold climates.
6.4 Summary
- Humans may respond to in four different ways: adaptation, developmental adjustment, acclimatization, and cultural responses.
- An adaptation is a genetically based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations evolve by natural selection in populations over a relatively long period to time. Examples of adaptations include sickle cell trait as an adaptation to the stress of endemic malaria and the ability to taste bitter compounds as an adaptation to the stress of bitter-tasting toxins in plants.
- A developmental adjustment is a non-genetic response to stress that occurs during infancy or childhood, and that may persist into adulthood. This type of change may be irreversible. Developmental adjustment is possible because humans have a high degree of phenotypic plasticity. It may be the result of environmental stresses (such as inadequate food), which may stunt growth, or cultural practices (such as orthodontic treatments), which re-align the teeth and jaws.
- Acclimatization is the development of reversible changes to environmental stress that develop over a relatively short period of time. The changes revert to the normal baseline state after the stress is removed. Examples of acclimatization include tanning of the skin and physiological changes (such as increased sweating) that occur with heat acclimatization.
- More than any other species, humans respond to environmental stress with learned behaviors and technology, which are cultural responses. These responses allow us to change our environment to control stress, rather than changing our bodies genetically or physiologically to cope with stress. Examples include using shelter, fire, and clothing to cope with a cold climate.
6.4 Review Questions
- List four different types of responses that humans may make to cope with environmental stress.
- Define adaptation.
- Explain how natural selection may have resulted in most human populations having people who can and people who cannot taste PTC.
- What is a developmental adjustment?
- Define phenotypic plasticity.
- Explain why phenotypic plasticity may be particularly important in a species with a long generation time.
- Why may stunting of growth occur in children who have an inadequate diet? Why is stunting preferable to the alternative?
- What is acclimatization?
- How does acclimatization to heat come about, and what are two physiological changes that occur in heat acclimatization?
- Give an example of a cultural response to heat stress.
- Which is more likely to be reversible — a change due to acclimatization, or a change due to developmental adjustment? Explain your answer.
6.4 Explore More
https://www.youtube.com/watch?v=upp9-w6GPhU
Could we survive prolonged space travel? - Lisa Nip, TED-Ed, 2016.
https://www.youtube.com/watch?v=hRnrIpUMyZQ&t=182s
How this disease changes the shape of your cells - Amber M. Yates, TED-Ed, 2019.
Attributions
Figure 6.4.1
Free_Awesome_Girl_With_Braces_Close_Up by D. Sharon Pruitt from Hill Air Force Base, Utah, USA on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Figure 6.4.2
Tongue by Mahdiabbasinv on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
Figure 6.4.3
- White cauliflower on brown wooden chopping board by Louis Hansel @shotsoflouis on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Broccoli on wooden chopping board by Louis Hansel @shotsoflouis on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Green cabbage close up by Craig Dimmick on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Cabbage hybrid/ brussel sprouts by Solstice Hannan on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Kale by Laura Johnston on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Tiny bok choy at the Asian market by Jodie Morgan on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 6.4.4
Scoliosis_patient_in_cheneau_brace_correcting_from_56_to_27_deg by Weiss H.R. from Scoliosis Journal/BioMed Central Ltd. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.
Figure 6.4.5
- Tan Lines by k.steudel on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
- Twin tan lines (all sizes) by Quinn Dombrowski on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
- Wedding ring tan line by Quinn Dombrowski on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
- Tan by Evil Erin on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 6.4.6
Nepalese base camp by Mark Horrell on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
References
TED-Ed. (2016, October 4). Could we survive prolonged space travel? - Lisa Nip. YouTube. https://www.youtube.com/watch?v=upp9-w6GPhU&feature=youtu.be
TED-Ed. (2019, May 6). How this disease changes the shape of your cells - Amber M. Yates. YouTube. https://www.youtube.com/watch?v=hRnrIpUMyZQ&feature=youtu.be
Weiss, H. (2007). Is there a body of evidence for the treatment of patients with Adolescent Idiopathic Scoliosis (AIS)? [Figure 2 - digital photograph], Scoliosis, 2(19). https://doi.org/10.1186/1748-7161-2-19
The central nervous system organ inside the skull that is the control center of the nervous system.
As per caption
Image shows a diagram of the bladder. There is smooth muscle in the bladder walls which are under involuntary control. There is a sphincter between the bladder and the urethra which can inhibit urination.
Created by CK-12 Foundation/Adapted by Christine Miller
Figure 16.3.1 The surprising uses of pee.
Surprising Uses
What do gun powder, leather, fabric dyes and laundry service have in common? This may be surprising, but they all historically involved urine. One of the main components in gun powder, potassium nitrate, was difficult to come by pre-1900s, so ingenious gun-owners would evaporate urine to concentrate the nitrates it contains. The ammonium in urine was excellent in breaking down tissues, making it a prime candidate for softening leathers and removing stains in laundry. Ammonia in urine also helps dyes penetrate fabrics, so it was used to make colours stay brighter for longer.
What is the Urinary System?
The actual human , also known as the renal system, is shown in Figure 16.3.2. The system consists of the kidneys, ureters, bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting . Typically, between one and two litres of urine are produced every day in a healthy individual.
Organs of the Urinary System
The urinary system is all about urine. It includes organs that form urine, and also those that transport, store, or excrete urine.
Kidneys
is formed by the , which filter many substances out of the , allow the blood to reabsorb needed materials, and use the remaining materials to form urine. The human body normally has two paired kidneys, although it is possible to get by quite well with just one. As you can see in Figure 16.3.3, each kidney is well supplied with blood vessels by a major artery and vein. Blood to be filtered enters the kidney through the renal artery, and the filtered blood leaves the kidney through the renal vein. The kidney itself is wrapped in a fibrous capsule, and consists of a thin outer layer called the cortex, and a thicker inner layer called the medulla.
is filtered and is formed by tiny filtering units called . Each kidney contains at least a million nephrons, and each nephron spans the cortex and medulla layers of the kidney. After urine forms in the nephrons, it flows through a system of converging collecting ducts. The collecting ducts join together to form minor calyces (or chambers) that join together to form major calyces (see Figure 16.3.3 above). Ultimately, the major calyces join the , which is the funnel-like end of the where it enters the kidney.
Ureters, Bladder, Urethra
After urine forms in the kidneys, it is transported through the ureters (one per kidney) via to the sac-like urinary bladder, which stores the urine until . During urination, the urine is released from the bladder and transported by the urethra to be excreted outside the body through the external urethral opening.
Functions of the Urinary System
Waste products removed from the body with the formation and elimination of urine include many water-soluble metabolic products. The main waste products are — a by-product of — and , a by-product of catabolism. Excess water and mineral ions are also eliminated in urine.
Besides the elimination of waste products such as these, the urinary system has several other vital functions. These include:
- Maintaining homeostasis of mineral ions in extracellular fluid: These ions are either excreted in urine or returned to the blood as needed to maintain the proper balance.
- Maintaining homeostasis of blood pH: When pH is too low (blood is too acidic), for example, the kidneys excrete less bicarbonate (which is basic) in urine. When pH is too high (blood is too basic), the opposite occurs, and more bicarbonate is excreted in urine.
- Maintaining homeostasis of extracellular fluids, including the blood volume, which helps maintain blood pressure: The kidneys control fluid volume and blood pressure by excreting more or less salt and water in urine.
Control of the Urinary System
The formation of must be closely regulated to maintain body-wide homeostasis. Several help control this function of the urinary system, including antidiuretic hormone, parathyroid hormone, and aldosterone.
- (ADH), also called vasopressin, is secreted by the posterior pituitary gland. One of its main roles is conserving body water. It is released when the body is dehydrated, and it causes the kidneys to excrete less water in urine.
- is secreted by the parathyroid glands. It works to regulate the balance of mineral ions in the body via its effects on several organs, including the kidneys. Parathyroid hormone stimulates the kidneys to excrete less calcium and more phosphorus in urine.
- is secreted by the cortex of the adrenal glands, which rest atop the kidneys, as shown in Figure 16.3.4. Through its effect on the kidneys, it plays a central role in regulating blood pressure. It causes the kidneys to excrete less sodium and water in urine.
Once urine forms, it is excreted from the body in the process of , also sometimes referred to as micturition. This process is controlled by both the and the nervous systems. As the bladder fills with urine, it causes the autonomic nervous system to signal smooth muscle in the bladder wall to contract (as shown in Figure 16.3.5), and the sphincter between the bladder and urethra to relax and open. This forces urine out of the bladder and through the urethra. Another sphincter at the distal end of the urethra is under control. When it relaxes under the influence of the somatic nervous system, it allows urine to leave the body through the external urethral opening.
16.3 Summary
- The consists of the kidneys, ureters, bladder, and urethra. The main function of the urinary system is to eliminate the waste products of from the body by forming and excreting .
- Urine is formed by the kidneys, which filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine. Blood to be filtered enters the kidney through the renal artery, and filtered blood leaves the kidney through the renal vein.
- Within each , blood is filtered and urine is formed by tiny filtering units called , of which there are at least a million in each kidney.
- After urine forms in the kidneys, it is transported through the via to the . The bladder stores the urine until , when urine is transported by the urethra to be excreted outside the body.
- Besides the elimination of waste products (such as , , excess water, and mineral ions), the urinary system has other vital functions. These include maintaining of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling .
- The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including from the posterior , from the , and from the .
- The process of urination is controlled by both the and the nervous systems. The autonomic system causes the bladder to empty, but conscious relaxation of the at the distal end of the allows urine to leave the body.
16.3 Review Questions
- State the main function of the urinary system.
- What are nephrons?
- Other than the elimination of waste products, identify functions of the urinary system.
- How is the formation of urine regulated?
- Explain why it is important to have voluntary control over the sphincter at the end of the urethra.
- In terms of how they affect the kidneys, compare aldosterone to antidiuretic hormone.
- If your body needed to retain more calcium, which of the hormones described in this concept is most likely to increase? Explain your reasoning.
16.3 Explore More
https://youtu.be/dxecGD0m0Xc
The Urinary System - An Introduction | Physiology | Biology | FuseSchool, 2017.
https://youtu.be/pyMcTUQYMQw
Maple Syrup Urine Disease, Alexandria Doody, 2016.
https://youtu.be/3z-xjfdJWAI
How Accurate Are Drug Tests? Seeker, 2016.
https://youtu.be/xt1Tj5eeS0k
Three Ways Pee Could Change the World, Gross Science, 2015.
Attributions
Figure 16.3.1
- File:Pyrodex powder ffg.jpg by Hustvedt on Wikimedia Commons is used under a CC BY SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en).
- Brown leather satchel bag by Álvaro Serrano on Unsplash is used under the Unsplash Licence (https://unsplash.com/license).
- Laundry basket by Andy Fitzsimon on Unsplash is used under the Unsplash Licence (https://unsplash.com/license).
- Tags: Wool Skeins Natural Dyed Colorful Himalayan Weavers by on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
Figure 16.3.2
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 16.3.3
2610_The_Kidney by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 16.3.4
Adrenal glands on Kidney by Alan Hoofring (Illustrator)/ NCI Visuals Online is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 16.3.5
Urinary_Sphincter by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
References
Alexandria Doody. (2016, March 29). Maple syrup urine disease. YouTube. https://www.youtube.com/watch?v=pyMcTUQYMQw&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, June 19). Figure 25.8 Left kidney [digital image]. In Anatomy and Physiology (Section 25.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/25-3-gross-anatomy-of-the-kidney
FuseSchool. (2017, June 19). The urinary system - An introduction | Physiology | Biology | FuseSchool. YouTube. https://www.youtube.com/watch?v=dxecGD0m0Xc&feature=youtu.be
Gross Science. (2015, September 15). Three ways pee could change the world. YouTube. https://www.youtube.com/watch?v=xt1Tj5eeS0k&feature=youtu.be
Seeker. (2016, January 16). How accurate are drug tests? YouTube. https://www.youtube.com/watch?v=3z-xjfdJWAI&feature=youtu.be
The ability of an organism to maintain constant internal conditions despite external changes.
The chemical processes that occur in a living organism to sustain life.
A specific reactant in a chemical reaction which works with a specific enzyme.
A hollow, tube-like structure through which blood flows in the cardiovascular system; vein, artery, or capillary.