Introduction Does evolution lead to the perfect animal form?

Introduction Does evolution lead to the perfect animal form?
Physical structures are adaptations that enhance an animal’s chances of survival and reproduction. The correlation of structure and function is an overarching theme of biology. Animal structures are often just “good enough” to function and not the ultimate in design.

Figure Figure Does evolution lead to the perfect animal form?

Chapter 20: Big Ideas Structure and Function in Animal Tissues
Figure Chapter 20: Big Ideas Structure and Function in Animal Tissues Organs and Organ Systems Figure Chapter 20: Big Ideas External Exchange and Internal Regulation

Structure and Function in Animal Tissues

20.1 EVOLUTION CONNECTION: 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. The throat is about 1 foot away from the brain. Why, then, does the laryngeal nerve make about a 15-foot journey? 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. • 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. 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. 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 two 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, and (c) walking on two legs, which frees up our hands for other tasks, but makes us less stable than walking on four legs. • 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.

Brain Laryngeal nerve Aorta Heart Figure 20.1a
Figure 20.1a The lengthy laryngeal nerve in the giraffe’s neck

Adaptations that led to the varying lengths of the laryngeal nerve in tetrapods can be illustrated with an analogy. If a table is moved away from an outlet, two options are obvious for plugging in the lamp: unplug the cord and reposition the cord so that it runs directly from the lamp to the outlet, causing a temporary loss of light, or keep the cord plugged in while simply extending the cord to reach the outlet. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. • 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. 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. 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 two 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, and (c) walking on two legs, which frees up our hands for other tasks, but makes us less stable than walking on four legs. • 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.

A table close to the wall outlet
Figure 20.1b A table close to the wall outlet Moving the table far from the wall outlet: Option 1 Moving the table far from the wall outlet: Option 2 Figure 20.1b An analogy for evolutionary adaptation of the laryngeal nerve in the giraffe

The early embryos of fish and tetrapods are very similar. In their embryos, the laryngeal nerve connects the brain to a rudimentary structure that in fish will become the gills and in tetrapods will develop into the larynx. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. • 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. 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. 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 two 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, and (c) walking on two legs, which frees up our hands for other tasks, but makes us less stable than walking on four legs. • 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.

In these embryos, the nerve hooks under the aorta. This is not problematic in fish because they do not have necks. But in tetrapods, the aorta ends up in the chest, resulting in an elongated laryngeal nerve in tetrapods. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. • 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. 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. 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 two 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, and (c) walking on two legs, which frees up our hands for other tasks, but makes us less stable than walking on four legs. • 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.

20.2 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 an integrated group of similar cells that perform a common function. Organs perform a specific task and consist of two or more tissues. Organ systems consist of multiple organs 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. Teaching 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, while molars are more effective at crushing. • As the authors note, homes, like humans, reflect levels of structural hierarchy. Challenge your students to identify levels of structural organization in automobiles or the room where you teach!

A Cellular level Muscle cell
Figure 20.2 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

20.3 Tissues are groups of cells with a common structure and function
are an integrated group of similar cells that perform a common function and 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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.

20.4 Epithelial tissue covers the body and lines its organs and cavities
Epithelial tissues, or epithelia, are sheets of closely packed cells that cover body surfaces and line internal organs and cavities. Epithelial cells come in three shapes: squamous, like a fried egg, cuboidal, as tall as they are wide, and columnar, taller than they are wide. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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. • 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 Epithelial tissue covers the body and lines its 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. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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. • 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!

Apical surface of epithelium
Figure 20.4 Apical surface of epithelium Basal lamina Underlying tissue Cell nuclei A Simple squamous epithelium D Stratified squamous epithelium Figure 20.4 Types of epithelial tissue B Simple cuboidal epithelium C Simple columnar epithelium

20.5 Connective tissue binds and supports other tissues
Connective tissue can be grouped into six major types. Loose connective tissue is the most widespread, consists of ropelike collagen and elastic fibers that are strong and resilient, and helps to join skin to underlying tissues. Fibrous connective tissue has densely packed collagen fibers and forms tendons that attach muscle to bone. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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.

Adipose tissue stores fat in large, closely packed cells held in a matrix of fibers. Cartilage is a strong and flexible skeletal material and commonly surrounds the ends of bones. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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.

Bone has a matrix of collagen fibers embedded in a hard mineral substance containing calcium, magnesium, and phosphate. Blood transports substances throughout the body. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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.

A Loose connective tissue (under the skin)
Figure White blood cells Red blood cell Central canal Plasma Matrix F Blood Bone- forming cells Cell nucleus E Bone Collagen fiber Cartilage- forming cells Elastic fibers A Loose connective tissue (under the skin) Matrix Figure Types of connective tissue D Cartilage (at the end of a bone) Cell nucleus Fat droplets Collagen fibers C Adipose tissue B Fibrous connective tissue (forming a tendon)

Loose connective tissue (under the skin)
Figure Cell nucleus Collagen fiber Elastic fibers Loose connective tissue (under the skin) Figure Types of connective tissue: loose (part 1)

Fibrous connective tissue (forming a tendon)
Figure Cell nucleus Collagen fibers Fibrous connective tissue (forming a tendon) Figure Types of connective tissue: fibrous (part 2)

Fat droplets Adipose tissue Figure 20.5-3
Figure Types of connective tissue: adipose (part 3) Adipose tissue

Cartilage- forming cells
Figure Cartilage- forming cells Matrix Cartilage (at the end of a bone) Figure Types of connective tissue: cartilage (part 4)

Central canal Matrix Bone- forming cells Bone Figure 20.5-5
Figure Types of connective tissue: bone (part 5) Bone

White blood cells Red blood cell Plasma Blood Figure 20.5-6
Figure Types of connective tissue: blood (part 6) Blood

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. 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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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 two 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.)

Junction between two cells Unit of muscle contraction
Figure Junction between two cells Unit of muscle contraction Muscle fiber (cell) Muscle fiber Nuclei Nucleus B Cardiac muscle A Skeletal muscle Muscle fiber Nucleus Figure The three types of muscle tissue C Smooth muscle

Unit of muscle Muscle fiber (cell) contraction Nuclei Skeletal muscle
Figure Unit of muscle contraction Muscle fiber (cell) Nuclei Figure The three types of muscle tissue: skeletal (part 1) Skeletal muscle

Junction between two cells
Figure Junction between two cells Muscle fiber Nucleus Figure The three types of muscle tissue: cardiac (part 2) Cardiac muscle

Muscle fiber Nucleus Smooth muscle Figure 20.6-3
Figure The three types of muscle tissue: smooth (part 3) Smooth muscle

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.” • 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. Student practice with common examples helps to build a better understanding of these important relationships. 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 neurons become self-aware!

Figure 20.7 Dendrites Cell body Axon Figure 20.7 A neuron

Organs and Organ Systems

20.8 Organs are made up of tissues
Each tissue performs specific functions. The heart has extensive muscle that generates contractions, epithelial tissues that line the heart chambers, prevent leaks, and form a smooth surface for blood flow, connective tissues that make the heart elastic and strong, and neurons that regulate contractions. 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. • 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)! 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 Organs are made up of tissues
The small intestine is lined by a columnar epithelium, includes connective tissues that contain blood vessels, and has two layers of smooth muscle that help propel food. The inner surface of the small intestine has many finger-like projections that increase the surface area for absorption. 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. • 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)! 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.

Epithelial tissue (columnar epithelium)
Figure 20.8 Small intestine Lumen Epithelial tissue (columnar epithelium) Connective tissue Figure 20.8 Tissue layers of the wall of the small intestine Smooth muscle tissue (two layers) Connective tissue Epithelial tissue

20.9 CONNECTION: Bioengineers are learning to produce organs for transplants
Bioengineering is 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 desktop printers to create layers of different cells resembling the structure of 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. • 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)! 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.

Figure 20.9 Figure 20.9 A decellularized pig heart

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. 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. • 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)! 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.

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

Lymphatic and immune systems
Figure Endocrine system Lymphatic and immune systems Hypothalamus Pituitary gland Thymus Thyroid gland Parathyroid gland Lymph nodes Thymus Adrenal gland Spleen Pancreas Testis (male) Ovary (female) Appendix Bone marrow Lymphatic vessels Reproductive system Nervous system Brain Figure Human organ systems and their component parts (part 2) Sense organ (ear) Seminal vesicles Female Male Spinal cord Oviduct Prostate gland Nerves Ovary Vas deferens Uterus Vagina Penis Urethra Testis

The circulatory system delivers oxygen and nutrients to body cells, transports carbon dioxide to the lungs, and carries metabolic wastes to the kidneys. The respiratory system exchanges gases with the environment, supplying the blood with oxygen and disposing of carbon dioxide. 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. • 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)! 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.

Circulatory system Respiratory system
Figure Circulatory system Respiratory system Nasal cavity Pharynx Larynx Bronchus Trachea Heart Lung Blood vessels Figure Human organ systems and their component parts: circulatory and respiratory (part 3)

The integumentary system protects against physical injury, infection, excessive heat or cold, and drying out. 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. • 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)! 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.

Integumentary system Hair Skin Nails Figure 20.10-4
Figure Human organ systems and their component parts: integumentary (part 4)

The skeletal system supports the body, protects organs such as the brain and lungs, and provides the framework for muscle movement. 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. • 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)! 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.

Skeletal system Bone Cartilage Figure 20.10-5
Figure Human organ systems and their component parts: skeletal (part 5)

The muscular system moves the body, maintains posture, and produces heat. 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. • 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)! 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.

Muscular system Skeletal muscles Figure 20.10-6
Figure Human organ systems and their component parts: muscular (part 6)

The urinary system removes waste products from the blood, excretes urine, and regulates the chemical makeup, pH, and water balance of blood. The digestive system ingests and digests food, absorbs nutrients, and eliminates undigested material. 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. • 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)! 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.

Digestive system Urinary system
Figure Digestive system Urinary system Mouth Esophagus Liver Kidney Stomach Ureter Small intestine Figure Human organ systems and their component parts: urinary and digestive (part 7) Urinary bladder Large intestine Urethra Anus

The endocrine system secretes hormones that regulate body activities. 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. • 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)! 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.

Endocrine system Hypothalamus Pituitary gland Thyroid gland Thymus
Figure Endocrine system Hypothalamus Pituitary gland Thyroid gland Thymus Parathyroid gland Adrenal gland Pancreas Testis (male) Ovary (female) Figure Human organ systems and their component parts: endocrine (part 8)

The lymphatic and immune systems protect the body from infection and cancer. The lymphatic system also returns excess body fluid to the circulatory 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. • 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)! 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.

Lymphatic and immune systems
Figure Lymphatic and immune systems Lymph nodes Thymus Spleen Appendix Bone marrow Figure Human organ systems and their component parts: lymphatic and immune (part 9) Lymphatic vessels

The nervous system coordinates body activities by detecting stimuli, integrating information, and directing responses. 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. • 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)! 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.

Nervous system Brain Sense organ (ear) Spinal cord Nerves
Figure Nervous system Brain Sense organ (ear) Spinal cord Nerves Figure Human organ systems and their component parts: nervous (part 10)

The reproductive system produces gametes and sex hormones. The female reproductive system supports a developing embryo and produces milk. 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. • 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)! 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.

Reproductive system Seminal vesicles Female Male Oviduct
Figure Reproductive system Seminal vesicles Female Male Oviduct Prostate gland Ovary Vas deferens Uterus Figure Human organ systems and their component parts: reproductive (part 11) Vagina Penis Urethra Testis

20.11 The integumentary system protects the body
The skin consists of two layers. The epidermis is a stratified squamous epithelium and forms the surface of the skin. The dermis forms a deeper skin layer, is composed of dense connective tissue with many resilient elastic fibers and strong collagen fibers, and contains hair follicles, oil and sweat glands, muscle cells, nerves, sensory receptors, and blood vessels. 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. • 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)! Teaching Tips • The stratified squamous epithelium on most outside surfaces of our body resists abrasions in part because it is keratinized. However, the nonkeratinized epithelial tissue that lines our body cavities, such as the mouth, pharynx, esophagus, and anus, is also resistant to abrasion as a result of its mucus coatings, which provide friction-reducing lubrication. Students may not realize that the same type of tissue performs similar functions in very different parts of the body. • Students might have noticed that their skin wrinkles when soaked in water. Some students may have noticed that their hands wrinkle even faster in soapy water. Skin absorbs water through osmosis (just as a freshwater fish gains water). The wrinkling occurs because the skin can expand only in certain areas, creating puckers. Oils on our skin reduce the influx of water. Therefore, soapy water, which washes away these oils, speeds up the movement of water into our keratinized skin.

Hypodermis (under the skin)
Figure 20.11 Hair Epidermis Sweat pore Muscle Dermis Nerve Sweat gland Figure A section of skin, the major organ of the integumentary system Hypodermis (under the skin) Adipose tissue Blood vessels Oil gland Hair follicle

Skin has many functions. The epidermis resists physical damage, decreases water loss, and prevents penetration by microbes. The dermis collects sensory information, synthesizes vitamin D, and helps regulate body temperature. 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. • 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)! Teaching Tips • The stratified squamous epithelium on most outside surfaces of our body resists abrasions in part because it is keratinized. However, the nonkeratinized epithelial tissue that lines our body cavities, such as the mouth, pharynx, esophagus, and anus, is also resistant to abrasion as a result of its mucus coatings, which provide friction-reducing lubrication. Students may not realize that the same type of tissue performs similar functions in very different parts of the body. • Students might have noticed that their skin wrinkles when soaked in water. Some students may have noticed that their hands wrinkle even faster in soapy water. Skin absorbs water through osmosis (just as a freshwater fish gains water). The wrinkling occurs because the skin can expand only in certain areas, creating puckers. Oils on our skin reduce the influx of water. Therefore, soapy water, which washes away these oils, speeds up the movement of water into our keratinized skin.

Exposure of the skin to ultraviolet light causes skin cells to release melanin, which contributes to a visible tan, and damages DNA of skin cells, which can lead to premature aging of the skin, cataracts, and skin cancers. 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. • 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)! Teaching Tips • The stratified squamous epithelium on most outside surfaces of our body resists abrasions in part because it is keratinized. However, the nonkeratinized epithelial tissue that lines our body cavities, such as the mouth, pharynx, esophagus, and anus, is also resistant to abrasion as a result of its mucus coatings, which provide friction-reducing lubrication. Students may not realize that the same type of tissue performs similar functions in very different parts of the body. • Students might have noticed that their skin wrinkles when soaked in water. Some students may have noticed that their hands wrinkle even faster in soapy water. Skin absorbs water through osmosis (just as a freshwater fish gains water). The wrinkling occurs because the skin can expand only in certain areas, creating puckers. Oils on our skin reduce the influx of water. Therefore, soapy water, which washes away these oils, speeds up the movement of water into our keratinized skin.

Hair is an important component of the integumentary system of mammals, helps to insulate their bodies, and consists of a shaft of keratin-filled dead cells. Oil glands release oils that are associated with hair follicles, lubricate hair, condition surrounding skin, and inhibit the growth of bacteria. 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. • 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)! Teaching Tips • The stratified squamous epithelium on most outside surfaces of our body resists abrasions in part because it is keratinized. However, the nonkeratinized epithelial tissue that lines our body cavities, such as the mouth, pharynx, esophagus, and anus, is also resistant to abrasion as a result of its mucus coatings, which provide friction-reducing lubrication. Students may not realize that the same type of tissue performs similar functions in very different parts of the body. • Students might have noticed that their skin wrinkles when soaked in water. Some students may have noticed that their hands wrinkle even faster in soapy water. Skin absorbs water through osmosis (just as a freshwater fish gains water). The wrinkling occurs because the skin can expand only in certain areas, creating puckers. Oils on our skin reduce the influx of water. Therefore, soapy water, which washes away these oils, speeds up the movement of water into our keratinized skin.

20.12 SCIENTIFIC THINKING: Well-designed studies help answer scientific questions
As consumers, we are bombarded with claims daily. To make informed decisions and behave as responsible consumers, we should evaluate information as scientists do. For example, consider claims for acne treatment. 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. • 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)! Teaching Tips • Prior to the Internet, learning often involved going to a dictionary, encyclopedia, or library where trusted resources were stored. Now, with the enormity of the Internet, we are instead challenged by the need to “filter” abundant sources of information to identify those more reliable. Our task has shifted from “finding” to “filtering.” Module helps to address this important and difficult challenge of identifying reliable sources of information. Active Lecture Tips • See the Activity Learning to Think Critically Like a Scientist on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Acne results when the hair follicles that produce oil become clogged with dead cells and oil. When the pore is plugged, bacteria of the species Propionibacterium acnes (P. acnes) become trapped in the follicle. If the follicle ruptures into the dermis and white blood cells are recruited from the immune system, the pore is said to be inflamed in what we commonly call a “pimple” or “zit.” 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. • 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)! Teaching Tips • Prior to the Internet, learning often involved going to a dictionary, encyclopedia, or library where trusted resources were stored. Now, with the enormity of the Internet, we are instead challenged by the need to “filter” abundant sources of information to identify those more reliable. Our task has shifted from “finding” to “filtering.” Module helps to address this important and difficult challenge of identifying reliable sources of information. Active Lecture Tips • See the Activity Learning to Think Critically Like a Scientist on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Ruptured, inflammed pore
Figure 20.12a Hair Skin surface Pus White blood cells Ruptured, inflammed pore Bacteria (P. acnes) Figure 20.12a The anatomy of a pimple Oil gland Hair follicle (pore)

Consider a study involving 19 individuals, all chosen because they had at least five pimples. After three treatments, all participants had a significant reduction in the total number of pimples. 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. • 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)! Teaching Tips • Prior to the Internet, learning often involved going to a dictionary, encyclopedia, or library where trusted resources were stored. Now, with the enormity of the Internet, we are instead challenged by the need to “filter” abundant sources of information to identify those more reliable. Our task has shifted from “finding” to “filtering.” Module helps to address this important and difficult challenge of identifying reliable sources of information. Active Lecture Tips • See the Activity Learning to Think Critically Like a Scientist on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Average number of pimples
Figure 20.12b 40 30 Average number of pimples 20 10 Figure 20.12b Reduction in pimples before and after laser therapy treatment in an uncontrolled study Before laser therapy After 1st treatment After 2nd treatment After 3rd treatment Reprinted from P. M. Friedman et al., Treatment of inflammatory facial acne vulgaris with the 1450-nm diode laser: A pilot study, Dermatologic Surgery 30: 147–51 (2004), with permission.

But the study did not have a control group that could reveal if individuals who had undergone laser therapy would have had the same reduction in pimples if they had not had the procedure. The study also failed to control variables. The participants were allowed to continue using acne medications over the course of the study. 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. • 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)! Teaching Tips • Prior to the Internet, learning often involved going to a dictionary, encyclopedia, or library where trusted resources were stored. Now, with the enormity of the Internet, we are instead challenged by the need to “filter” abundant sources of information to identify those more reliable. Our task has shifted from “finding” to “filtering.” Module helps to address this important and difficult challenge of identifying reliable sources of information. Active Lecture Tips • See the Activity Learning to Think Critically Like a Scientist on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Average number of red pimples
Figure 20.12c 14 12 10 Average number of red pimples 8 Key Treated side of face 6 4 Untreated side of face 2 Before laser therapy 4 weeks after 3rd treatment 10 weeks after 3rd treatment Figure 20.12c Reduction in a subtype of pimple before and after laser therapy in a controlled, randomized, single-blind study Data from J. S. Orringer et al., Photodynamic therapy for acne vulgaris: A randomized, controlled, split-faced clinical trial of topical animolevulinic acid and pulsed dye therapy, Journal of Cosmetic Dermatology 9: 28–34 (2010).

External Exchange and Internal Regulation

20.13 Structural adaptations enhance exchange with the environment
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. 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 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. • 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. 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. • 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.

As organisms increase in size, the surface area is too small for the corresponding volume and is too far away from the deepest cells of the body. In these larger organisms, evolutionary adaptations consist of extensively branched or folded surfaces, which increase the surface area, and provide for sufficient environmental exchange. 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 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. • 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. 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. • 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.

The respiratory system exchanges gases between the external environment and blood. The digestive system acquires food and eliminates wastes. The urinary system eliminates metabolic waste. The circulatory system distributes gases, nutrients, and wastes throughout the body and exchanges materials between blood and body cells through the interstitial fluid that bathes body cells. 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 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. • 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. 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. • 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.

External environment CO2 Food O2 Mouth Animal Respiratory system
Figure 20.13a External environment CO2 Food O2 Mouth Animal Respiratory system Digestive system Interstitial fluid Heart Nutrients Circulatory system Body cells Figure 20.13a A schematic representation showing indirect exchange between the environment and the cells of a complex animal Urinary system Intestine Anus Unabsorbed matter (feces) Metabolic waste products (urine)

Figure 20.13b Trachea Figure 20.13b A resin model of the finely branched air tubes (white) and blood vessels (red) of the human lungs

20.14 Animals regulate their internal environment
Homeostasis is the active maintenance of a steady state within the body. External environmental conditions may fluctuate wildly. Homeostatic mechanisms regulate internal conditions. 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 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. • 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. • 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. 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. • 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. 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 two 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).

Homeostatic mechanisms
Figure 20.14 External environment 10C Internal environment −30C 40C 38C Large fluctuations Homeostatic mechanisms Small fluctuations Figure A model of homeostasis in a white-tailed ptarmigan in its snowy habitat

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. 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 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. • 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. • 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. 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.

Animation: Negative Feedback

Animation: Positive Feedback

Glands secrete sweat that evaporates, cooling the body
Figure Glands secrete sweat that evaporates, cooling the body The hypothalamus activates cooling mechanisms Blood vessels in the skin dilate, increasing heat loss Temperature decreases The hypothalamus shuts off the cooling mechanisms Temperature rises above set point Homeostasis: Body temperature approximately 37C Temperature increases The hypothalamus shuts off the warming mechanisms Temperature falls below set point Figure Feedback control of body temperature Blood vessels in the skin constrict, minimizing heat loss The hypothalamus activates warming mechanisms Skeletal muscles contract; shivering generates heat

Glands secrete sweat that evaporates, cooling the body
Figure Glands secrete sweat that evaporates, cooling the body The hypothalamus activates cooling mechanisms Blood vessels in the skin dilate, increasing heat loss Temperature decreases The hypothalamus shuts off the cooling mechanisms Temperature rises above set point Figure Feedback control of body temperature (part 1) Homeostasis: Body temperature approximately 37C

Homeostasis: Body temperature approximately 37C
Figure Homeostasis: Body temperature approximately 37C Temperature increases The hypothalamus shuts off the warming mechanisms Temperature falls below set point Blood vessels in the skin constrict, minimizing heat loss Figure Feedback control of body temperature (part 2) The hypothalamus activates warming mechanisms Skeletal muscles contract; shivering generates heat

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 artificial tissues and organs are being created in laboratories. Explain how organ systems work together to perform life’s functions. Describe the general structures and functions of the 12 major vertebrate organ systems. Relate the structure of the skin to its functions.

Describe the components of well-designed scientific studies. 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.

Figure 20.UN01 Two cell layers Figure 20.UN01 Hydra cell layers

Function Structure Example
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 Sheets of closely packed cells Sparse cells in extracellular matrix Long cells (fibers) with contractile proteins Neurons with branching extensions; supporting cells Structure Example Figure 20.UN02 Reviewing the concepts, 20.4–20.7 Columnar epithelium Loose connective tissue Skeletal muscle Neuron

Figure 20.UN03 a. b. Figure 20.UN03 Connecting the concepts, question 1 c. d. e.

Introduction Does evolution lead to the perfect animal form?

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