1. Chapter 20 Animal Structure and Function

2. 20.1 An animal’s form is not the perfect design
The laryngeal nerve of an adult giraffe travels from the brain, makes a U-turn around the aorta in the chest, and then extends back up the neck to muscles in the throat.
Why does the laryngeal nerve make about a 15-foot journey?
Student Misconceptions and Concerns
Another misconception involves the use of the term “design.” In biology, “design” does not imply conscious invention. Instead, the term identifies the arrangement of the parts and their interrelated functions. Natural selection is not a deliberate process. (20.1)
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
As the example of the laryngeal nerve illustrates, form and function relationships represent the remodeling of organisms. Like fixing up an old home, new functions require the revision of older structures. (20.1)
Active Lecture Tips
Most adaptations represent compromise. Ask students to turn to someone near them to think of examples of functional compromises in their own bodies. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. Common examples include (a) chewing, which can interfere with hearing; (b) our flexible skin that is more likely to be cut or punctured than a hard outer shell; (c) walking on two legs, which frees up our hands for other tasks but makes us less stable than walking on four legs. (20.1)
Students often fail to consider the significance of body size. Consider asking your students to think about the impact of being small. Have they ever had difficulty emerging from a swimming pool because of the adhesive properties of water? Of course not—and yet, small insects that land on a pond’s surface may find these forces to be lethal, preventing them from breaking away from the water’s surface! Ask students if they are ever unable to leave their homes because of high winds, which make it impossible for them to walk around outside. The movements of small insects are often hampered by winds that would do little more than toss around our hair! Many campers know that mosquitoes and flies are less of a nuisance on days when there is a good breeze. (20.1)

3. 20.1 An animal’s form is not the perfect design
The surprising length of the laryngeal nerve illustrates a major concept in evolution: Through natural selection, a structure in an ancestral organism can be modified to function in a descendant organism.
Explain why the process of natural selection in tetrapods resulted in a long, looped laryngeal nerve instead of a short nerve following a more direct route.
Checkpoint Question Response
A more direct (shorter) connection from the brain to the throat would require severing and rejoining this nerve, which would be incompatible with survival.
Student Misconceptions and Concerns
Another misconception involves the use of the term “design.” In biology, “design” does not imply conscious invention. Instead, the term identifies the arrangement of the parts and their interrelated functions. Natural selection is not a deliberate process. (20.1)
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
As the example of the laryngeal nerve illustrates, form and function relationships represent the remodeling of organisms. Like fixing up an old home, new functions require the revision of older structures. (20.1)
Active Lecture Tips
Most adaptations represent compromise. Ask students to turn to someone near them to think of examples of functional compromises in their own bodies. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. Common examples include (a) chewing, which can interfere with hearing; (b) our flexible skin that is more likely to be cut or punctured than a hard outer shell; (c) walking on two legs, which frees up our hands for other tasks but makes us less stable than walking on four legs. (20.1)
Students often fail to consider the significance of body size. Consider asking your students to think about the impact of being small. Have they ever had difficulty emerging from a swimming pool because of the adhesive properties of water? Of course not—and yet, small insects that land on a pond’s surface may find these forces to be lethal, preventing them from breaking away from the water’s surface! Ask students if they are ever unable to leave their homes because of high winds, which make it impossible for them to walk around outside. The movements of small insects are often hampered by winds that would do little more than toss around our hair! Many campers know that mosquitoes and flies are less of a nuisance on days when there is a good breeze. (20.1)

4. EVOLUTION CONNECTION: An animal’s form reflects natural selection
The body plan or design of an organism
reflects the relationship between form and function,
results from natural selection, and
does not imply a process of conscious invention
For example- Streamlined and tapered bodies
increase swimming speeds and
have similarly evolved in fish, sharks,
and aquatic birds and mammals,
representing convergent evolution.
Student Misconceptions and Concerns
1. Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe,” or “because we need oxygen.” Need, however, does not cause evolution. Natural selection involves editing rather than creating diversity. A better answer might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.”
2. Relationships between form and function are found all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Practice with examples helps to build a better understanding of these important relationships.
3. As noted in Module 20.2, the use of the term design in biology does not imply conscious invention. Instead, the term identifies the arrangement of the parts and their interrelated functions. Natural selection is not a deliberate process.
Teaching Tips
Students often fail to consider the significance of body size. Consider asking your students to think about the impact of being small. Have they ever had difficulty emerging from a swimming pool because of the adhesive properties of water? Of course not—and yet, small insects that land on a pond’s surface may find these forces to be lethal, preventing them from breaking away from the water’s surface! Ask students if they are ever unable to leave their homes because of high winds, which make it impossible for them to walk around outside. The movements of small insects are often hampered by winds that would do little more than toss around our hair! Many campers know that mosquitoes and flies are less of a nuisance on days when there is a good breeze. 4

5. 20.2 Structure fits function at all levels of organization in the animal body
Anatomy is the study of structure.
Physiology is the study of function.
Animals consist of a hierarchy of levels of organization.
Tissues are integrated groups of _____________that perform a common function.
Organs perform a specific task and consist of _______________.
Organ systems consist of __________________that together perform a vital body function.
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Active Lecture Tips
When you discuss form and function relationships, ask students to consider their own teeth as an example. Ask them to use their tongues to feel their teeth and relate the shape of the teeth to the human diet. Incisors and canines slice, whereas molars are more effective at crushing. (20.2)
Houses like humans, reflect levels of structural hierarchy. Ask students to turn to someone near them to identify levels of structural organization in automobiles or the room where you teach. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. (20.2)

6. A Cellular level Muscle cell B Tissue level Muscle tissue C
Organ level
Heart
Figure 20.2 The structural hierarchy of animals D
Organ system level
Circulatory system E
Organism level
Many organ systems
functioning together
© 2018 Pearson Education, Inc.

7. 20.3 Tissues are groups of cells with a common structure and function
Tissues are an integrated group of similar cells that perform a common function; they combine to form organs.
Animals have four main categories of tissues:
epithelial tissue,
connective tissue,
muscle tissue, and
nervous tissue.
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
Extracellular substances, such as collagen fibers, are the source of the main functional properties of many connective tissues such as tendons, ligaments, cartilage, and bone. (20.3)

8. 20.4 Epithelial tissue Epithelial tissues, or epithelia,
are sheets of closely packed cells that
cover body surfaces and
line internal organs and cavities.
Epithelial tissues are named according to the number of cell layers they have and the shape of the cells on their apical surface.
Epithelial cells come in ___ shapes.
Checkpoint Question Response
All contain tightly packed cells situated on top of an extracellular matrix. They form protective barriers or exchange surfaces that line body structures.
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
Simple squamous cells have a shape that is generally similar to a fried egg: flattened, with a bump in the middle representing the nucleus or yolk. (20.4)
Students might misunderstand how the cilia lining our respiratory passages work. Cilia do not “filter” the air like a comb. Instead, cilia are covered by a layer of mucus. Dust particles adhere to sticky mucus, which is then swept up the respiratory tract by the cilia. If students clear their throats, they will identify the fate of this mucus. We “dispose” of this dirty mucus by swallowing after clearing our throats! (20.4)

9. 20.5 Connective tissue binds and supports other tissues
Connective tissue -six major types.
Loose connective tissue
Fibrous connective tissue
Adipose tissue
Cartilage
Bone
Blood
Why does blood qualify
as a type of connective tissue?
Checkpoint Question Response
Because it consists of a population of cells surrounded by a noncellular matrix, which in this case is a fluid called plasma
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
The elastic cartilage in the human ear is a wonderful example of a form and function relationship. Elastic fibers are abundant in the extracellular matrix, increasing the flexibility of elastic cartilage. Have students bend their own ears to feel the effects. (20.5)

10. 20.6 Muscle tissue functions in movement
Muscle tissue is the most abundant tissue in most animals
There are three types of vertebrate muscle tissue:
skeletal muscle causes voluntary movements,
cardiac muscle pumps blood, and
smooth muscle moves walls of internal organs, such as the intestines.
Checkpoint Question Response
Smooth muscle
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Active Lecture Tips
Muscle cells are only able to contract. None can actively relengthen. Ask students to turn to someone near them to try to explain how muscle cells return to their extended length. After perhaps 2 minutes, have pairs of students contribute their explanations for a quick discussion. (Answer: Opposing muscles or other forces, such as gravity, act in opposition to relengthen muscle cells when they relax.) (20.6)

11. 20.7 Nervous tissue forms a communication network
senses stimuli and
rapidly transmits information.
Neurons carry signals by
conducting electrical impulses.
Other cells in nervous tissue
insulate axons,
nourish neurons, and
regulate the fluid around neurons.
Student Misconceptions and Concerns
Students often find it challenging to gain a proper understanding of the evolution of form and function relationships. Such relationships appear to have been “constructed” to meet a purpose, a consequence of deliberate planning and design. Ask students to explain why we have lungs, and they will typically answer something along the line of “because we need to breathe” or “because we need oxygen.” However, “need” does not cause evolution. Natural selection involves editing rather than creative construction. A better answer to the question about why we have lungs might be “Because lung-like structures conveyed an advantage in gas exchange in our ancestors.” (20.1–20.7)
Relationships between form and function are all around us. For some of us, noticing the connections is easy. However, many students have spent little time considering why any particular structure has its characteristic shape. Student practice with common examples helps to build a better understanding of these important relationships. (20.1–20.7)
Teaching Tips
Students might enjoy this simple observation when discussing neurons. As we consider the structure and functions of neurons, we are using our own neurons to think about them. Our collection of neurons becomes self-aware! (20.7)

12. 20.8 Organs are made up of tissues- Each tissue performs specific functions.
Checkpoint Question Response
Connective tissue is a component of most organs.
Student Misconceptions and Concerns
It can be difficult for students to think of their own bodies in such simple terms as surfaces and tubes. Perceiving the digestive tract as one continuous tube, in which food that passes through never technically enters the body, is one such challenge. Illustrate these fundamental principles first using less complex animals such as earthworms. Then apply these principles to humans as a final test of comprehension. (20.8–20.12)
Students exploring form and function relationships should be cautioned to avoid confusing properties of an adaptation with its biological role(s). What a particular form can do may be quite different from how it is used by an organism. For example, the long canine tooth of a saber-toothed cat might make a great letter opener, but these teeth were not used by these cats for that function (biological role)! (20.8–20.12)
Teaching Tips
Like the small intestine with its many villi, the highly porous structure of sponges dramatically increases the surface area available for water filtering. (20.8)
Explain why a disease that damages connective tissue can impair most of the body’s organs.

13. 20.9 CONNECTION: Bioengineers are learning to produce organs for transplants
Bioengineers are seeking ways to repair or replace damaged tissues and organs.
New tissues and organs are being grown on a scaffold of connective tissue from donated organs.
Other researchers are using 3D printers to create layers of cells resembling the structure of organs.
Why is a windpipe or a bladder easier to “print” than with a heart?
Checkpoint Question Response
A windpipe functions simply as a pipe—it has a less complicated structure and function than a heart.
Student Misconceptions and Concerns
It can be difficult for students to think of their own bodies in such simple terms as surfaces and tubes. Perceiving the digestive tract as one continuous tube, in which food that passes through never technically enters the body, is one such challenge. Illustrate these fundamental principles first using less complex animals such as earthworms. Then apply these principles to humans as a final test of comprehension. (20.8–20.12)
Students exploring form and function relationships should be cautioned to avoid confusing properties of an adaptation with its biological role(s). What a particular form can do may be quite different from how it is used by an organism. For example, the long canine tooth of a saber-toothed cat might make a great letter opener, but these teeth were not used by these cats for that function (biological role)! (20.8–20.12)
Teaching Tips
Researchers are also bioengineering artificial skin using cells derived from newborns’ foreskin, removed during circumcision of newborn baby boys. Such tissues are widely available as a by-product of this procedure. (20.9)

14. 20.10 Organ systems work together to perform life’s functions
Each organ system typically
consists of many organs,
has one or more functions,
and
works with other organ
systems to create a
functional organism.
Which two organ systems are
most directly involved in regulating
all other systems?
Checkpoint Question Response
The nervous system and the endocrine system
Student Misconceptions and Concerns
It can be difficult for students to think of their own bodies in such simple terms as surfaces and tubes. Perceiving the digestive tract as one continuous tube, in which food that passes through never technically enters the body, is one such challenge. Illustrate these fundamental principles first using less complex animals such as earthworms. Then apply these principles to humans as a final test of comprehension. (20.8–20.12)
Students exploring form and function relationships should be cautioned to avoid confusing properties of an adaptation with its biological role(s). What a particular form can do may be quite different from how it is used by an organism. For example, the long canine tooth of a saber-toothed cat might make a great letter opener, but these teeth were not used by these cats for that function (biological role)! (20.8–20.12)
Active Lecture Tips
To help your students appreciate the functional integration of the major systems of the body, have students turn to someone near them, pick a body system, and discuss its relationships with perhaps three other body systems. Then, working as a class, have your students help create a concept map noting the nature of the interrelationships between body systems. (20.10)

15. Circulatory
system
Respiratory
system
Skeletal system
Nasal cavity
Pharynx
Bronchus
Larynx
Bone
Trachea
Heart
Cartilage
Integumentary
system
Lung
Blood
vessels
Hair
Skin
Nails
Urinary
system
Digestive
system
Figure 20.10_1 Human organ systems and their components (part 1)
Muscular system
Mouth
Skeletal muscles
Esophagus
Liver
Kidney
Stomach
Ureter
Small
Urinary
bladder
intestine
Large
Urethra
intestine
Anus

16. 20.13 Structural adaptations enhance exchange with the environment- work to maintain homeostasis
Every organism is an open system that must exchange matter and energy with its surroundings.
Cells in small and flat animals can exchange materials directly with the environment.
Complex animals have specialized internal structures that increase surface area.
Exchange of materials between blood and body cells takes place through the interstitial fluid.
How do the structures of the lungs, small intestine, and kidneys relate to the function of exchange with the environment?
Student Misconceptions and Concerns
If students have not previously addressed surface-to-volume ratios, discussed in Module 4.2, they may not understand the consequences of increasing body size. One simple explanation is to compare the relative surface-to-volume ratio of a closed human fist versus an open hand with spread fingers. The volume remains the same, but the surface area exposed is minimized in a fist. Ask your students how they might shape their hands when exposed to a cold winter’s day. (20.13–20.15)
If students have not previously studied the diversity of animals, consider giving a brief overview of the basic animal body plans before explaining how the fundamental principles of form and function generally apply to the animal kingdom. (20.13–20.15)
Teaching Tips
Large organisms must transport and exchange material throughout their entire structure, including their inner core. This principle applies equally to natural organisms, such as a whale or a redwood tree, and to collective “organisms,” such as the United States. The U.S. railway and highway networks are analogous to animal transport and exchange systems. Railroads and highways move essential products from their point of entry (ocean ports) into the country’s interior, where they can be stored or sold. (20.13)
Organisms and individual cells need sufficient surface exchange and transport systems to support their surface-to-volume ratios. Cell size is limited, in part, by the ability of a cell to exchange materials efficiently with its surface. Thus, adaptations that increase surface area can permit cells to reach larger sizes. (20.13)

17. Checkpoint Question Response
By its exchanges with the digestive, respiratory, and urinary systems, the blood helps maintain the proper balance of materials in the interstitial fluid surrounding body cells.
What are some ways in which the circulatory system contributes to homeostasis?

18. 20.14 Animals regulate their internal environment
Conditions often fluctuate widely in the external environment, but homeostatic mechanisms regulate internal conditions, resulting in much smaller changes in the animal’s internal environment.
Student Misconceptions and Concerns
If students have not previously addressed surface-to-volume ratios, discussed in Module 4.2, they may not understand the consequences of increasing body size. One simple explanation is to compare the relative surface-to-volume ratio of a closed human fist versus an open hand with spread fingers. The volume remains the same, but the surface area exposed is minimized in a fist. Ask your students how they might shape their hands when exposed to a cold winter’s day. (20.13–20.15)
If students have not previously studied the diversity of animals, consider giving a brief overview of the basic animal body plans before explaining how the fundamental principles of form and function generally apply to the animal kingdom. (20.13–20.15)
The concept of homeostasis may be new to many students who have never considered how organisms maintain their structure and physiology. Analogies to other systems that engage in self-regulation (noted in the text and the following) can help. (20.14–20.15)
Teaching Tips
The heat generated by aerobic metabolism is analogous to the heat generated by the engine of an automobile. In both cases, the heat is a by-product of the process. In the winter, this excess heat helps keep a human body and an automobile warm. In the summer, both the body and the automobile’s engine must work to keep from overheating. (20.14)
Ask students to explain how blood vessel constriction near the body’s surface, shivering, and a general increase in metabolism help a person to keep warm in a cold environment. (20.14)
Active Lecture Tips
Ask students to turn to someone near them to list at least four factors that affect heat gain and loss during periods of physical activity. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. These examples will demonstrate how much our homeostatic mechanisms must work to maintain a steady body temperature. These factors include (a) the person’s physical condition, (b) the level of physical activity, (c) the age of the person (younger people tend to have higher metabolic rates), (d) the person’s level of hydration (which in turn affects the amount of sweating and evaporative cooling), (e) the external level of humidity (higher levels decrease evaporative cooling), (f) the intensity of the wind (greater intensity promotes evaporative cooling), (g) the intensity of sunlight, and (h) the color of the person’s clothing (which affects the amount of light energy the body absorbs). (20.14–20.15)

19. 20.15 Homeostasis depends on negative feedback
Control systems
detect change and
direct responses.
Negative-feedback mechanisms
keep internal variables steady and
permit only small fluctuations around set points.
Checkpoint Question Response
Without a thermostat, there is no control center to initiate negative feedback. The heater will be on continuously, making the room warmer and warmer until the owner manually turns it off.
Student Misconceptions and Concerns
If students have not previously addressed surface-to-volume ratios, discussed in Module 4.2, they may not understand the consequences of increasing body size. One simple explanation is to compare the relative surface-to-volume ratio of a closed human fist versus an open hand with spread fingers. The volume remains the same, but the surface area exposed is minimized in a fist. Ask your students how they might shape their hands when exposed to a cold winter’s day. (20.13–20.15)
If students have not previously studied the diversity of animals, consider giving a brief overview of the basic animal body plans before explaining how the fundamental principles of form and function generally apply to the animal kingdom. (20.13–20.15)
The concept of homeostasis may be new to many students who have never considered how organisms maintain their structure and physiology. Analogies to other systems that engage in self-regulation (noted in the text and the following) can help. (20.14–20.15)
Teaching Tips
Challenge your students to think of other examples of negative feedback in their environments, including the filling of a toilet tank with water after flushing. Students from diverse disciplines may think of many new examples. (20.15)
Active Lecture Tips
Ask students to turn to someone near them to list at least four factors that affect heat gain and loss during periods of physical activity. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. These examples will demonstrate how much our homeostatic mechanisms must work to maintain a steady body temperature. These factors include (a) the person’s physical condition, (b) the level of physical activity, (c) the age of the person (younger people tend to have higher metabolic rates), (d) the person’s level of hydration (which in turn affects the amount of sweating and evaporative cooling), (e) the external level of humidity (higher levels decrease evaporative cooling), (f) the intensity of the wind (greater intensity promotes evaporative cooling), (g) the intensity of sunlight, and (h) the color of the person’s clothing (which affects the amount of light energy the body absorbs). (20.14–20.15)

20. Animation: Negative Feedback
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21. Animation: Positive Feedback
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22. You should now be able to
Explain why evolution does not lead to perfection.
Describe the levels of organization in an animal’s body.
Describe the four main types of animal tissues. Note their structures and their functions.
Explain how the structure of organs is based on the cooperative interactions of tissues.
Explain how organ systems work together to perform life’s functions.

23. You should now be able to
Describe the general structures and functions of the 12 major vertebrate organ systems.
Describe the systems that help an animal exchange materials with its environment.
Define the concept of homeostasis and illustrate it with examples.
Explain how negative feedback is used to regulate internal body temperature.

24. covers the body and lines its organs and cavities.
Figure 20.UN02
20.4 Epithelial tissue
covers the body and lines
its organs and cavities.
20.5 Connective tissue
binds and supports
other tissues.
20.6 Muscle tissue
functions in
movement.
20.7 Nervous tissue
forms a communication
network.
Function
Structure
Sheets of closely
packed cells
Sparse cells in extra-
cellular matrix
Long cells (fibers)
with contractile
proteins
Neurons with
branching extensions;
supporting cells
Example
Figure 20.UN02 Reviewing the concepts, 20.4–20.7
Columnar epithelium
Loose connective tissue
Skeletal muscle
Neuron
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