Part VI Animal Structure and Function
Being Organized 
and Steady
Chapter
22
Outline
	22.1	The Body’s Organization
	•	An animal’s body follows these levels of biological organization: cells  tissues  
organ  organ systems  organism.378
	•	Each of four types of animal tissue (epithelial, connective, muscular, nervous) has a specific structure and carries out particular functions.379
	•	Epithelial tissue, which lines body cavities and covers surfaces, plays a protective role as well as other roles in the body.380
	•	Connective tissue, which connects and supports other tissues, includes cartilage, bone, and blood, the only liquid tissue.382
	•	Muscular tissue, which makes the body and its parts move, can be involuntary (smooth and cardiac) or voluntary (skeletal).384
	•	Nervous tissue coordinates the activities of the other tissues and body parts.385
	22.2	Organs and Organ Systems
	 •	Organs, which are composed of several types of tissues, are also specialized in structure and function. The body’s organs are a part of a system, and each system has particular functions.386
	22.3	Homeostasis
	•	Cells function best in an environment that stays relatively constant so that their needs are served.388
	•	The body’s systems contribute in various ways to maintaining homeostasis.388
	•	Negative feedback mechanisms are the primary way that complex animals maintain their internal environment.389
To keep you healthy, your body is always maintaining homeostasis, the relative constancy of the internal environment. Your cells are bathed by tissue fluid, which continually exchanges substances with the blood. Therefore, the internal environment consists of blood and tissue fluid.
How might homeostasis be maintained? Let’s take an example. Despite extreme fluctuations in our diet, the blood glucose (sugar) concentration usually stays within a normal range, day in and day out. This is primarily due to the work of two hormones, called insulin and glucagon. Just after you have eaten, the blood glucose level usually rises. In response, your pancreas releases insulin, which causes the body’s cells, including those of the liver, to take up glucose from the blood. Now the blood glucose level lowers. The liver ordinarily stores glucose, so if your blood runs out of glucose before you eat again, the other hormone, glucagon, causes the liver to release sugar into the blood. 
If by chance the liver runs out of stored glucose, it will convert other molecules such as amino acids from protein to glucose in order to maintain the blood glucose level constant. This might happen if you have diabetes mellitus, and your pancreas doesn’t produce enough insulin or if you are on a low-carbohydrate diet. The inability of the pancreas to produce insulin or the inability of the liver to maintain the glucose constant are serious conditions that require good medical care. If homeostasis cannot be maintained, death can occur due to either an extremely high or extremely low blood glucose level. Of all the organs in the body, the brain requires a constant supply of glucose. 
Understanding the structure and function of the various types of tissues in the body is of critical importance to understanding the functions of various organ systems. In this chapter, you will learn about basic tissue types, and how they come together to form organs and organ systems. In addition, you will learn how the body maintains homeostasis.
22.1		The Body’s Organization
In Part One of this text, you studied the general structure and function of a plant cell and an animal cell. Now it is time to consider that an animal’s body is composed of specialized cells that perform particular functions. Your body contains trillions of cells, of which there are a hundred different types. Cells of the same type occur within a tissue. A tissue is a group of similar cells performing a similar function. An organ contains different types of tissues arranged in a certain fashion. In other words, the structure and function of an organ are dependent on the tissues it contains. That’s why it is sometimes said that tissues, not organs, are the structural and functional units of the body. Several organs are found within an organ system, and the organs of a system work together to perform necessary functions for the organism. 
Let’s take an example (Fig. 22.1). In humans, the urinary system contains these organs: kidneys, ureters, bladder, and urethra. The kidneys remove waste molecules from the blood and produce urine. The tubular shape of the ureters is suitable for passing urine to the bladder, which can store urine because it is expandable. Urine passes out of the body by way of the tubular urethra. The functions of the urinary system—to produce, store, and rid the body of metabolic wastes—are dependent on its organs, which in turn are dependent on the tissues making up these organs. 
Other examples also show that the work of an organ is dependent on its tissues. In the digestive system, the intestine absorbs nutrients. The cells of the tissue lining the lumen (cavity) of the small intestine have microvilli that increase the available surface area for absorption. Muscle cells, within muscle tissue, shorten when they contract because they have intracellular components that move past one another. Nerve cells, within nervous tissue, have long, slender projections that carry impulses to distant body parts. The biological axiom that “structure suits function” and vice versa begins with the specialized cells within a tissue. And thereafter, this truism applies also to organs and organ systems. 
From the many different types of animal cells, biologists have been able to categorize tissues into just four major types: 
Epithelial tissue (epithelium) covers body surfaces and lines body cavities.
Connective tissue binds and supports body parts.
Muscular tissue moves the body and its parts.
Nervous tissue receives stimuli and conducts nerve impulses.
Except for nervous tissue, each type of tissue is subdivided into even more types (Fig. 22.2). We look at each of these in the sections that follow to see how their particular structure allows them to perform their functions. 
Epithelial Tissue Protects
Epithelial tissue, also called epithelium, forms the external and internal linings of many organs and covers the surface of the body. Therefore, for a substance to enter or exit the body at the digestive tract, the lungs, or the genital tract, it must cross an epithelial tissue. Epithelial cells adhere to one another, but are generally only one cell thick. This characteristic enables them to fulfill a protective function and yet allows substances to pass through to a tissue beneath them. 
The cells of epithelial tissue differ in shape (Fig. 22.3). Squamous epithelium, such as that lining the air spaces of the lungs and blood vessels, is composed of flattened cells. Cuboidal epithelium, which lines the lumen (cavity) of the kidney tubules, contains cube-shaped cells. Columnar epithelium has cells resembling rectangular pillars or columns, with nuclei usually located near the bottom of each cell. Columnar epithelium is found lining the lumen of the digestive tract. 
One or more epithelial cells are the primary components of glands, which produce products and secrete them by exocytosis. During secretion, the contents of a vesicle are released when the vesicle fuses with the plasma membrane. Some glands have a duct, and others are ductless. Each mucus-secreting goblet cell within the lining of the digestive tract is a single-celled gland that produces a protective mucus (see Fig. 22.3). Pseudostratified epithelium lines the trachea (windpipe) (Fig. 22.3d). Along the trachea, mucus traps foreign particles, and the upward motion of cilia carries the mucus to the back of the throat, where it may be either swallowed or expelled. Smoking can cause a change in mucus secretion and inhibit ciliary action, and the result is an inflammatory condition called chronic bronchitis. 
The outer region of the skin, called the epidermis, is a stratified (layered) epithelium in which the cells have been reinforced by keratin, a protein that provides strength (Fig. 22.4). A stratified epithelium allows skin to protect the body from injury, drying out, and possible pathogen (virus and bacterium) invasion. 
The lining of the urinary bladder is a transitional epithelium, whose structure suits its function. When the walls of the bladder are relaxed, the transitional epithelium consists of several layers of cuboidal cells. When the bladder is distended with urine, the epithelium stretches, and the outer cells take on a squamous appearance. The cells are able to slide in relation to each other, while at the same time forming a barrier that prevents any part of urine from diffusing into other regions of the body. 
Epithelial tissue is well known for constantly replacing its cells. This feature is particularly useful along the digestive tract, where rough food particles and enzymes can damage the lining. The liver, which is composed of cells of epithelial origin, can regenerate whole portions removed during surgery. But there is a price to pay for the ability of epithelial tissue to constantly divide—it is more likely to become cancerous than are other tissues. Cancers of epithelial tissue within the digestive tract, lungs, and breast are called carcinomas. 
Connective Tissue Connects and Supports
The many different types of connective tissue are all involved in binding organs together and providing support and protection. As a rule, connective tissue cells are widely -separated by a matrix, a noncellular -material that varies from solid to semifluid to fluid. The matrix usually has fibers, notably collagen fibers. Collagen is the most common protein in the human body, which gives you some idea of how prevalent connective tissue is.
Loose Fibrous and Related Connective Tissues
Let’s consider loose fibrous connective tissue first and then compare the other types to it (see Fig. 22.2). This tissue occurs beneath an epithelium and connects it to other tissues within an organ. It also forms a protective covering for many -internal organs, such as muscles, blood vessels, and nerves. Its cells are called fibroblasts because they produce a matrix that contains fibers, including collagen fibers and elastic fibers. The presence of loose fibrous connective tissue in the walls of lungs and arteries allows these organs to expand.
Adipose tissue is a type of loose connective tissue in which the fibroblasts enlarge and store fat, and there is limited matrix (Fig. 22.5). Adipose tissue is located beneath the skin and around organs such as the heart and kidneys.
Compared to loose fibrous connective tissue, dense fibrous connective tissue contains more collagen fibers, and they are packed closely together (Fig. 22.5). This type of tissue has more specific functions than does loose fibrous connective tissue. For example, dense fibrous connective tissue is found in -tendons, which connect muscles to bones, and in ligaments, which connect bones to other bones at joints.
In cartilage, the cells lie in small chambers called lacunae, separated by a matrix that is solid yet flexible. Hyaline cartilage, the most common type of cartilage, contains only very fine collagen fibers (Fig. 22.5). The matrix has a white, translucent appearance. Hyaline cartilage is found in the nose and at the ends of the long bones and the ribs, and it forms rings in the walls of respiratory passages. The human fetal skeleton is also made of this type of -cartilage, which is later replaced by bone. Cartilaginous fishes, such as sharks, have a cartilaginous skeleton throughout their lives. 
Bone is the most rigid connective tissue (Fig. 22.5). It consists of an -extremely hard matrix of inorganic salts, notably calcium salts, deposited around collagen fibers. The inorganic salts give bone rigidity, and the protein fibers provide elasticity and strength, much as steel rods do in reinforced concrete. Compact bone, the most common type, consists of cylindrical structural units called -osteons. The central canal of each osteon is surrounded by rings of hard matrix. Bone cells are located in spaces called lacunae between the rings of matrix. Blood vessels in the central canal carry nutrients that allow bone to renew itself. 
Blood
Blood is composed of several types of cells suspended in a liquid matrix called plasma. Blood is unlike other types of connective tissue in that the matrix (i.e., plasma) is not made by the cells (Fig. 22.6). Some people do not classify blood as connective tissue; instead, they suggest a separate tissue category called vascular tissue.
Blood serves the body well. It transports nutrients and oxygen to cells and removes their wastes. It helps distribute heat and also plays a role in fluid, ion, and pH balance. Also, various components of blood help protect us from disease, and blood’s ability to clot prevents fluid loss.
Blood contains two types of cells. Red blood cells are small, biconcave, disk-shaped cells without nuclei. The presence of the red pigment hemoglobin makes the cells red, and in turn, makes the blood red. Hemoglobin combines with oxygen, and in this way red blood cells transport -oxygen.
White blood cells may be distinguished from red blood cells by the fact that they are usually larger, have a nucleus, and without staining would appear translucent. White blood cells fight infection in two primary ways. Some white blood cells are phagocytic and engulf infectious pathogens, while other white blood cells produce antibodies, molecules that combine with foreign substances to inactivate them.
Platelets are another component of blood, but they are not complete cells; rather, they are fragments of giant cells present only in bone marrow. When a blood vessel is damaged, platelets form a plug that seals the vessel, and injured tissues release molecules that help the clotting process.
Muscular Tissue Moves the Body
Muscular tissue and nervous tissue account for the ability of animals and their parts to move. Muscle tissue is also sometimes called contractile tissue because it contains contractile protein filaments, called actin and myosin filaments, that interact to produce movement. The three types of vertebrate muscles are skeletal, cardiac, and smooth.
Skeletal muscle, also called voluntary muscle, is attached by tendons to the bones of the -skeleton, and when it contracts, bones move. -Contraction of skeletal muscle is under voluntary con-trol and occurs faster than in the other muscle types. The cells of skeletal muscle, called fibers, are cylindrical and quite long—sometimes they run the length of the muscle (Fig. 22.7a). They arise during -development when several cells fuse, resulting in one fiber with multiple nuclei. The nuclei are located at the -periphery of the cell, just inside the plasma mem-brane. The fibers have alternating light and dark bands that give them a striated appearance. These bands are due to the placement of actin filaments and myosin filaments in the cell.
Cardiac muscle is found only in the walls of the heart, and its contraction pumps blood and accounts for the heartbeat. Like skeletal muscle, cardiac muscle has striations, but the contraction of the heart is autorhythmic and invol-untary (Fig. 22.7b). Cardiac muscle cells also differ from skeletal muscle cells in that they have a single, centrally placed nucleus. The cells are branched and seemingly fused one with the other, and the heart appears to be composed of one large interconnecting mass of muscle cells. Actually, cardiac muscle cells are separate and individual, but they are bound end to end at intercalated disks, areas where folded plasma membranes allow the contraction impulse to spread from one cell to the other.
Smooth muscle is so named because the cells lack striations. The spindle-shaped cells form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue (Fig. 22.7c). Smooth muscle is not under voluntary control, and therefore is said to be -involuntary. Smooth muscle is also sometimes called visceral muscle because it is found in the walls of the viscera (intestine, stomach, and other internal organs) and blood vessels. Smooth muscle contracts more slowly than skeletal muscle but can remain contracted for a longer time. When the smooth muscle of the intestine contracts, food moves along its lumen (central cavity). When the smooth muscle of the blood vessels contracts, blood vessels constrict, helping to raise blood pressure.
Nervous Tissue Communicates
Nervous tissue coordinates body parts and allows an animal to respond to the environment. The nervous system depends on (1) sensory -input, (2) integration of data, and (3) motor output to carry out its functions. Nerves conduct impulses from sensory receptors to the spinal cord and the brain, where integration occurs. The phenomenon called sensation occurs only in the brain, however. Nerves then conduct nerve impulses away from the spinal cord and brain to the muscles and glands, causing them to contract and secrete, respectively. In this way, a coordinated response to both internal and external stimuli is achieved.
A nerve cell is called a neuron. Every neuron has three parts: dendrites, a cell body, and an axon (Fig. 22.8). A dendrite is an extension that conducts signals toward the cell body. The cell body contains the -major concentration of the cytoplasm and the nucleus of the neuron. An axon is an extension that conducts nerve impulses. The brain and spinal cord contain many neurons, whereas nerves contain only axons of neurons. The dendrites and cell bodies of these neurons are located in the spinal cord or brain, depending on whether it is a spinal nerve or a cranial nerve. A nerve is like a land-based telephone trunk cable, because just like telephone wires, each axon is a communication channel independent of the others. 
In addition to neurons, nervous tissue contains neuroglia, cells that support and nourish neurons. They outnumber neurons nine to one and take up more than half the volume of the brain. -Although their primary function is support, research is currently being conducted to determine how much neuroglia directly contribute to brain function. Schwann cells are the type of neuroglia that encircle long nerve fibers within nerves, forming a protective myelin sheath. 
22.2		Organs and Organ Systems
Organs are composed of a number of tissues, and the structure and function of an organ are dependent on the tissues that make it up. Since an organ has many types of tissues, it can perform a function that none of the tissues can do alone. To take an example, the function of the bladder is to store urine and to expel it when convenient. But this function is dependent on the individual tissues making up the bladder. The epithelium lining the bladder helps the organ store urine by stretching while still preventing urine from leaking into the body proper. The muscles of the bladder propel the urine forward so that it is voided from the body. Similarly, the functions of an organ system are dependent on its organs. The function of the urinary system is to produce urine, store it, and then transport it out of the body. The kidneys produce urine, the bladder stores it, and various tubes transport it from the kidneys to the bladder and also out of the body. 
This text divides the systems of the body into those involved in (1) transport of fluids about the body, (2) maintenance of the body, (3) control of the body’s systems, (4) sensory input and motor output, and (5) reproduction. All these systems have functions that contribute to homeostasis, the relative constancy of the internal environment (see Section 22.3).
Transport 
The cardiovascular system (Fig. 22.9a) consists of the heart and the blood vessels that carry blood throughout the body. The body’s cells are surrounded by a liquid called tissue fluid; blood transports nutrients and oxygen to tissue fluid for the cells, and removes waste molecules excreted by cells from the tissue fluid. Recall that the internal environment of the body consists of the blood within the blood vessels and the tissue fluid that surrounds the cells. 
The lymphatic system (Fig. 22.9b) consists of lymphatic vessels, lymph, lymph nodes, and other lymphatic organs. Lymphatic vessels absorb fat from the digestive system and collect excess tissue fluid, which is returned to the blood in the cardiovascular system. 
These two systems are also involved in defense against disease. Certain blood cells in the lymph and blood are part of an immune system, which specifically protects the body from disease.
Maintenance of the Body 
Three systems (digestive, respiratory, and urinary) add substances to and/or remove substances from the blood. If the composition of the blood remains constant, so does that of the tissue fluid.
The digestive system (Fig. 22.10a) consists of the various organs along the digestive tract together with the associated organs, such as teeth, salivary glands, the liver, and the pancreas. This system receives food and digests it into nutrient molecules that enter the blood.
The respiratory system (Fig. 22.10b) consists of the lungs and the tubes that take air to and from the lungs. The respiratory system brings oxygen into the body and takes carbon dioxide out of the body through the lungs. It also exchanges gases with the blood.
The urinary system (Fig. 22.10c) contains the kidneys and the urinary bladder along with tubes that transport urine. This system rids blood of wastes and also helps regulate the fluid level and chemical content of the blood.
Control 
The nervous system (Fig. 22.11a) consists of the brain, spinal cord, and associated nerves. The nerves conduct nerve impulses from receptors to the brain and spinal cord. They also conduct nerve impulses from the brain and spinal cord to the muscles and glands, allowing us to respond to both external and internal stimuli. (Sensory receptors and sense organs are sometimes considered a part of the nervous system.) 
The endocrine system (Fig. 22.11b) consists of the hormonal glands, which secrete chemicals that serve as messengers between body parts. Both the nervous and endocrine systems coordinate and regulate the functions of the body’s other systems. The endocrine system also helps maintain the proper functioning of the male and female reproductive organs.
Sensory Input and Motor Output 
The integumentary system (Fig. 22.12a) consists of the skin and its accessory structures. The sensory receptors in the skin, and in organs such as the eyes and ears, are sensitive to certain external stimuli and communicate with the brain and spinal cord by way of nerve fibers. These messages may cause the brain to bring about a response to a stimulus.
The skeletal system (Fig. 22.12b) and the muscular system (Fig. 22.12c) enable the body and its parts to move as a result of motor output. The skeleton, as a whole, serves as a place of attachment for the skeletal muscles. Contraction of muscles in the muscular system actually accounts for movement of body parts.
These systems, along with the integumentary system, also protect and support the body. The skeletal system, consisting of the bones of the skeleton, protects body parts. For example, the skull forms a protective encasement for the brain, as does the rib cage for the heart and lungs. The skin and muscles assist in this endeavor because they are exterior to the bones. 
Reproduction
The reproductive system (Fig. 22.13) involves different organs in the male and female. The male reproductive system consists of the testes, other glands, and various ducts that conduct semen to and through the penis. The testes produce sex cells called sperm. The female reproductive system consists of the ovaries, oviducts, uterus, vagina, and external genitals. The ovaries produce sex cells called eggs. When a sperm fertilizes an egg, an offspring begins development.
22.3		Homeostasis
It is likely you are most familiar with the use of the word environment to mean the external environment. There is much concern these days about our external environment because of the need to keep pollution to a minimum in order to maintain the health of ecosystems and the organisms, including ourselves, that live in them. Your body, however, is composed of many cells, and therefore its internal environment is the environment of the cells. Cells live in a liquid environment called tissue fluid. Tissue fluid is constantly renewed by exchanges with the blood (Fig. 22.14). Therefore, blood and tissue fluid are the internal environment of the body. -Tissue fluid remains relatively constant only as long as blood com-position remains near normal levels. Relatively constant means that the composition of both tissue fluid and blood usually falls within a certain range of normality. 
All the systems of the body contribute to maintaining homeostasis, the relative constancy of the internal environment. The cardiovascular system conducts blood to and away from capillaries, where exchange occurs. The heart pumps the blood, and thereby keeps it moving toward the capillaries. Red blood cells transport oxygen and participate in the transport of carbon dioxide. White blood cells fight infection, and platelets participate in the clotting process. The lymphatic system is accessory to the cardiovascular system. Lymphatic capillaries collect excess tissue fluid and return it via lymphatic vessels to the car-diovascular system. Lymph nodes help purify lymph and keep it free of pathogens.
The digestive system takes in and digests food, providing nutrient molecules that enter the blood to replace those that are constantly being used by the body cells. The respiratory system removes carbon dioxide from and adds oxygen to the blood. The chief regulators of blood com-position are the kidneys and the liver. Urine formation by the kidneys is extremely critical to the body, not only because it rids the body of metabolic wastes, but also because the kidneys carefully regulate blood volume, salt balance, and pH. The liver, among other functions, regulates the glucose concentration of the blood. -Immediately after glucose enters the blood, the liver -removes the excess for storage as glycogen. Later, the glycogen is broken down to replace the glucose that was used by body cells. In this way, the glucose composition of the blood remains constant. The liver also removes toxic chemicals, such as ingested alcohol and other drugs. The liver makes urea, a nitrogenous end product of protein metabolism. 
The nervous system and the endocrine system regulate the other systems of the body. They work together to control body systems so that homeostasis is maintained. In negative feedback mechanisms involving the nervous system, sensory receptors send nerve impulses to control centers in the brain, which then direct effectors to become active. -Effectors can be muscles or glands. Muscles bring about an immediate change. Endocrine glands secrete hormones that bring about a slower, more lasting change that keeps the -internal environment relatively stable.
Because of homeostasis, even though external conditions may change dramatically, internal conditions stay within a narrow range. For example, the temperature of the body is maintained near 37°C (97° to 99°F), even if the surrounding temperature is lower or higher. If you eat acidic foods, the pH of your blood still stays about 7.4, and even if you eat a candy bar, the amount of sugar in your blood remains at just about 0.1%.
Negative Feedback
Negative feedback is the primary homeostatic mechanism that allows the body to keep the internal environment constant. A negative feedback mechanism has at least two components: a sensor and a control center (Fig. 22.15). The sensor detects a change in the internal environment (a stimulus); the control center initiates an effect that brings conditions back to normal again. Now, the sensor is no longer activated. In other words, a negative feedback mechanism is present when the output of the system dampens the original stimulus.
Let’s take a simple example. When the pancreas detects that the blood glucose level is too high, it secretes insulin, a hormone that causes cells to take up glucose. Now the blood sugar level returns to normal, and the pancreas is no longer stimulated to secrete insulin. 
When conditions exceed their limits and feedback mechanisms cannot compensate, illness results. For example, if the pancreas is unable to produce insulin, the blood sugar level becomes dangerously high, and the individual can become seriously ill. The study of homeo-static mechanisms is therefore medically important.
Mechanical Example
A home heating system is often used to illustrate how a more complicated negative feedback mechanism works (Fig. 22.16). You set the thermostat at, say, 20°C (68°F). This is the set point. The thermostat contains a thermometer, a sensor that detects when the room temperature is above or below the set point. The thermostat also contains a control center; it turns the -furnace off when the room is warm and turns it on when the room is cool. When the furnace is off, the room cools a bit, and when the furnace is on, the room warms a bit. In other words, a negative feedback system results in fluctuation above and below the set point.
Human Example: Regulation of Body Temperature
The thermostat for body temperature is located in a part of the brain called the hypothalamus. When the core body temperature falls below normal, the control center directs (via nerve impulses) the blood vessels of the skin to constrict (Fig. 22.17). This action conserves heat. If the core body temperature falls even lower, the control center sends nerve impulses to the skeletal muscles, and shivering occurs. Shivering generates heat, and gradually body temperature rises to 37°C (98.6°F). When the temperature rises to normal, the control center is inactivated.
When the body temperature is higher than normal, the control center directs the blood vessels of the skin to dilate. More blood is then able to flow near the -surface of the body, where heat can be lost to the envi-ronment. In addition, the nervous system activates the sweat glands, and the evaporation of sweat helps lower body temperature. Gradually, body temperature decreases to 37°C (98.6°F).
Notice that a negative feedback mechanism prevents change in the same direction; body temperature does not get warmer and warmer, because warmth brings changes that decrease body temperature. Also, body temperature does not get colder and colder, because a body temperature below normal causes changes that bring body temperature up.
The Chapter In Review
Summary
22.1		The Body’s Organization
Levels of animal organization:
Four groups of vertebrate tissues:
Epithelial Tissue Protects
Epithelial tissue covers the body and lines its cavities.
	•	Types of epithelial tissue are: squamous, cuboidal, and columnar.
	•	Epithelial cells sometimes form glands that secrete either into ducts or into the blood.
Connective Tissue Connects and Supports
In connective tissue, cells are separated by a matrix that contains fibers (e.g., collagen fibers). The four types are:
	•	Loose fibrous connective tissue, including adipose tissue
	•	Dense fibrous connective tissue (tendons and ligaments)
	•	Cartilage and bone (matrix for cartilage is more flexible than that for bone)
	•	Blood (matrix is a liquid called plasma)
Muscular Tissue Moves the Body
Muscular tissue is of three types: skeletal, cardiac, smooth
	•	Both skeletal muscle (attached to bones) and cardiac muscle (forms the heart wall) are striated.
	•	Both cardiac muscle (wall of heart) and smooth muscle (wall of internal organs) are involuntary.
Nervous Tissue Communicates
	•	Nervous tissue is composed of neurons and several types of neuroglia.
	•	Each neuron has dendrites, a cell body, and an axon. Axons conduct nerve impulses.
22.2		Organs and Organ Systems
Organs make up organ systems, which are summarized in the table below.
22.3		Homeostasis
Homeostasis is the relative constancy of the internal environment. The internal environment consists of blood and tissue fluid. Due to exchange of nutrients and wastes with the blood, tissue fluid stays constant:
All organ systems contribute to the relative constancy of tissue fluid and blood.
	•	The cardiovascular system transports nutrients to cells and wastes from cells.
	•	The lymphatic system absorbs excess tissue fluid and functions in immunity.
	•	The digestive system takes in food and adds nutrients to the blood.
	•	The respiratory system carries out gas exchange with the external environment and the blood.
	•	The urinary system (i.e., the kidneys) removes metabolic wastes and regulates the pH and salt content of the blood.
	•	The nervous system and endocrine system regulate the other systems.
Negative Feedback
Negative feedback mechanisms keep the internal environment relatively stable. When a sensor detects a change above or below a set point, a control center brings about an effect that reverses the change and brings conditions back to normal again. Examples include:
	•	Regulation of blood glucose level by insulin.	
	•	Regulation of room temperature by a thermostat and furnace.
	•	Regulation of body temperature by the brain and sweat glands.
Thinking Scientifically
	1.	Patients with muscular dystrophy often develop heart disease. However, heart disease in these patients may not be detected until it has progressed past the point at which treatment options are effective. New research indicates that the link between muscular dystrophy and heart disease is so strong that muscular dystrophy patients should be screened for heart disease early, so that treatment may be begun. What do you suppose is the link between muscular dystrophy and heart disease? 
	2.	According to a recent study, African Americans are 34% more likely to die from a liver transplant than are white Americans. What might be potential causes for this differential survival rate? How would you test your hypotheses?
Testing Yourself
Choose the best answer for each question.
	1.	Label the levels of biological organization in the following illustration. 
	2.	Which of the following is not a feature of epidermis?
	a. contains keratin for strength
	b. stratified
	c. contains a matrix
	d. composed of epithelial cells
For questions 3-–9, identify the tissue in the key that matches the description. Each answer may be used more than once. Each question may have more than one answer.
Key:
	a. epithelial tissue
	b. connective tissue
	c. muscular tissue
	d. nervous tissue
	3.	Contains cells that are separated by a matrix.
	4.	Has a protective role.
	5.	Covers the body surface and forms linings of organs.
	6.	Composed of a group of cells with a similar function.
	7.	Capable of replacing its cells.
	8.	Allows animals to respond to their environment.
	9.	Contains actin and myosin filaments.
	10.	Label the blood smear in the following illustration. 
	11.	White blood cells fight infection by
	a. producing antibodies.
	b. producing toxins.
	c. engulfing pathogens.
	d. More than one of these are correct.
	e. All of these are correct.
	12.	Which of the following systems does not add or remove substances from the blood?
	a. digestive
	b. cardiovascular
	c. urinary
	d. respiratory
	13.	Identify the type of muscle associated with each of the following illustrations.
	14.	The major function of tissue fluid is to
	a. provide nutrients and oxygen to cells and remove wastes.
	b. keep cells hydrated.
	c. prevent cells from touching each other.
	d. provide flexibility by allowing cells to slide over each other.
	15.	Label the parts of a neuron in the following illustration. 
	16.	Which of the following is a function of skin?
	a. temperature regulation
	b. protection against water loss
	c. collection of sensory input
	d. protection from invading pathogens
	e. All of these are correct.
	17.	Endocrine glands secrete 
	a. tissue fluid.
	b. wastes.
	c. toxins.
	d. hormones.
	18.	When a human being is cold, the blood vessels
	a. dilate, and the sweat glands are inactive.
	b. dilate, and the sweat glands are active.
	c. constrict, and the sweat glands are inactive.
	d. constrict, and the sweat glands are active.
	e. contract so that shivering occurs.
	19.	The loss of _______________ will prevent proper blood clotting.
	a. red blood cells
	b. white blood cells
	c. platelets
	d. oxygen
	20.	Blood is a(n) _____________ tissue because it has a _____________.
	a. connective, gap junction
	b. muscle, matrix
	c. epithelial, gap junction
	d. connective, matrix
	21.	Skeletal muscle is 
	a. striated.
	b. under voluntary control.
	c. multinucleated.
	d. All of these are correct.
	22.	A reduction in red blood cells would cause problems with
	a. fighting infection.
	b. carrying oxygen.
	c. blood clotting.
	d. None of these are correct.
	23.	Which choice is true of both cardiac and skeletal muscle?
	a. striated
	b. single nucleus per cell
	c. multinucleated cells
	d. involuntary control
	24.	The skeletal system functions in
	a. blood cell production.
	b. mineral storage.
	c. movement.
	d. All of these are correct.
	25.	The correct order for a negative feedback mechanism is:
	a.	
sensory detection, control center, effect brings about change in environment.
	b.	
control center, sensory detection, effect brings about change in environment.
	c.	
sensory detection, control center, effect causes no change in environment.
	d.	None of these are correct.
	26.	Which of the following is an example of negative feedback?
	a. Air conditioning goes off when room temperature lowers.
	b. Insulin decreases blood sugar levels after eating a meal.
	c. Heart rate increases when blood pressure drops.
	d. All of these are examples of negative feedback.
	27.	Which of these is not a type of epithelial tissue?
	a. cuboidal and columnar
	b. bone and cartilage
	c. squamous 
	d. pseudostratified
	e. All of these are epithelial tissue.
	28.	Chemical messengers in the body are secreted by the
	a. integumentary system.
	b. nervous system.
	c. lymphatic system.
	d. digestive system.
	e. endocrine system.
	29.	Urine formation is critical for the body because it
	a. removes metabolic wastes.
	b. helps regulate blood volume.
	c. helps regulate salts.
	d. helps regulate pH.
	e. All of these are correct.
	30.	Which of the following act as slow effectors in the negative feedback system?
	a. muscles
	b. epidermal cells
	c. endocrine glands
	d. red blood cells
	e. senses
	31.	Which tissue is more apt to line a lumen?
	a. epithelial tissue
	b. connective tissue
	c. nervous tissue
	d. muscular tissue
	e. only smooth muscle
	32.	Tendons and ligaments
	a. are connective tissue.
	b. are associated with the bones.
	c. are found in vertebrates.
	d. contain collagen.
	e. All of these are correct.
	33.	Which tissue has cells in lacunae?
	a. epithelial tissue
	b. cartilage
	c. bone
	d. smooth muscle
	e. Both b and c are correct.
	34.	Cardiac muscle is
	a. striated.
	b. involuntary.
	c. smooth.
	d. composed of many fibers fused together.
	e. Both a and b are correct.
	35.	Which of these components of blood fight infection?
	a. red blood cells
	b. white blood cells
	c. platelets
	d. hydrogen ions
	e. All of these are correct.
	36.	Which of these body systems contribute to homeostasis?
	a. digestive and urinary systems
	b. respiratory and nervous systems
	c. nervous and endocrine systems
	d. immune and cardiovascular systems
	e. All of these are correct.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
Despite widespread efforts to convince people of the need for organ donors, supply continues to lag far behind demand. One proposed strategy to bring supply and demand into better balance is to develop an “insurance” program for organs. In this program, participants would pay the “premium” by promising to donate their organs at death. They, in turn, would receive priority for transplants as their “benefit.” To avoid the problem of too many high-risk people applying for this type of insurance, a medical exam would be required so that only people with a normal risk of requiring a transplant would be accepted. Do you suppose this system would be an improvement over the current system in which organ donation is voluntary, with no tangible benefit? Can you think of any other strategy that would be more effective for increasing organ donations?
Understanding the Terms
blood383
bone383
cardiac muscle384
cardiovascular system386
cartilage383
compact bone383
connective tissue382
dense fibrous connective 
tissue382
digestive system386
endocrine system387
epithelial tissue380
fibroblast382
homeostasis388
hyaline cartilage383
immune system386
integumentary system387
intercalated disks384
lacuna383
ligament382
loose fibrous connective 
tissue382
lumen380
lymphatic system386
matrix382
muscular system387
muscle tissue384
negative feedback389
nerve385
nervous system387
nervous tissue385
neuroglia385
neuron385
platelet383
red blood cell383
reproductive system387
respiratory system386
skeletal muscle384
skeletal system387
smooth (visceral) muscle384
striated384
tendon382
tissue378
tissue fluid386
urinary system386
white blood cell383
Match the terms to these definitions:
a.	_______________	Connect bones to bones at joints. 
b.	_______________	The most rigid connective tissue.
c.	_______________	Fragments of cells that seal damaged blood vessels.
d.	_______________	Called voluntary muscle; attached to bones by tendons. 
e.	_______________	Characterized by alternating light and dark bands. 
f.	_______________	Nervous tissue cells that support and nourish neurons. 
g.	_______________	The organ system that consists of skin and its accessory structures. 
h.	_______________	The system that consists of hormonal glands. 
i.	_______________	The relative constancy of the body’s internal environment. 
j.	_______________	The homeostatic mechanism that is composed of a sensor, a control center, and an effect. 

Insulin maintains blood glucose levels.
Blood is a liquid tissue.
The internal environment is composed of blood and tissue fluid.
Figure 22.1Levels of biological organization.
A tissue is composed of specialized cells all having the same structure and performing the same functions. An organ is composed of those types of tissues that help it perform particular functions. An organ system contains several organs and has functions necessary to the continued existence of an organism. 
Check Your Progress
	1.	List the levels of biological organization, and explain how they are related to each other.
	2.	
List the four major types of tissues in animals.
Answers:1. An organism contains organ systems, which are composed of groups of organs that work together. An organ is composed of tissues, which determine its structure and function. Each tissue is composed of cells.
2. The four tissue types are epithelial, connective, muscular, and nervous.
Figure 22.2Types of vertebrate tissues. 
The four classes of vertebrate tissues are epithelial (pink), connective (blue), muscular (tan), and nervous (yellow). 
Figure 22.3Epithelial tissue.
Epithelial tissue covers the surfaces and lines the cavities of internal organs. It also makes up the outer layer of skin, called the epidermis. The functions of epithelial tissue are associated with protection, secretion, and absorption. 
a. Squamous epithelium. b. Cuboidal epithelium. 
c. Columnar epithelium. d. Pseudo-stratified (appears to be layered but is not) epithelium. 
Check Your Progress
	1.	Describe the composition and activity of a gland.
	2.	
Explain why cancers are more likely to develop in epithelial tissue than in other types of tissue.
Answers:1. A gland is usually composed primarily of epithelium. The epithelial cells produce and secrete a product, such as mucus.2. Epithelial tissue is constantly dividing, so errors that might occur during cell division and result in cancer are more likely to occur.
Figure 22.4Skin.
The outer portion of skin, called the epidermis, is a stratified epithelium. The many layers of tightly packed cells reinforced by the protein keratin make the skin protective against water loss and pathogen invasion. New epidermal cells arise in the most inner layer and are shed at the outer layer. In reptiles, such as this gila monster, the epidermis forms scales, which are simply projections hardened with keratin. When the epidermis is shed, the scales have to be replaced. Pigmented epidermal cells account for coloration of the skin.
Figure 22.6Blood, a liquid tissue.
Blood is classified as connective tissue because the cells are separated by a matrix—plasma. Plasma, the liquid portion of blood, usually contains several types of cells. The components of blood are red blood cells, white blood cells, and platelets (which are actually fragments of larger cells).
Figure 22.5Types of connective tissue.
The human knee provides examples of most types of connective tissue.
Check Your Progress
	1.	Compare and contrast loose fibrous connective tissue with dense fibrous connective tissue.
	2.	Contrast the function of tendons with that of ligaments.
	3.	Contrast the structure and function of red blood cells with those of white blood cells.
Answers:1. Both loose and dense fibrous connective tissue are involved in binding organs together and providing support and protection. Loose fibrous connective tissue contains fewer collagen fibers, and these are less closely packed than those in dense fibrous connective tissue.2. Tendons connect muscles to bones, while ligaments connect bones to other bones.3. Red blood cells are smaller and lack a nucleus. The presence of hemoglobin for carrying oxygen makes them red. White blood cells contain a nucleus and without staining would appear translucent. They play a role in fighting infection.
Figure 22.7Types of muscular tissue.
Check Your Progress
	1.	Contrast the functions of skeletal, cardiac, and smooth muscle tissues.
	2.	
Describe the components of a neuron, and give a function for each.
Answers:1. Skeletal muscle causes bones to move when it contracts. Cardiac muscle causes the heart to beat. Contraction of smooth muscle causes the walls of internal organs to constrict.2. The dendrite is an extension that conducts signals toward the cell body. The cell body contains most of the cytoplasm and the nucleus. It carries on the usual functions of the cell. The axon is an extension that conducts nerve impulses.
Figure 22.8Nervous tissue. 
Neurons are surrounded by neuroglia, such as Schwann cells, which envelop axons and form the myelin sheath. Only axons conduct nerve impulses.
Figure 22.11The control systems.
The nervous and endocrine systems coordinate the other systems of the bodFigure 22.12Sensory input and motor output.
Sensory receptors in the skin (integumentary system) and the sense organs send input to the skeletal and muscular systems, the control systems that cause a response to stimuli. 
Figure 22.12Sensory input and motor output.
Sensory receptors in the skin (integumentary system) and the sense organs send input to the skeletal and muscular systems, the control systems that cause a response to stimuli. 
Figure 22.13The reproductive system.
The reproductive organs in the male and female differ. The reproductive system functions to ensure survival of the species.
Figure 22.9The body’s transport systems.
Aside from transport, the cardiovascular and lymphatic systems are involved in defense against disease.
Figure 22.10The body’s maintenance systems.
The digestive, respiratory, and urinary systems keep the internal environment constant. 
Check Your Progress
	1.	Compare and contrast the function of the cardiovascular system with that of the lymphatic system.
	2.	Describe the common function of the nervous and endocrine systems.
Answers:1. Both are transport systems. The cardiovascular system carries blood throughout the body, while the lymphatic system absorbs fat from the digestive system and collects excess tissue fluid and returns it to the blood.2. Both the nervous and endocrine systems coordinate and regulate the functions of other systems.
Figure 22.15Negative feedback mechanism.
This diagram shows how the basic elements of a negative feedback mechanism work.
Figure 22.16Regulation of room temperature.
This diagram shows how room temperature is returned to normal when the room becomes too hot. A contrary cycle in which the furnace turns on and gives off heat returns the room temperature to normal when the room becomes too cold.
Figure 22.14Constancy of the internal environment (blood and tissue fluid).
The respiratory system exchanges gases with the external environment and with the blood. The digestive system takes in food and adds nutrients to the blood, and the urinary system removes metabolic wastes from the blood and excretes them. When blood exchanges nutrients and oxygen for carbon dioxide and other wastes with tissue fluid, the composition of the tissue fluid stays within normal limits. 
Figure 22.17
Regulation of body temperature.
This diagram shows how normal body temperature is maintained by a negative feedback system.
Check Your Progress
	1.	Describe the components of a negative feedback mechanism.
	2.	
Describe how the control center in the brain responds to a drop in internal temperature.
	3.	Describe how the control center in the brain responds to a rise in internal temperature.
Answers:1. A sensor detects a change in the internal environment; the control center initiates an effect that brings conditions back to normal and the sensor is no longer activated.2. First, the control center brings about constriction of the blood vessels of the skin. If the temperature continues to drop, shivering begins in order to generate heat.3. Blood vessels of the skin dilate, and sweat glands are activated.

Organ Systems	
Transport
Cardiovascular (heart and blood vessels)
Lymphatic (lymphatic vessels)	Sensory 
Integumentary (skin)	
Maintenance
Digestive (e.g., stomach, intestines)
Respiratory (tubes and lungs)
Urinary (tubes and kidneys)
	Motor
Skeletal (bones and cartilage)
Muscular (muscles)	
Control
Nervous (brain, spinal cord, and nerves)
Endocrine (glands)	Reproduction
Reproductive (tubes and testes in males; tubes and ovaries 
in females)	

Check Your Progress
	1.	Why are blood and tissue fluid considered the internal environment? 
	2.	How do the digestive, urinary, and respiratory systems contribute to homeostasis?
Answers:1. Cells are bathed in a liquid environment called tissue fluid, and tissue fluid exchanges substances with the blood. Therefore, both are considered the internal environment of the body.2. The digestive system adds nutrients to the blood, the urinary system removes metabolic wastes from the blood, and the respiratory system takes away carbon dioxide and adds oxygen from the blood.