The Maintenance Systems
Chapter
24
Outline
	24.1	Digestive System
	•	The digestive system, the respiratory system, and the excretory system exchange molecules with the blood so that it can meet the needs of cells.414
	•	In a complex digestive system, a number of organs participate in breaking down food to small nutrient molecules that can enter the bloodstream.414	
	•	Humans are omnivores, and their digestive system can be contrasted to that of mammalian herbivores and carnivores.415
	24.2	Respiratory System
	•	In complex animals, the surface for external gas exchange participates with a circulatory system to make internal gas exchange possible.422 
	•	In humans, oxygen and carbon dioxide cross the extensive external exchange surface in the lungs by diffusion.424
	•	The respiratory pigment hemoglobin transports oxygen from the lungs to the tissues and aids the transport of carbon dioxide from the tissues to the lungs.426
	•	Cigarette smoking contributes to two major lung disorders—emphysema and cancer.422, 423, 425
	24.3	Urinary System and Excretion
	•	The human kidney, like that of other vertebrates, excretes nitrogenous wastes, maintains the water-salt balance, and helps regulate the acid-base balance.427
	•	The urinary system of humans consists of organs that produce, store, and rid the body of urine.427
	•	Nephrons within the human kidney produce urine utilizing a multistep process.428
	•	Hemodialysis and kidney transplant can treat patients with renal failure.430
During human development, organ systems begin to function on an “as needed” basis. Within the first month of embryonic development, many organ systems (such as the cardiovascular and nervous systems) are present and functioning. However, some organ systems that appear may not be refined until much later because they are not critical to early development. When babies are born prematurely (before the 37th week of gestation), numerous health problems can occur, particularly with regard to these systems. One condition that is common in very premature babies (those born at 32 weeks or earlier) is respiratory distress syndrome (RDS). Because the lungs of a fetus do not normally function until full term (40 weeks), lung development is not complete when a baby is born prematurely. Usually, a natural surfactant that helps keep the air sacs of the lungs inflated so that gas exchange can occur is not being produced. Without treatment, RDS is often fatal because the blood is not being oxygenated. Now, an artificial surfactant is available, and premature babies are given the surfactant they lack. This treatment greatly increases the likelihood that premature babies will survive. Other medical breakthroughs are also now available to encourage the functioning of a premature baby’s organ systems.
The maintenance systems have vital roles in the overall functioning of the body, and a malfunction in one of these systems can lead to problems in other systems. In this chapter, you will learn about the structure and function of the digestive, respiratory, and urinary systems, and how they support the body’s other organ systems.
24.1		Digestive System
The cells of your body are bathed in tissue fluid. They acquire oxygen and nutrients and get rid of carbon dioxide and other wastes through exchanges with tissue fluid. In turn, tissue fluid makes these same exchanges with the blood. Blood is refreshed because the digestive, respiratory, and urinary systems make exchanges with the external environment. Only in this way is blood supplied with the oxygen and nutrients cells require and cleansed of waste molecules. In this first section, we consider how the digestive system contributes to homeostasis (Fig. 24.1). 
Tube-Within-a-Tube Body Plan
Hydra and planarians (see Fig. 23.1) have a sac body plan—that is, the mouth, like the top of a sack, serves as both an entrance and an exit. Most other animals, such as the earthworm, have a tube-within-a-tube plan, so called because the inner tube has both an entrance and an exit—the mouth and the anus. Therefore, it is called a complete digestive tract (Fig. 24.2a). Notice that the inner tube (the digestive tract, sometimes called the alimentary canal) is separated from the outer tube (the body wall) by the coelom. The digestive tract of humans and other vertebrates consists of so many specialized organs that it might be hard to realize that the basic plan of vertebrates is the same as that of the earthworm (Fig. 24.2b). The digestive tract of humans exemplifies that the tube-within-a-tube plan and a complete tract result in specialization of parts. 
Digestion of food in earthworms and humans is an extracellular process. Digestive enzymes, produced by glands in the wall of the tract or by accessory glands that lie nearby, enter the tract. Food is never found within these accessory glands, only within the tract itself. 
Digestion contributes to homeo-stasis by providing the body’s cells with the nutrients they need to continue living. A digestive tract performs the following functions: 
	1.	Ingests food.
	2.	Breaks food down into small molecules that can cross plasma membranes. 
	3.	Absorbs nutrient molecules. 
	4.	Eliminates indigestible remains.
Mouth
In humans, the digestive system begins with the mouth, which chews food into pieces convenient for swallowing. Many vertebrates have teeth, an exception being birds, which lack teeth and depend on the churning of small pebbles within a gizzard to break up their food. The teeth (dentition) of mammals reflect their diet (Fig. 24.3). Carnivores eat meat, which is easily digestible because the cells don’t have a cellulose wall. Herbivores eat plant material, which needs a lot of chewing and other processing to break up the cellulose walls. Humans are omnivores; they eat both meat and plant material. The four front teeth (top and bottom) of humans are sharp, chisel-shaped incisors used for biting. On each side of the incisors are the sharp, pointed canines used for tearing food. The premolars and molars grind and crush food. It is as though humans are carnivores in the front of their mouths and herbivores in the back. 
Food contains cells composed of molecules of carbohydrates, proteins, nucleic acids, and lipids. During digestion, hydrolytic enzymes break down these large molecules to smaller molecules. The process begins in the mouth. Three pairs of salivary glands send saliva by way of ducts to the mouth. Saliva contains the enzyme salivary amylase, which breaks down starch, a carbohydrate, to maltose, a disaccharide. While in the mouth, food is manipulated by a muscular tongue, which has touch and pressure receptors similar to those in the skin. Taste buds that allow us to “taste” our food are also on the tongue as well as on the surface of the mouth. The tongue mixes the chewed food with saliva and then forms this mixture into a mass called a bolus that is swallowed.
Swallowing
The human digestive and respiratory passages come together in the pharynx and then separate (Fig. 24.4a). When food is swallowed, the soft palate, the rear portion of the mouth’s roof, moves back to close off the nasal cavities (Fig. 24.4b). A flap of tissue called the epiglottis covers the glottis, an opening into the larynx. Now the bolus must move through the pharynx into the esophagus because the air passages are blocked.
The esophagus is a tubular structure that takes food to the stomach, which lies below the diaphragm, a muscular, membranous partition that divides the thoracic (upper) cavity from the abdominal (lower) cavity of the body. When food enters the esophagus, peristalsis begins. Peristalsis is a rhythmic contraction that moves the contents along in tubular organs—in this case, those of the digestive tract.
Stomach
The human stomach is a thick-walled, J-shaped organ that stretches and stores food (Fig. 24.5). It also begins the digestion of proteins, and regulates the entrance of food into the small intestine. 
The wall of the stomach has deep folds, which disappear as the stomach fills to an approximate capacity of 1 liter. Therefore, humans can periodically eat relatively large meals and spend the rest of their time at other activities. But the stomach is much more than a mere storage organ, as was discovered by William Beaumont in the mid-nineteenth century. Beaumont, an American doctor, had a French Canadian patient, Alexis St. Martin, who had been shot in the stomach. When the wound healed, St. Martin was left with an opening that allowed Beaumont to look inside the stomach and to collect gastric (stomach) juices produced by gastric glands. Beaumont was able to determine that the muscular walls of the stomach contract vigorously and mix food with juices that are secreted whenever food enters the stomach. He found that gastric juice contains hydrochloric acid (HCl) and a substance, now called pepsin, that is active in digestion.
We now know that the epithelial lining of the stomach, called a mucosa, has millions of gastric glands. These gastric glands produce gastric juice containing so much hydro-chloric acid that the stomach routinely has a pH of about 2. Such high acidity is usually sufficient to kill any microbes that might be in food. This low pH also promotes the activity of pepsin, a hydrolytic enzyme that acts on protein to produce peptides. In addition, high acidity causes heartburn and gastric reflux disease when gastric juice backs up into the esophagus.
As with the rest of the digestive tract, a thick layer of mucus protects the wall of the stomach from enzymatic action. Still, an ulcer, which is an open sore in the wall caused by the gradual destruction of tissues, may occur in some individuals. Ulcers are due to an infection by an acid-resistant bacterium, Helicobacter pylori, which is able to attach to the epithelial lining. Wherever the bacterium attaches, the lining stops producing mucus, and the area becomes exposed to digestive action. Then an ulcer develops. 
Peristalsis pushes food along in the stomach as it does in other digestive organs (Fig. 24.5b). At the base of the stomach is a narrow opening controlled by a sphincter, a muscle that surrounds a tube and closes or opens it by contracting and relaxing. Whenever this sphincter relaxes, a small quantity of material passes through the opening into the small intestine. 
RuminantsRuminants, a type of mammal that includes cattle, sheep, goats, deer, and buffalo, are named for a part of their stomachs, the rumen (Fig. 24.6). The rumen contains symbiotic bacteria and protozoans that, unlike the mammal, can digest cellulose. After these herbivores feed on grass, it goes to the rumen, where it is broken down and then formed into small balls of cud. The cud then returns to the mouth where the animal “chews the cud.” The cud may return to the rumen for a second go-round before passing through the other chambers of the stomach. The last chamber is analogous to the human stomach, being the place where protein is digested to peptides. 
Small Intestine
Processing food in humans is more complicated than one might think. So far, the food has been chewed in the mouth and worked on by the enzyme salivary amylase, which digests starch to maltose. In addition, the digestion of protein has begun in the stomach as pepsin digests proteins to peptides. By now, the contents of the digestive tract are called chyme. Chyme passes from the stomach to the small intestine, a long, coiled tube that has two functions: (1) digestion of all the molecules in chyme, including polymers of carbohydrate, protein, nucleic acid, and fat, and (2) absorption of the nutrient molecules into the body.
The first part of the small intestine is called the duodenum. Two important accessory glands, the liver, the largest organ in the body, and the pancreas, located behind the stomach, send secretions to the duodenum by way of ducts (Fig. 24.7). The liver produces bile, which is stored in the gallbladder. Bile looks green because it contains pigments that are the products of hemoglobin breakdown. This green color is familiar to anyone who has observed how bruised tissue changes color. Hemoglobin within the bruise is breaking down into the same types of pigments found in bile. Bile also contains bile salts, which break up fat into fat droplets by a process called emulsification. Fat droplets mix with water and have more surface area for digestion by enzymes.
The pancreas produces pancreatic juice, which contains sodium bicarbonate (NaHCO3) and digestive enzymes. NaHCO3 neutralizes chyme and makes the pH of the small intestine slightly basic. The higher pH helps prevent autodigestion of the intestinal lining by pepsin and optimizes the pH for pancreatic enzymes. Pancreatic amylase digests starch to maltose; trypsin digests proteins to peptides; lipase digests fat droplets to glycerol and fatty acids; and nuclease digests nucleic acids to nucleotides. 
Still more digestive enzymes are present in the small intestine. The wall of the small intestine contains fingerlike projections called villi (Fig. 24.8b). The epithelial cells of the villi produce intestinal -enzymes, which remain attached to them. These enzymes complete the digestion of peptides and sugars. Peptides, which result from the first step in protein digestion, are digested by peptidase to amino acids. Maltose, which results from the first step in starch digestion, is digested by maltase to glucose. Other disaccharides, each of which is acted upon by a specific enzyme, are digested to simple sugars also. 
Finally, these small nutrient molecules can be absorbed into the body. Our cells use these molecules as a source of energy and as building blocks to make their own macromolecules. 
Absorption by VilliThe wall of the small intestine is adapted to absorbing nutrient molecules because it has an extensive surface area—approximately that of a tennis court! First, the mucous membrane layer of the small intestine has circular folds that give it an almost corrugated appearance (Fig. 24.8a). Second, on the surface of these circular folds are the villi. Finally, the cells on the surface of the villi have minute projections called microvilli (Fig. 24.8c). If the human small intestine were simply a smooth tube, it would have to be 500 to 600 meters long to have a comparable surface area for absorption. Carnivores have a much shorter digestive tract than herbivores because meat is easier to process than plant material (Fig. 24.9).
The villi of the small intestine absorb small nutrient molecules into the body. Each villus contains an extensive network of blood capillaries and a lymphatic capillary called a lacteal. As discussed in Chapter 23, the lymphatic system is an adjunct to the cardiovascular system—its vessels carry a fluid called lymph to the cardiovascular veins. Sugars and amino acids enter the blood capillaries of a villus. In contrast, glycerol and fatty acids (digested from fats) enter the epithelial cells of the villi, and within them are joined and packaged as lipoprotein droplets, which enter a lacteal. Absorption continues until almost all nutrient molecules have been absorbed. Absorption occurs by diffusion, as well as by active transport, which requires an expenditure of cellular energy. Lymphatic vessels transport lymph to cardiovascular veins. Eventually, the bloodstream carries the nutrients absorbed by the digestive system to all the cells of the body.
Large Intestine
The word bowel technically means the part of the digestive tract between the stomach and the anus, but it is sometimes used to mean only the large intestine. The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated at the anus. The large intestine takes its name from its diameter rather than its length, which is shorter than that of the small intestine. The large intestine has a blind pouch below the entry of the small intestine with a small projection called the appendix. In humans, the appendix may play a role in fighting infections. In the condition called appendicitis, the appendix becomes infected and so filled with fluid that it may burst. If an infected appendix bursts before it can be removed, a serious, generalized infection of the abdominal lining, called peritonitis, may result.
About 1.5 liters of water enter the digestive tract daily as a result of eating and drinking. An additional 8.5 liters also enter the digestive tract each day, carrying the various substances secreted by the digestive glands. About 95% of this water is absorbed by the small intestine, and much of the remaining portion is absorbed by the large intestine. If this water is not reabsorbed, diarrhea can occur, leading to serious dehydration and ion loss, especially in children.
The large intestine has a large population of bacteria, notably Escherichia coli. The bacteria break down indigestible material, and they also produce some vitamins, including vitamin K. Vitamin K is necessary for blood clotting. Digestive wastes (feces) eventually leave the body through the anus, the opening of the anal canal. Feces are about 75% water and 25% solid matter. Almost one-third of this solid matter is made up of intestinal bacteria. The remainder is indigestible plant material (also called fiber), fats, waste products (such as bile pigments), inorganic material, mucus, and dead cells from the intestinal lining. A diet that includes fiber adds bulk to the feces, improves regularity of elimination, and prevents constipation.
The large intestine (also called the colon) is subject to the development of polyps, which are small growths arising from the mucosa. Polyps, whether they are benign or cancerous, can be removed surgically.
Figure 24.10 reviews the digestive process. 
Accessory Organs
The pancreas and the liver are accessory organs of digestion, along with the teeth, salivary glands, and gallbladder.
Pancreas
The pancreas (see Fig. 24.7) functions as both an endocrine gland and an exocrine gland. Endocrine glands are ductless and secrete their products into the blood. The pancreas is an endocrine gland when it produces and secretes the hormones insulin and glucagon into the bloodstream. Exocrine glands secrete into ducts. The pancreas is an exocrine gland when it produces and secretes pancreatic juice into the duodenum through the common bile duct.
Liver
The liver has numerous functions, including the following: 
	1.	 Detoxifies the blood by removing and metabolizing poisonous substances. 
	2.	Produces the plasma proteins, such as albumin and fibrinogen. 
	3.	Destroys old red blood cells and converts hemoglobin to the breakdown products in bile (bilirubin and biliverdin).
	4.	Produces bile, which is stored in the gallbladder before entering the small intestine, where it emulsifies fats.
	5.	Stores glucose as glycogen and breaks down glycogen to glucose between meals to maintain a constant glucose concentration in the blood.
	6.	Produces urea from amino groups and ammonia.
Blood vessels from the large and small intestines merge to form the hepatic portal vein, which leads to the liver (Fig. 24.11). The liver helps maintain the glucose concentration in blood at about 0.1% by removing excess glucose from the hepatic portal vein and storing it as glycogen. Between meals, glycogen is broken down to glucose, and glucose enters the hepatic veins. Like plant-made starch, glycogen is made up of glucose molecules, and thus it is sometimes called animal starch. If the supply of glycogen and -glucose runs short, the liver converts amino acids to glucose molecules.
Amino acids contain nitrogen in the form of amino groups, whereas glucose contains only carbon, oxygen, and hydrogen. Therefore, before amino acids can be converted to glucose molecules, deamination, the removal of amino groups from amino acids, must occur. By a complex metabolic pathway, the liver converts the amino groups to urea, the most common nitrogenous (nitrogen-containing) waste product of humans. After urea is formed in the liver, it is transported by the bloodstream to the kidneys, where it is excreted.
Liver DisordersWhen a person is jaundiced, the skin has a yellowish tint due to an abnormally large quantity of bile pigments in the blood. In hemolytic jaundice, red blood cells are broken down in abnormally large amounts; in obstructive jaundice, the bile duct is obstructed, or the liver cells are damaged. Obstructive jaundice often occurs when crystals of cholesterol precipitate out of bile and form gallstones.
Jaundice can also result from viral infection of the liver, called hepatitis. Hepatitis A is most often caused by eating contaminated food. Hepa-titis B and C are commonly spread by blood transfusions, kidney dialysis, and injection with unsterilized needles. These three types of hepatitis can also be spread by sexual contact.
Cirrhosis is a chronic liver disease in which the organ first becomes fatty, and later liver tissue is replaced by inactive fibrous scar tissue. Alcoholics often get cirrhosis, most likely due at least in part to the excessive amounts of alcohol the liver is forced to break down.
Regulation of Digestive Juices
Does your mouth water when you smell food cooking? Even the thought of food can sometimes cause the nervous system to order the secretion of digestive juices. The secretion of these juices is also under the influence of several peptide hormones, so called because each is a small sequence of amino acids. When you eat a meal rich in protein, the stomach wall produces a peptide hormone that enters the bloodstream and doubles back to cause the stomach to produce more gastric juices. When protein and fat are present in the small intestine, another peptide hormone made in the intestinal wall stimulates the secretion of bile and pancreatic juices. In this way, the organs of digestion regulate their own needs. 
24.2		Respiratory System
A respiratory system conducts oxygen-laden air to an exchange surface and conducts carbon dioxide-laden air out of the body. Respiration contributes to homeostasis by providing the body’s cells with oxygen and removing carbon dioxide (Fig. 24.12). Respiration in complex animals requires these steps: 
	1.	Breathing: inspiration (entrance of air into the lungs) and expiration (exit of air from the lungs).
	2.	External exchange of gases between the air and the blood within the lungs.
	3.	Internal exchange of gases between blood and tissue fluid: The body’s cells exchange gases with tissue fluid.
Regardless of the particular gas-exchange surfaces of animals and the manner in which gases are delivered to the cells, in the end oxygen enters mitochondria, where cellular respiration takes place. Without the delivery of oxygen to the body’s cells, ATP is not produced, and life ceases. Carbon dioxide, a waste molecule given off by cells, is a by-product of cellular respiration.
The Human Respiratory Tract 
The human respiratory system includes all of the structures that conduct air in a continuous pathway to and from the lungs (see Fig. 24.14). As air moves through the respiratory tract, it is filtered so that it is free of debris, warmed, and humidified. By the time the air reaches the lungs, it is at body temperature and saturated with water. In the nose, hairs and cilia act as screening devices. In the respiratory passages, cilia beat upward, carrying mucus, dust, and occasional bits of food that “went down the wrong way” into the throat, where the accumulation may be swallowed or spit out (Fig. 24.13). Smoking cigarettes and cigars inactivates and eventually destroys these cilia, so that the lungs become laden with soot and debris. This is the first step toward various lung disorders. 
Conversely, as air moves out of the tract, it cools and loses its moisture. As air cools, it deposits its moisture on the lining of the tract, and the nose many even drip as a result of this condensation. However, the air still retains so much moisture that on a cold day, it forms a small cloud when we breathe out. 
The Upper Respiratory Tract
The upper respiratory tract consists of the nasal cavities, pharynx, and larynx (Fig. 24.14). The nose, a prominent feature of the face, is the only external portion of the respiratory system. The nose contains the nasal cavities, narrow canals separated from one another by a septum composed of bone and cartilage. Tears from the eyes drain into the nasal cavities by way of tear ducts. For this reason, crying produces a runny nose. The nasal cavities communicate with the sinuses, air-filled spaces that reduce the weight of the skull and act as resonating chambers for the voice. If the ducts leading from the sinuses become inflamed, fluid may accumulate, causing a sinus headache. The nasal cavities are separated from the mouth by a partition called the palate. The palate has two portions. Anteriorly, the hard palate is supported by bone, and posteriorly, the soft palate is not so supported.
The pharynx is a funnel-shaped passageway that connects the nasal cavity and mouth to the larynx, or voice box. The tonsils form a protective ring of lymphatic tissue at the junction of the mouth and the pharynx. Tonsillitis occurs when the tonsils become inflamed and enlarged. If tonsillitis occurs frequently and enlargement makes breathing difficult, the tonsils can be removed surgically, a procedure called a tonsillectomy. In the pharynx, the air passage and food passage cross because the larynx, which receives air, is anterior to the esophagus, which receives food. This arrangement may seem inefficient, since there is danger of choking if food accidentally enters the trachea, but it does have the advantage of letting you breathe through your mouth in case your nose is plugged up. In addition, it permits greater intake of air during heavy exercise, when greater gas exchange is required.
Air passes from the pharynx through the glottis, an opening into the larynx. The larynx is always open because it is formed by a complex of cartilages, among them the Adam’s apple. At the edges of the glottis, embedded in mucous membrane, are the vocal cords. These flexible bands of connective tissue vibrate and produce sound when air is expelled past them through the glottis from the larynx. Laryngitis is an infection of the larynx with accompanying hoarseness leading to the inability to speak audibly. 
Lower Respiratory Tract
The lower respiratory tract contains the respiratory tree, consisting of the trachea, bronchi, and bronchioles (Fig. 24.14). The trachea, commonly called the windpipe, is a tube connecting the larynx to the bronchi. The trachea is held open by a series of C-shaped, cartilaginous rings that do not completely meet in the rear. The trachea divides into two primary bronchi, which enter the right and left lungs. Bronchitis is an infection of the bronchi. As bronchitis develops, a nonproductive cough becomes a deep cough that produces mucus and perhaps pus. The deep cough of smokers indicates that they have bronchitis and that the respiratory tract is irritated. When a person stops smoking, this progression reverses, and the airways become healthy again. Chronic bronchitis is the second step toward emphysema and lung cancer caused by smoking cigarettes. Lung cancer often begins in the bronchi, and from there it spreads to the lungs. 
The bronchi continue to branch until there are a great number of smaller passages called bronchioles. The two bronchi resemble the trachea in structure, but as the passages divide and subdivide, their walls become thinner, and rings of cartilage are no longer present. During an attack of asthma, the smooth muscle of the bronchioles contracts, causing constriction of the bronchioles and characteristic wheezing. Each bronchiole terminates in an elongated space enclosed by a multitude of little air pockets, or sacs, called alveoli (sing., alveolus), which make up the lungs (see Fig. 24.18).
Respiration in InsectsWhile humans have one trachea, insects have many tracheae, little air tubes supported by rings of chitin that branch into every part of the body (Fig. 24.15). The tracheal system begins at spiracles, openings that perforate the insect’s body wall, and ends in very fine, fluid-filled tubules, which may actually indent the plasma membranes of cells to come close to mitochondria. Ventilation is assisted by the presence of air sacs that merely assist the drawing in of air. The hemolymph in an insect does not transport oxygen, and no oxygen-carrying pigment is required. 
Breathing
The breathing mechanism of humans is the same as that of other -terrestrial vertebrates except birds. As in other mammals, the volume of the thoracic cavity and lungs is increased by muscle contractions that lower the diaphragm and raise the ribs (Fig. 24.16). These movements create a negative pressure in the thoracic cavity and lungs, and air then flows into the lungs, a process called inspiration. It is important to realize that air comes in because the lungs have already opened up; air does not force the lungs open. When the ribs and diaphragm muscles relax, the lungs recoil, and air moves out as a result of increased pressure in the lungs, a process called expiration.
Because air moves in and out by the same route, some residual air is always left in the lungs of humans. In contrast, birds use a one-way ventilation mechanism (Fig. 24.17). Incoming air is carried past the lungs by a trachea that takes it to a set of abdominal air sacs. Then air passes forward through the lungs into a set of thoracic air sacs. Fresh air never mixes with used air in the lungs of birds, and thereby gas-exchange efficiency is greatly improved. 
Increased concentrations of hydrogen ion (H) and carbon dioxide (CO2) in the blood are the primary stimuli that increase the breathing rate in humans. The chemical content of the blood is monitored by chemoreceptors called the aortic bodies and carotid bodies, which are specialized structures in the walls of certain arteries. These receptors are very sensitive to changes in H and CO2 concentrations, but they are only minimally -sensitive to a lower oxygen (O2) concentration. Information from the chemoreceptors goes to the breathing center in the brain, which increases the breathing rate when concentrations of hydrogen ions and carbon dioxide rise (see Fig. 24.21). The breathing center is also directly -sensitive to the chemical content of the blood, including its oxygen content. 
Lungs and External Exchange of Gases
The lungs of humans and other mammals are more elaborately subdivided than those of amphibians and reptiles. Frogs and salamanders have a moist skin that allows them to use the surface of their body for gas exchange in addition to the lungs. It has been estimated that human lungs have a total surface area at least 50 times the skin’s surface area because of the presence of alveoli (Fig. 24.18). 
An alveolus, like the capillary that surrounds it, is bounded by squamous epithelium. Diffusion alone accounts for gas exchange between the alveolus and the capillary. Carbon dioxide, being more plentiful in the pulmonary venule, diffuses from a pulmonary capillary to enter an alveolus while oxygen, being more plentiful in the lungs, diffuses from an alveolus into a pulmonary capillary. The process of diffusion requires a gas-exchange -region to be not only large, but also moist and thin. The alveoli are lined with surfactant, a film of lipoprotein that lowers the surface tension of water, thereby preventing the alveoli from collapsing. Some newborns, especially if premature, lack this film. Surfactant replacement therapy is now thankfully available to treat this condition.
The blood within pulmonary capillaries is indeed spread thin, and the red blood cells are pressed up against their narrow walls. The alveolar epithelium and the capillary epithelium are so close that together they are called the respiratory membrane. Hemoglobin in the red blood cells quickly picks up oxygen molecules as they diffuse into the blood. 
Emphysema is a serious lung condition in which the walls of many alveoli have been destroyed. The lungs have less recoil and there is less surface area for gas exchange to occur. 
Gills of FishIn contrast to the lungs of terrestrial vertebrates, fish and other aquatic animals use gills as their respiratory organ (Fig. 24.19). In fish, water is drawn into the mouth and out from the pharynx across the gills. The flow of blood in gill capillaries is opposite the flow of water across the gills, and therefore the blood is always exposed to water having a higher oxygen content. In the end, about 80% to 90% of the dissolved oxygen in water is extracted.
Transport and Internal Exchange of Gases
Recall from Chapter 23 that hemoglobin molecules within red blood cells carry oxygen to the body’s tissues. If hemoglobin didn’t transport oxygen, it’s been estimated it would take three years for an oxygen molecule to move from your lungs to your toes. Each hemoglobin molecule contains four polypeptide chains, and each chain is folded around an iron-containing group called heme. It is actually the iron that bonds with oxygen and carries it to the tissues (Fig. 24.20). Since there are about 250 million hemoglobin molecules in each red blood cell, each cell is capable of carrying at least one billion molecules of oxygen. Hemoglobin gives up its oxygen in the tissues during internal exchange primarily because tissue fluid always has a lower oxygen concentration than blood does. This difference occurs because cells take up and utilize oxygen when they carry on cellular respiration. Another reason hemoglobin gives up oxygen is due to the warmer temperature and lower pH in the tissues, environmental conditions that are also caused by cellular respiration. When cells respire, they give off heat and carbon dioxide as by-products. 
Carbon dioxide enters the blood during -internal exchange because the tissue fluid always has a higher concentration of carbon dioxide. Most of the carbon dioxide is transported in the form of the bicarbonate ion (HCO3). First, carbon dioxide combines with water, forming carbonic acid, and then this acid dissociates to a hydrogen ion (H) and HCO3:
	CO2	1	H2O		H2CO3		H1	1	HCO3
	carbon		water		carbonic		hydrogen		bicarbonate
	dioxide				acid		ion		ion
The H does cause the pH to lower, but only slightly because much of the H is absorbed by the globin portions of -hemoglobin. The HCO3 is carried in the plasma. 
What happens to the above equation in the lungs? As blood enters the pulmonary capillaries, carbon dioxide diffuses out of the blood into the alveoli. Hemoglobin gives up the H it has been carrying as this reaction occurs:
	H1	1	HCO3		H2CO3		H2O	1	CO2
Now, much of the carbon dioxide diffuses out of the blood into the -alveoli of the lungs. Should the blood level of H rise, the breathing center in the brain increases the breathing rate, and as more CO2 leaves the blood, the pH of blood is corrected (Fig. 24.21). 
24.3		Urinary System and Excretion
In complex animals, kidneys excrete nitrogenous wastes and are involved in regulating the water-salt balance of the body (Fig. 24.22). In addition, the mammalian kidney helps regulate the pH of blood. To summarize, these three functions can be associated with a kidney:
	1.	Excretion of nitrogenous wastes such as urea and uric acid.
	2.	Maintenance of the water-salt balance of the blood. 
	3.	Maintenance of the acid-base balance of the blood.
In humans and other mammals, the kidneys are bean-shaped, reddish-brown organs, each about the size of a fist. They are located on either side of the vertebral column just below the diaphragm, where they are partially protected by the lower rib cage. Urine made by the kidneys is conducted from the body by the other organs in the urinary system. Each kidney is connected to a ureter, a tube that takes urine from the kidney to the urinary bladder, where it is stored until it is voided from the body through the single urethra (Fig. 24.23a). In males, the urethra passes through the penis, and in females, it opens in front of the opening of the vagina. In females, there is no connection between the genital (reproductive) and urinary systems, but there is a connection in males—that is, the urethra also carries sperm during ejaculation.
In amphibians, birds, reptiles, and some fishes, the bladder empties into the cloaca, a common chamber and outlet for the digestive, urinary, and genital tracts. 
Kidneys
If a kidney is sectioned longitudinally, three major parts can be distinguished (Fig. 24.23b). The renal cortex, the outer region of a kidney, has a somewhat granular appearance. The renal medulla consists of the cone-shaped renal pyramids, which lie inside the renal cortex. The innermost part of the kidney is a hollow chamber called the renal pelvis. Urine collects in the renal pelvis and then is carried to the bladder by a ureter. Microscopically, each kidney is composed of about one million tiny tubules called nephrons (Fig. 24.23c). The nephrons of a kidney produce urine.
Nephrons
Each nephron is made of several parts (Fig. 24.24). The blind end of a nephron is pushed in on itself to form the nephron capsule. The inner layer of the capsule is composed of specialized cells that allow easy passage of molecules. Leading from the capsule is a portion of the nephron called the proximal tubule, which is lined by cells with many mitochondria and tightly packed microvilli. Then comes the nephron loop with a descending limb and an ascending limb. This is followed by the distal tubule. Several distal tubules enter one collecting duct. A collecting duct delivers urine to the renal pelvis. The nephron loop and the collecting duct give the pyramids of the -renal medulla their striped appearance (see Fig. 24.23b).
Each nephron has its own blood supply, and various exchanges occur between parts of the nephron and a blood capillary as urine forms. 
Urine Formation
Urine formation requires three steps: filtration, reabsorption, and secretion (Fig. 24.24).
FiltrationFiltration occurs whenever small substances pass through a filter and large substances are left behind. During urine formation, filtration is the movement of small molecules from a blood capillary to the inside of the capsule as a result of adequate blood pressure. Small molecules, such as water, nutrients, salts, and urea, move to the inside of the capsule. Plasma proteins and blood cells are too large to be part of this filtrate, so they remain in the blood.
If the composition of the filtrate were not altered in other parts of the nephron, death from loss of nutrients (starvation) and loss of water (dehydration) would quickly follow. The next step, reabsorption, helps prevent this from happening.
Reabsorption of SolutesReabsorption takes place when substances from the proximal tubule move into the blood. Nutrients such as glucose and amino acids also return to the blood. This process is selective because some molecules such as glucose are both passively and actively reabsorbed. The cells of the proximal tubule have numerous microvilli, which increase the surface area, and numerous mitochondria, which supply the energy needed for active transport. However, if there is more glucose in the filtrate than there are carriers to handle it, glucose will appear in the urine. Glucose in the urine is a sign of diabetes mellitus, sometimes caused by lack of the hormone insulin. 
Sodium ions (Na) are also actively pumped into the peritubular capillary, and then chloride ions (Cl) follow passively. Now water moves by osmosis from the tubule into the blood. About 60–70% of salt and water are reabsorbed at the proximal tubule.
Urea, the primary nitrogenous waste product of human metabolism, and other types of nitrogenous wastes excreted by humans are passively reabsorbed, and most remain in the filtrate.
SecretionSecretion refers to the transport of substances into the nephron by means other than filtration. For our purposes, secretion may be particularly associated with the distal tubule. Substances such as uric acid, hydrogen ions, ammonia, and penicillin are eliminated by secretion. The process of secretion helps rid the body of potentially harmful compounds that were not filtered into the capsule.
Regulation of Water-Salt Balance and pHTypically, animals have some means of regulating the osmolarity of the internal environment so that the water-salt balance stays within normal limits. Insects have a unique excretory system consisting of long, thin tubules called Malpighian tubules attached to the gut (Fig. 24.25). Uric acid, their primary nitrogenous waste product, simply flows from the surrounding hemolymph into these tubules, and water follows a salt gradient established by active transport of potassium (K). Water and other useful substances are reabsorbed at the rectum, but the uric acid leaves the body at the anus. Insects that live in water or eat large quantities of moist food reabsorb little water. But insects in dry environments reabsorb most of the water and excrete a dry, semisolid mass of uric acid. Most animals can regulate the blood level of both water and salt. For example, freshwater fishes take up salt in the digestive tract and gills and produce large amounts of dilute urine. Saltwater fishes take in salt water by drinking, but then they pump ions out at the gills, and produce only small amounts of a concentrated urine. 
In mammals, the long nephron loop allows secretion of a hypertonic urine (see Fig. 24.24). The ascending limb of the nephron loop pumps salt and also urea into the renal medulla, and water follows by osmosis both at the descending limb of the nephron loop and at the collecting duct. As you will see in Chapter 27, at least three hormones are involved in regulating water-salt reabsorption by the kidneys. Drinking coffee interferes with one of these hormones, and that’s why coffee is a diuretic, a substance that causes the production of more urine. 
Most mammals can also regulate the pH of the blood. The bicarbonate (HCO3) buffer system of the blood and regulation of the breathing rate (see Fig. 24.21) to rid the body of CO2 both contribute to maintaining blood pH. As helpful as these mechanisms might be, only the kidneys can excrete a wide range of acidic and basic substances. The kidneys are slower acting than the buffer/breathing mechanism, but they have a more powerful effect on pH. For the sake of simplicity, we can think of the kidneys as reabsorbing bicarbonate ions and excreting hydrogen ions as needed to maintain the normal pH of the blood (Fig. 24.26). If the blood is acidic, hydrogen ions are excreted and bicarbonate ions are reabsorbed. If the blood is basic, hydrogen ions are not excreted and bicarbonate ions are not reabsorbed. The fact that urine is most often acidic shows that usually an excess of hydrogen ions are excreted. Ammonia (NH3) provides a means for buffering these hydrogen ions in urine: (NH3 1H £ NH4). Ammonia (the presence of which is quite obvious in the diaper pail or kitty litter box) is produced in tubule cells by the deamination of amino acids. Phosphate provides another means of buffering -hydrogen ions in urine.
Problems with Kidney Function
Many types of illnesses, especially diabetes, hypertension, and inherited conditions, cause progressive renal disease and renal failure. Urinary tract infections, an enlarged prostate gland, pH imbalances, or simply an intake of too much calcium can lead to kidney stones. Kidney stones form in the renal pelvis and usually pass unnoticed in the urine flow. If they grow to several centimeters and block the renal pelvis or ureter, back pressure builds up and destroys nephrons. One of the first signs of nephron damage is the presence of albumin, white blood cells, or even red blood cells in the urine, as detected by a urinalysis. If damage is so extensive that more than two-thirds of the nephrons are inoperative, urea and other waste substances accumulate in the blood. Although nitrogenous waste in the blood is a threat to homeostasis, the retention of water and salts is of even greater concern. Edema, fluid accumulation in the body tissues, may occur. Imbalance in the ionic composition of body fluids can lead to loss of consciousness and even to heart failure.
Hemodialysis and Kidney Replacement
Patients with renal failure can undergo hemodialysis, utilizing -either an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD). Dialysis is defined as the diffusion of dissolved molecules through a semipermeable membrane with pore sizes that allow only small molecules to pass through. In an artificial kidney machine, the patient’s blood is passed through a membranous tube, which is in contact with a dialysis solution, or dialysate (Fig. 24.27). Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood. The dialysate is continuously replaced to maintain favorable concentration gradients. In this way, the artificial kidney can be utilized either to extract substances from the blood, including waste products or toxic chemicals and drugs, or to add substances to the blood—for example, bicarbonate ions (HCO3) if the blood is acidic. In the course of a three- to six-hour hemodialysis procedure, from 50 to 250 grams of urea can be removed from a patient, which greatly exceeds the amount excreted by our kidneys within the same time frame. Therefore, a patient needs to undergo treatment only about twice a week.
CAPD is so named because the peritoneum, the epithelium that lines the abdominal cavity, is the dialysis membrane. A fresh amount of dialysate is -introduced directly into the abdominal cavity from a bag -that is temporarily attached to a permanently implanted plastic tube. The dialysate flows into the peritoneal cavity by gravity. Waste and salt molecules pass from the blood vessels in the -abdominal wall into the dialysate before the fluid is collected four or eight hours later. The solution is drained into a bag from the abdominal cavity by gravity, and then it is discarded. One advantage of CAPD over an artificial kidney machine is that the individual can go about his or her normal activities during CAPD.
Patients with renal failure may undergo a transplant operation in which a functioning kidney from a donor is received. A person only needs one functioning kidney. As with all organ transplants, the possibility of organ rejection exists. Receiving a kidney from a close relative has the highest chance of success. The current one-year survival rate is 97% if the kidney comes from a relative, and 90% if it is from a nonrelative. In the future, it may be possible to use kidneys from pigs or kidneys created in the laboratory for transplant operations.
The Chapter in Review
Summary
24.1		Digestive System
The digestive, respiratory, and urinary systems make exchanges with the external environment and blood, thereby supplying the blood with nutrients and oxygen and cleansing it of waste molecules. Then the blood makes exchanges with the tissue fluid, and in this way cells acquire nutrients and oxygen and rid themselves of wastes.
Tube-Within-a-Tube Body Plan
Like the earthworm, humans have a complete digestive system that (1) ingests food; (2) breaks food down to small molecules that can cross plasma membranes; (3) absorbs these nutrient molecules; and (4) eliminates indigestible remains.
The digestive tract consists of several specialized parts:
	•	Mouth: Teeth chew the food, saliva contains salivary amylase for digesting starch, and the tongue forms a bolus for swallowing.
	•	Pharynx: The air and food passages cross in the pharynx. During swallowing, the air passage is blocked off by the soft palate and epiglottis; peristalsis begins.
	•	Stomach: The stomach expands and stores food and also churns, mixing food with the acidic gastric juices. This juice contains pepsin, an enzyme that digests protein. The stomach of ruminants has a special chamber, the rumen, where symbiotic bacteria and protozoans digest grass.
	•	Small intestine: The duodenum of the small intestine receives bile from the liver and pancreatic juice from the pancreas. Pancreatic juice contains trypsin (digests protein), lipase (digests fat), and pancreatic amylase (digests starch). The small intestine produces enzymes that finish digestion, breaking food down to small molecules that cross the villi. Amino acids and glucose enter blood capillaries. Glycerol and fatty acids are joined and packaged as lipoproteins before entering lymphatic vessels called lacteals.
	•	Large intestine: The large intestine stores the remains of digestion until they can be eliminated. It also absorbs water, salts, and some vitamins. Reduced water absorption results in diarrhea. The intake of water and fiber helps prevent constipation.
Accessory Organs
The pancreas is both an exocrine gland that produces pancreatic juice and an endocrine gland that produces the hormones insulin and glucagon. The liver produces bile, which is stored in the gallbladder. Pancreatic juice and bile enter the small intestine. The nervous system and the peptide hormones regulate the secretion of digestive juices and bile.
The hepatic portal vein carries absorbed molecules from the small intestine to the liver, an organ that performs many important functions.
24.2		Respiratory System
A respiratory system has these functions: 
(1) breathing; (2) external exchange of gases; and (3) internal exchange of gases.
The respiratory tract consists of several parts:
	•	Nasal cavities: In the nasal cavities of the nose, air is moistened, and hairs trap debris.
	•	Pharynx: Air crosses from front to back.
	•	Larynx: Contains the vocal cords.
	•	Respiratory tree: Consists of the trachea, bronchi, and bronchioles, which terminate in the alveoli of the lungs. 
			Insects have many tracheae. A tracheal system has no need of a respiratory pigment because air is delivered directly to cells.
	•	Lungs: Contain many alveoli, air sacs surrounded by a capillary network.
Breathing
	•	Inspiration: The diaphragm lowers, and the rib cage moves up and out; the lungs expand and air rushes in.
	•	Expiration: The diaphragm relaxes and moves up; the rib cage moves down and in; pressure in the lungs increases; air is pushed out of the lungs.
Lungs and External Exchange of Gases
	•	Alveoli are surrounded by pulmonary capillaries.
	•	CO2 diffuses from the blood into the alveoli, and O2 diffuses into the blood from the alveoli, because of their respective concentration gradients.
	•	Hemoglobin takes up oxygen.
Transport and Internal Exchange of Gases
Oxygen for cellular respiration follows this path:
	•	Transported by the iron portion of hemoglobin.
	•	Exits blood at tissues by diffusion.
	•	Given up by hemoglobin in capillaries because cellular respiration makes tissues warmer and more acidic.
Carbon dioxide, from cellular respiration, follows this path:
	•	Enters blood by diffusion.
	•	Taken up by red blood cells and joins with water to form carbonic acid.
	•	Carbonic acid breaks down to H and bicarbonate ion (HCO3): 
Bicarbonate ions are carried in plasma. H combines with globin of hemoglobin.
	•	In the lungs, H joins with bicarbonate ion to form carbonic acid, which breaks down to water and carbon dioxide:
24.3		Urinary System and Excretion
The kidneys perform the following functions:
	•	Excrete nitrogenous wastes, such as urea and uric acid.
	•	Maintain the normal water-salt balance of blood.
	•	Maintain the acid-base balance of blood.
The urinary system consists of these parts:
	•	Kidneys: Produce urine.
	•	Ureters: Take urine to the bladder.
	•	Urinary bladder: Stores urine.
	•	Urethra: Releases urine to the outside.
Kidneys
Macroscopically, the kidneys have three parts: renal cortex, renal medulla, and renal pelvis. Microscopically, they contain the nephrons. A nephron has a nephron capsule, proximal tubule, nephron loop, and distal tubule. 
Urine formation requires three steps:
	•	Filtration: Water, nutrients, and wastes move from the blood to the inside of the nephron capsule.
	•	Reabsorption: Primarily salts, water, and nutrients are reabsorbed at the proximal tubule.
	•	Secretion: Certain substances (e.g., hydrogen ions) are transported into the distal tubule from blood.
Regulation of Water-Salt Balance and pHAnimals regulate their osmolarity; examples are the Malpighian tubules of insects and the specialized organs of saltwater versus freshwater fishes. The reabsorption of water and the production of a hypertonic urine involve establishment of a solute gradient that pulls water from the descending limb of the nephron loop and from the collecting duct. The kidneys keep blood pH at about 7.4 by reabsorbing HCO3– and excreting H as needed. Ammonia buffers H in the urine.
Problems with Kidney FunctionVarious medical conditions, including diabetes, kidney stones, and kidney infections, can lead to renal failure. Renal failure can be treated by hemodialysis using a kidney machine or CAPD, or by a kidney transplant. 
Thinking Scientifically
	1.	Camels can survive for days or even weeks without taking a drink. They can lose an amount of water equivalent to up to 40% of their body weight with no ill effects. This type of water loss would be lethal to any other mammal. What physiological adaptations do you suppose allow a camel to survive so long without drinking water?
	2.	Participants in the Atkins diet avoid carbohydrates, but eat protein-rich foods until they are satiated. Stories of success told by followers of the Atkins diet have led to the current “low-carb” craze apparent in grocery stores, restaurants, and fast-food chains. Why would a diet high in proteins lead to weight loss? Do you think the weight loss is sustainable? What might be some negative side effects of a high-protein diet?
Testing Yourself
Choose the best answer for each question.
	1.	Label the components of the human digestive system in the following illustration.
	2.	If plants did not have cellulose in their cell walls,
	a.	an important nutrient would be missing in the diet of omnivores.
	b.	the teeth of herbivores would more closely resemble those of carnivores.
	c.	more birds would be carnivores.
	d.	herbivores would have a longer digestive tract.
	3.	The stomach
	a. is lined with a thick layer of mucus.
	b. contains sphincter glands.
	c. has a pH of about 6.
	d. digests pepsin.
	e. More than one of these are correct.
	4.	A rumen in cows 
	a. provides them with water.
	b. provides a home for symbiotic bacteria.
	c. helps them digest plant material.
	d. Both b and c are correct.
	5.	Which of the following is not a digestive enzyme?
	a. amylase
	b. trypsin
	c. lipase
	d. nuclease
	e. isomerase
	6.	Which of the following is not a function of the liver?
	a. removal of poisonous substances from the blood
	b. secretion of digestive juices
	c. production of albumin
	d. storage of glucose
	e. production of bile
	7.	Label the components of the human respiratory system in the following illustration.
	8.	Infection of one of the two branches just below the trachea is called
	a. tonsillitis.
	b. meningitis.
	c. a sinus infection.
	d. bronchitis.
	e. laryngitis.
	9.	Birds have a higher gas-exchange capacity than other vertebrates because they
	a. have more alveoli in their lungs.
	b. have a stronger diaphragm.
	c. have a shorter trachea.
	d. never mix fresh air with used air.
	e. breathe at a more rapid rate.
	10.	Label the components of the human urinary system in the following illustration.
For questions 11-–14, identify the kidney component in the key that matches the description. 
Key:
	a. nephron capsule
	b. proximal tubule
	c. nephron loop
	d. distal tubule
	11.	Pumps salt into the renal medulla, and water follows by osmosis.
	12.	Uric acid is eliminated by secretion here.
	13.	Microvilli reabsorb molecules such as glucose here.
	14.	Filtration occurs here. 
	15.	The presence of albumin in the urine is indicative of
	a. kidney stones.
	b. exposure to diuretics.
	c. the inability to adjust blood pH.
	d. hypertension.
	16.	Cleansing inhaled air involves
	a. hairs. 
	b. cilia.
	c. mucus.
	d. All of these are correct.
	17.	Food and air both travel through the
	a. lungs. 
	b. pharynx.
	c. larynx.
	d. trachea.
In questions 18–21, match the functions to the structures in the key.
Key:
	a. glottis
	b. larynx
	c. bronchi
	d. lungs
	18.	Passage of air into larynx.
	19.	Passage of air into lungs.
	20.	Sound production.
	21.	Gas exchange.
	22.	The use of an artificial kidney is known as
	a. filtration.
	b. excretion.
	c. hemodialysis.
	d. None of these are correct.
	23.	Which of the following materials would not be filtered from the blood at the nephron capsule?
	a. water
	b. urea
	c. protein
	d. nutrients
	e. salts
	24.	The renal medulla has a striped appearance due to the presence of which structures?
	a. nephron loop
	b. collecting duct
	c. capillaries
	d. Both a and b are correct.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
As a result of serious injury or illness, a person may enter a persistent vegetative state. In this condition, the person does not recognize anyone and feels no pain. Advances in medical care now allow us to use ventilators and feeding tubes to keep vegetative patients alive for years. A living will allows patients to declare that they do not wish to be maintained in this way, giving doctors the legal authority to discontinue life-sustaining treatments. But for a patient without a living will, family members are left with a difficult decision and sometimes a court battle over what they all think is best. One recent case involved a woman who was kept in a vegetative state since suffering a heart attack in 1990. If her feeding tube were removed, she would die within two weeks. Her husband asked to have the tube removed, saying that would have been his wife’s wish. Her parents, on the other hand, wanted her to be kept alive. They believed she was trying to communicate with noises and facial expressions, although her doctors said those activities were simply reflexes. What would you do if you were a judge and had to determine whether the husband’s or the parents’ wishes should be carried out?
Understanding the Terms
abdominal cavity416
alveolus (pl., alveoli)423
anus420
aortic body424
appendix419
bicarbonate ion426
bile418
bolus416
bronchiole423
bronchus (pl., bronchi)423
carnivore415
carotid body424
chyme418
coelom414
collecting duct428
continuous ambulatory peritoneal 
dialysis (CAPD)430
dialysate430
diaphragm416
diarrhea420
distal tubule428
duodenum420
edema430
emulsification418
endocrine gland420
epiglottis416
esophagus416
exocrine gland420
filtration428
gallbladder418
glottis423
glucagon420
heme426
hemodialysis430
herbivore415
insulin420
intestinal enzyme418
kidneys427
lacteal418
large intestine419
larynx423 
lipase418
liver418
lungs422
Malpighian tubule429
microvillus (pl., microvilli)418
mouth415
nasal cavity422
nephron427
nephron capsule428
nephron loop428
nuclease418
omnivore415
pancreas418
pancreatic amylase418
pepsin417
peristalsis416
pharynx416, 423
polyp420
proximal tubule428
reabsorption428
renal cortex427
renal medulla427
renal pelvis427
respiration422
salivary amylase416
salivary gland416
secretion428
sinus422
small intestine418
stomach417
thoracic cavity416
trachea (pl., tracheae)423
trypsin418
ureter427
urethra427
urinary bladder427
urine427
villus (pl., villi)418
vocal cord423
Match the terms to these definitions:
a.	_______________	The digestive and respiratory passages come together here.  
b.	_______________	A hydrolytic enzyme that breaks down proteins in the stomach. 
c.	_______________	The name for the contents of the digestive tract as they move from the stomach to the intestine. 
d.	_______________	The storage organ for bile.  
e.	_______________	The voice box. 
f.	_______________	The windpipe; it connects the larynx to the bronchi. 
g.	_______________	Small air pockets in bronchioles.  
h.	_______________	The iron-containing group in hemoglobin.  
i.	_______________	Tiny urine-producing tubules in the kidney. 

The respiratory system is one of the last organ systems to develop in the human fetus.
A urinalysis can involve any of 35 different tests. 
The small intestine of humans is about 6 meters (19.7 feet) long.
Figure 24.2Complete digestive system.
The earthworm (a) and humans (b) have complete digestive systems. A complete digestive system leads to specialization of organs along the digestive tract. 
Figure 24.1Keeping the internal environment steady.
The digestive system takes in food and digests it to nutrient molecules that enter the blood. The blood transports nutrients to the tissues where exchange occurs with tissue fluid. Later in this chapter we will also consider the manner in which the respiratory system and the urinary system help keep the internal environment relatively constant.
Figure 24.3Dentition among mammals.
Check Your Progress
	1.	List the three organ systems that make exchanges with the external environment and blood.
	2.	Compare and contrast the human digestive system with that of an earthworm.
	3.	List the functions of a digestive tract.
	4.	Explain why carnivores do not need teeth for grinding food.
	5.	Describe the function of peristalsis in digestion.
Answers:1. Digestive, respiratory, and urinary systems.2. Both are tube-within-a-tube plans, but the human system has many more specialized organs than that of an earthworm.3. Ingest and break down food, absorb nutrients, and eliminate indigestible material.4. Meat is easy to break down because it does not contain cellulose, so extensive chewing is not necessary.
5. Peristalsis moves materials through the digestive tract.
Figure 24.6A ruminant’s stomach.
Ruminants eat grass, which is made of cells with strong cellulose walls. The first chamber of a ruminant’s stomach, called the rumen, contains symbiotic bacteria and protozoans that can digest cellulose. After a first pass through the rumen, the “cud” returns to the mouth where it is leisurely chewed. Then, it may return to the rumen for a second go-round of digestion before passing through to the true stomach. 
Figure 24.5Anatomy of the human stomach.
a. The stomach has a thick wall that expands as it fills with food. The wall contains three layers of muscle, and their presence allows the stomach to churn and mix food with gastric juices. The mucosa of the stomach wall secretes mucus and contains gastric glands, which secrete gastric juices active in the digestion of protein. b. Peristalsis, a rhythmic contraction, occurs along the length of the digestive tract.
Figure 24.4The human mouth, pharynx, esophagus, and larynx.
a. The palates (both hard and soft) separate the mouth from the nasal cavities. The pharynx leads to the esophagus and the larynx. b. When food is swallowed, the soft palate closes off the nasal cavities, and the epiglottis closes off the larynx. 
Figure 24.8Small intestine and absorption of nutrients.
The surface area of the small intestine is increased by three modifications: (a) circular folds, (b) villi, and (c) microvilli. The blood vessels of the villi absorb amino acids and sugars. Lacteals, which are lymphatic capillaries, absorb glycerol and fatty acids.
Figure 24.9Digestive tract of a carnivore compared to a ruminant herbivore.
The digestive tract of a carnivore (a) is much shorter than that of a ruminant herbivore (b) because proteins can be more easily digested than plant matter.
Figure 24.7The pancreatic and bile ducts empty into the duodenum.
Bile, made by the liver and stored in the gallbladder, and pancreatic juice, which contains enzymes, enter the duodenum by way of ducts.
Check Your Progress
	1.	Describe the functions of the stomach.
	2.	
Describe the relationship between the duodenum and the liver and pancreas. 
Answers:1. The stomach stores food and continues digestion, kills microbes, and moves partially digested material into the intestines.2. The liver and pancreas secrete bile and pancreatic juice, respectively, into the duodenum.
Figure 24.10The digestive organs and their 
functions. 
A review of the processing of food to nutrient molecules and their absorption into the body. 
Figure 24.11Hepatic portal system.
The hepatic portal vein takes the products of digestion from the digestive system to the liver, where they are processed before entering hepatic veins. 
Check Your Progress
Contrast the functions of the small intestine and the large intestine.
Answer:The small intestine digests all types of food, and it produces enzymes that complete the breakdown of food to small molecules that can be absorbed across its villi. No digestion occurs in the large intestine, but it does store digestive remains until they are eliminated. The large intestine also absorbs water, salts, and some vitamins. 
Check Your Progress
List the functions of the liver.
Answer:The liver detoxifies the blood, produces plasma proteins, produces bile, stores glucose as glycogen, and produces urea. 
Figure 24.12Keeping the internal environment steady.
The respiratory system functions in gas exchange. CO2-laden blood enters the pulmonary capillaries and then CO2 diffuses into the lungs and exits the body by way of respiratory passages. O2-laden air enters the respiratory passages and lungs. Then O2 diffuses into the blood at the pulmonary capillaries.
Figure 24.13Ciliated cells of the respiratory 
passages. 
These cilia sweep impurities up away from the lungs toward the throat where they can be swallowed. Smoking first inactivates and then destroys these cilia. 
Figure 24.14The human respiratory tract.
To reach the lungs, air moves from the nasal cavities, through the pharynx, larynx, trachea, bronchi, and finally the bronchioles, which end in the lungs.
Figure 24.15Tracheae of insects.
A system of air tubes extending throughout the body of an insect carries oxygen to the cells. Air enters the tracheae at openings called spiracles. From here, the air moves to smaller tubes, which take it to the cells where gas exchange takes place. a. Diagram of tracheae. b. The photomicrograph shows how the walls of the trachea are stiffened with bands of chitin.
Check Your Progress
	1.	List the three steps of respiration in a complex animal.
	2.	Contrast respiration in insects with that in humans.
	3.	
Explain how air moves into the lungs during breathing.
Answers:1. The three steps of respiration are breathing, external exchange, and internal exchange.2. Insects have many tracheae that branch into tubes, which deliver oxygen to the cells. Humans have a single trachea that leads to the bronchi and then to the bronchioles, which enter the lungs where gas exchange occurs.3. Muscle contractions that lower the diaphragm and raise the ribs cause negative pressure that pulls air into the lungs.
Figure 24.16Inspiration versus expiration.
a. During inspiration, the thoracic cavity and lungs expand so that air is drawn in. b. During expiration, the thoracic cavity and lungs resume their original positions and pressures, forcing air out.
Figure 24.17Breathing in birds.
Two types of air sacs are attached to the lungs of birds. When a bird inhales, most of the air enters the abdominal air sacs, and when a bird exhales, air moves through the lungs to the thoracic air sacs before exiting the trachea. This one-way flow of air through the lungs allows more fresh air to be present in the lungs with each breath, and this leads to greater uptake of oxygen from one breath of air. 
Figure 24.18Gas exchange in the lungs.
Bronchioles lead to the alveoli, each of which is surrounded by an extensive capillary network. Notice that the pulmonary artery and arteriole carry O2-poor blood (colored blue) and the pulmonary venule and vein carry O2-rich blood (colored red). 
Figure 24.19Gills of bony fishes.
Gills are finely divided into filaments, and each filament has many thin, platelike lamellae. Gases are exchanged between the capillaries inside the lamellae and the water that flows between the lamellae. 
Check Your Progress
	1.	Explain why oxygen diffuses into the blood from the lungs during external exchange but diffuses out of the blood into tissue fluid during internal exchange.
	2.	Explain how carbon dioxide is carried in the blood from the tissues to the lungs.
Answers:1. Oxygen follows its concentration gradient; there is more oxygen in the lungs than the blood and there is more oxygen in the blood than in tissue fluid. Also, environmental conditions in the tissues causes hemoglobin to give up oxygen.2. Carbon dioxide combines with water to form carbonic acid. Carbonic acid breaks down to the bicarbonate ion (HCO3) and H, which combines with hemoglobin. 
Figure 24.20Hemoglobin.
Hemoglobin has four polypeptides, and each one is associated with a heme group. Oxygen bonds with the central iron atom of a heme group.
Figure 24.21Neural control of breathing rate.
The brain regulates the breathing rate by controlling contraction of the rib cage muscles and the diaphragm. When the breathing rate increases, [H] lowers as CO2 is removed from the blood, and the pH of the blood returns to normal. 
Figure 24.22Keeping the internal environment steady.
Excretion carried out by the kidneys serves three functions: excretion of nitrogenous wastes, maintenance of water-salt balance, and maintenance of pH. 
Figure 24.23The human urinary system.
a. The human urinary system produces, stores, and expels urine from the body. b. The human kidney has three sections, which can be correlated with (c) the presence of about one million nephrons. Nephrons do the work of the kidney.
Figure 24.24Urine formation.
Urine formation requires three steps: filtration, reabsorption, and secretion. In addition, humans are able to produce a hypertonic urine because water is reabsorbed into the blood along the length of the nephron and especially at the nephron loop.
Figure 24.26Acid-base balance.
In the kidneys, bicarbonate ions (HCO3) are reabsorbed and hydrogen ions (H) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH3), which becomes ammonium (NH4). Ammonia is produced in tubule cells by the deamination of amino acids. 
Figure 24.25Malpighian tubules.
The Malpighian tubules of insects are attached to the gut and surrounded by the hemolymph of the open circulatory system. K is secreted into these tubules, drawing in water by osmosis. Much of the water and K is reabsorbed across the wall of the rectum.
Check Your Progress
	1.	List the three functions of the kidney. 
	2.	List the three steps involved in urine formation.
Answers:1. The kidney functions are excretion of nitrogenous wastes, maintenance of the water-salt balance of blood, and maintenance of the acid-base balance of blood.2. Urine formation: filtration of small molecules from a blood capillary to the inside of a capsule; reabsorption of water and nutrients; and secretion of potentially harmful substances into the nephron.
Figure 24.27An artificial kidney machine. 
As the patient’s blood is pumped through dialysis tubing, the tubing is exposed to a dialysate (dialysis solution). Wastes exit from blood into the solution because of a preestablished concentration gradient. In this way, blood is not only cleansed, but its water-salt and acid-base balances can also be adjusted.
Check Your Progress
	1.	Explain how the kidneys regulate blood pH.
	2.	Explain how an artificial kidney machine works.
Answers:1. The kidneys reabsorb bicarbonate ions and excrete hydrogen ions as needed to maintain blood pH.2. The patient’s blood passes through a membranous tube filled with dialysate. The dialysate is continuously replaced to maintain favorable concentration gradients so that appropriate materials diffuse into and out of the blood.