Excretion and Water Balance

Transcription

1 Excretion and Water Balance 13 After you have finished reading this chapter, you should be able to: Discuss Compare Describe the importance of the kidneys, skin, and liver to excretion and to homeostasis. and contrast the process of excretion in different organisms. the structures and functions of kidneys in humans. Day after day, day after day, We struck, nor breath nor motion; As idle as a painted ship Upon a painted ocean. Water, water, everywhere, And all the boards did shrink; Water, water, everywhere Nor any drop to drink. Samuel Taylor Coleridge, The Rime of the Ancient Mariner Introduction Imagine yourself in the middle of a calm sea. No winds blow. Your sailing ship is unable to move. You have been at this place in the ocean for weeks. Food and water have been used up. You and your fellow sailors are becoming very thirsty. In every direction, stretching as far as the horizon, all you can see is water. And yet, as the Ancient Mariner said in the poem by Coleridge, there is not a drop to drink. How could this be? (See Figure 13-1 on page 276.) Ocean water is salty. When dried, ocean salt is almost chemically identical to table salt. Salt, a solute, is only one of many substances that can be dissolved in water. This is true of the water in the human body. The 275

2 Figure 13-1 You can be sailing in the middle of an ocean and still not have a drop to drink, since seawater is salty, and we can drink only freshwater. human body is about 70 percent water. This water contains many types of solutes, including table salt. The amounts of these solutes in our bodies must remain constant day by day, and hour by hour. We have seen how regulating our internal body temperature is an important example of homeostasis. An even more important function of homeostasis is maintaining a constant internal chemical environment. Drinking salt water disrupts the careful balance that our bodies need internally. The results could be very dangerous, perhaps even fatal. We cannot drink salt water, only freshwater. WATER COMPARTMENTS In the body, water is found in three areas, or compartments: Plasma, the liquid portion of the blood without the blood cells, makes up about 7 percent of body fluid. The intercellular fluid and lymph make up about 28 percent of body fluid. The intracellular compartment, the fluid located inside cells, makes up about 65 percent of body fluid. The real meaning of the cell theory is that we are only as healthy as our cells are. As you might expect, the chemical contents of our cells are therefore regulated carefully. We have seen that all body cells are surrounded by pools of intercellular fluid, which is mostly water. Because the level of chemicals in cells must remain within very narrow limits, it is essential that the composition of the intercellular fluid remains within

3 Chapter 13 / Excretion and Water Balance 277 narrow limits, too. How is the chemical composition of the intercellular fluid regulated? As you know, capillaries deliver blood to every cell in the body. Capillaries exchange materials with the intercellular fluids that surround each cell. It is the job of the blood, and more specifically the blood plasma, to control the levels of substances in the fluid that surrounds each cell. So, one of the most important jobs of homeostasis is regulating the chemical composition of blood plasma. (See Figure 13-2.) Chemical composition of body fluids + Healthy changes Unhealthy changes Normal Figure 13-2 If we are to remain healthy, the composition of our body fluids must remain within very narrow limits. In terrestrial vertebrates, including ourselves, the organ primarily responsible for regulating the chemical composition of plasma is the kidney. Approximately 25 percent of the total blood flow in the body passes through the kidneys at any moment, even though the kidneys make up only 1 percent of the body s total weight. When kidneys do not work properly, it results in blood that does not have the proper balance of chemicals. In time, this can cause death, unless the kidneys functions are taken over by mechanical means. Let us now examine what the kidneys do and learn how they function. THE KIDNEYS A single bag of potato chips contains enough salt to kill you. Of course it doesn t, because your body does not allow this salt to be added directly to your body s fluids. One of the jobs of the kidneys is to maintain the proper levels of both salt and water in the body. Improper levels of salt cause a number of potential problems. The sodium and chloride ions of salt can disrupt normal chemical activity. For example, if for some reason your body did increase the level of salt in its fluids, your heart might no longer beat properly. Salt also affects the rate at which osmosis occurs. Water diffuses across a cell membrane toward an area of higher salt concentration. Therefore, the job of regulating both salt and water levels is

4 278 Maintaining a Dynamic Equilibrium often called maintaining a water balance in the body. This is one of the functions of the kidneys. The kidneys are also part of the sanitation system of the body. All metabolic activities in the body produce wastes. These wastes are toxic and must not be allowed to accumulate in the body. So another important function of the kidneys is to carefully select the chemical wastes for removal while keeping useful nutrients. This process of ridding the body of metabolic wastes is called excretion. WASTES THAT NEED TO BE REMOVED You have already learned that cellular respiration produces carbon dioxide and water. The waste, carbon dioxide (CO 2 ), is released from the body primarily through gas exchange that occurs in the respiratory system. Carbon dioxide leaves the blood and enters the air in the alveoli of the lungs. Other wastes produced during metabolic activities include water from condensation synthesis and a variety of mineral salts from other metabolic processes. However, by far the most important and potentially dangerous metabolic wastes produced in the body contain the element nitrogen. These nitrogenous wastes result when amino acids, the building blocks of proteins, are broken down. The main nitrogenous waste produced during this process is ammonia (NH 3 ). (See Figure 13-3.) Ammonia is a toxic compound that can kill living cells. Open a bottle of household cleanser that contains ammonia and you will smell its unpleasant characteristic odor. All animals produce ammonia in their cells as they metabolize proteins. How do they get rid of this toxic waste? For an ameba, and some aquatic animals, ammonia diffuses into the water through the cell membrane. Other animals cannot get rid of ammonia so easily. These animals must change ammonia into a less harmful substance. Birds, terrestrial reptiles, and insects change ammonia into solid crystals of uric acid. This uric acid is mixed with solid, undigested wastes from the digestive system and passed out of the body. Any person trying to avoid being hit by bird droppings is trying to steer clear of these wastes. Nitrogen is a good fertilizer for plants, however. In some places, where large numbers of birds roost, these nitrogen-rich wastes accumulate in vast amounts. In the past, these droppings were shoveled into bags and sold for high prices as a potent plant fertilizer. Today, these natural sources of nitrogen have been replaced with nitrogen from ores that have been processed in factories. Mammals have evolved a different solution for dealing with ammonia

5 Chapter 13 / Excretion and Water Balance 279 Tanks-a-Lot Ammonia Out! INSTANTLY DETOXIFIES AMMONIA! WORKS INSTANTLY TO: Detoxify Ammonia Formed by Fish Waste Eliminate Ammonia Stress; Protect Healthy Gill Function Remove Chlorine and Chloramine from Tap Water Treats 60 Gallons (227 L) of Freshwater or Salt Water HAPPY FISH PRODUCTS, INC. NET 1 FL OZ. (30 ml) PRODUCT NO. 13 Figure 13-3 All animals produce ammonia a toxic compound when they metabolize amino acids. Ammonia that is formed by fish waste in an aquarium must be chemically treated to prevent damage. wastes. In mammals, ammonia is mostly changed into urea, not uric acid. Unlike uric acid, which does not dissolve in water, urea is very soluble in water. To be removed from the body, urea must be dissolved in water. The ammonia, now in the form of urea, is diluted and removed from the body in urine. EVOLUTION AND EXCRETION A variety of solutions to the problem of maintaining a constant internal chemical environment can be seen in different organisms. We have seen that single-celled organisms, such as an ameba and a paramecium, rid themselves of wastes by simple diffusion across their cell membrane. However, these protists have evolved a structure that handles the problem of maintaining water balance. These protists live in freshwater, where the level of solutes inside them is higher than in the external freshwater. As a result, water tends to diffuse into these organisms. A unique organelle, the contractile vacuole, takes in this excess water in the cell and pumps it out of the organism and back into the environment. In this way, the proper level of water is maintained in the cell. Because the diffusion of

6 280 Maintaining a Dynamic Equilibrium Figure 13-4 The paramecium rids itself of wastes by diffusion across its cell membrane. The contractile vacuole aids it in osmoregulation (see star-shaped organelle in upper half of paramecium). water is called osmosis, the process of maintaining water balance is called osmoregulation. (See Figure 13-4.) Some multicellular animals, such as sponges, sea anemones, and sea stars, have no special organs for osmoregulation and excretion. Instead, the exchange of water and wastes between their cells and the environment occurs by diffusion through the membranes of the cells on their body surfaces. Most other animals, from earthworms to crabs to polar bears, have some type of internal organ that contains tubelike structures to carry out chemical regulation. By examining this structure in earthworms, we can learn about the kidneys in humans. One thing you notice when you examine an earthworm is its segments often, more than 100 of them. Each segment contains a pair of twisting tubes, each called a nephridium. The tube has an opening into the previous segment. The earthworm s body fluids pass into the opening. As these fluids move through the nephridia (tubes), useful materials are absorbed out of the tubes and into the capillaries that surround the tubes. The useful materials stay in the earthworm s blood. The wastes remain in the nephridia and pass out to the external environment through openings in the earthworm s body wall. (See Figure 13-5.) The waste gas CO 2 diffuses directly from capillaries just below the earthworm s moist skin cells into the air between particles of soil. This reminds us of the process of excretion that occurs in the alveoli in the human lungs. During and just after a soaking rain, earthworms crawl out of their burrows in the ground onto the soil s surface. When the soil is saturated with water, an earthworm cannot get rid of waste gases. It cannot take in oxygen either. If it remains in wet soil, it might suffocate.

7 Chapter 13 / Excretion and Water Balance 281 Blood vessel Liquid wastes Capillaries Septum Ciliated funnel Excretory tube Body cavity Skin Opening to outside Figure 13-5 Nephridia tubes remove wastes from the body fluids of the earthworm. Having seen the structure of the nephridia tubes in earthworms, we can now examine how the human kidneys work. Check Your Understanding What are some ways in which an excretory system helps organisms maintain homeostasis? THE HUMAN KIDNEY: STRUCTURE AND FUNCTION The kidneys are the most important part of the excretory system in humans. There are two kidneys, located in the lower, rear portion of the abdominal cavity. (See Figure 13-6 on page 282.) Every boxer knows which punches are illegal a punch to the lower back area, on either side of the spinal column, can damage a kidney and is therefore banned. Large blood vessels bring blood to the kidneys. Other blood vessels leave the kidneys with the newly cleansed blood. As blood passes through the kidneys, metabolic wastes are removed, and the correct balance of salt and water in the blood is maintained. As a result, the kidneys produce urine. Urine leaves each kidney through a tube, the ureter. The two ureters constantly drip urine into the urinary bladder. Here the urine is stored until it is convenient to pass it from the body. Urine leaves the body through the urethra when instructions from the brain are given. A close look at the kidney s structure gives some idea of how this important organ works. If you examined a section of the kidney, you would see three large tubes at the center of the concave surface. One of these tubes is the renal artery, which brings blood to the kidney from the

8 282 Maintaining a Dynamic Equilibrium Solid region Cortex Medulla Hollow region Artery Kidney (cut) Vein Kidney (uncut) Figure 13-6 Human kidneys are located in the lower, rear portion of the abdominal cavity. Urethra Ureter Urinary bladder body; another is the renal vein, which returns blood to the body; and the third tube is the ureter, which connects each kidney to the urinary bladder. The inner portion of the kidney is hollow. This is where the urine is collected. The outer portion of the kidney consists of the medulla and the cortex. (See Figure 13-7.) If you examined some tissue from the kidney under a microscope, you would see that the blood vessels go through the medulla and into the cortex. In the cortex are the beginnings of many tiny, individual tubular structures that extend into the medulla. These structures are called nephrons. It is in the nephrons that the real work of the kidney occurs. There are about a million nephrons in each kidney. So you have to jump in even closer for a microscopic view of an individual nephron. Medulla Cortex Renal artery Renal vein Figure 13-7 The outer portion of the kidney consists of the medulla and cortex. The inner portion, where urine collects, is hollow. Renal pelvis Ureter

9 Chapter 13 / Excretion and Water Balance 283 Under a microscope, you can observe the structure of a nephron. It is a tube whose walls are a single cell layer thick. You can see the complex network of capillaries wrapped around each nephron. Blood is cleansed, water balance is maintained, and urine is produced through the exchange of materials between the capillaries and the nephron. The processes of excretion and chemical regulation begin in the nephron as blood from the renal artery enters a tight bundle of capillaries called the glomerulus. Inside these capillaries the blood is under high pressure. Filtration of the blood occurs here. Twenty percent of the liquid material of the blood, including the chemicals in the fluid, is forced out of the glomerular capillaries into a cuplike structure of the nephron that surrounds them. Almost all that remains behind in the capillaries are the relatively large blood cells and protein molecules. The cuplike structure is called Bowman s capsule. It is named after Sir William Bowman ( ), an English physician who gained fame because of his work on how the kidneys function. (See Figure 13-8.) Is this filtration sufficient to clean the blood and return it to the body? Definitely not, for two important reasons. First, so much fluid is forced out of the blood here that we would have to drink 7.5 liters of liquid every hour 180 liters per day to replace the lost liquid. We would do little else than drink and urinate all day! Second, the process of filtration does not sort materials. Harmful wastes and valuable nutrients alike are forced out. Amino acids, sugar, salts, vitamins, and other nutrients are removed from the blood at this point. Therefore, the process of filtration alone is not enough to maintain homeostasis. Glomerulus Capsule Tubule Solid outer region of kidney Artery Urine flow Capillary Collecting tubule Vein Hollow inner region of kdney Figure 13-8 The structure of the nephron.

10 284 Maintaining a Dynamic Equilibrium Living with Kidney Failure Several years ago, a young woman named Jeanette fell and ruptured her patella, the knee bone. Surgery was planned to repair it, but this was abruptly cancelled when blood tests showed that Jeanette was in kidney failure. A few decades ago, this would have meant certain death. Now, however, Jeanette had options. The one she chose was at-home hemodialysis. Kidney failure, now called ESRD (end-stage renal disease), leads to a buildup of wastes in the blood, higher concentrations of salt and water, and unbalanced ph levels. Homeostasis is seriously disrupted. Without kidney function, life cannot continue. Fortunately, however, treatments have been developed to artificially cleanse the blood. The treatments involve the exchange of substances across membranes in a process called dialysis. In the 1960s, when the first artificial kidney was developed, a patient had to be connected to a dialysis machine for 8 to 10 hours at a time, several times a week. There were also serious side effects from the treatment, such as severe infections and inflammation around the heart. The many options that Jeanette and other ESRD patients today can choose from are remarkable. The two basic choices are hemodialysis and peritoneal dialysis. With hemodialysis, the patient s blood passes through tubes into a machine next to him/her, where materials are exchanged between the blood and fluids in the machine. The patient has the option to undergo hemodialysis at home, like Jeanette did, or in the hospital, and at different times during the week. Some people have dialysis treatment every day, others two or three times a week. The other kind of treatment, peritoneal dialysis, amazingly uses the lining of the patient s own abdomen to filter the blood. A cleansing solution is transported into the abdomen, where it stays for a while. As the person s blood passes in capillaries through the solution, the blood gets cleansed. Then the solution is drained out, and a fresh solution is put into the patient s abdomen to start the process again. Peritoneal dialysis can be done in several ways. One method uses no machine; it just goes on continuously, even as the patient walks around. Another way uses a machine to move the cleansing solution through a tube, in and out of the person s abdomen. The machine does this at night while the person sleeps. Another method also uses a machine, but only at certain times, and usually in the hospital. So there are choices for people with kidney failure, choices that allow those who no longer have the use of an essential internal organ the kidney to go on living.

11 Chapter 13 / Excretion and Water Balance 285 The next stage, reabsorption, occurs as the filtrate passes through the nephron tubule. By the process of active transport, glucose, salts, and other valuable nutrients are moved back from the filtrate into the blood in the capillaries. As these solutes move out of the filtrate, the concentration of water inside the nephron tubule becomes higher than outside it. As a result, by osmosis, water automatically moves out of the filtrate. In fact, almost 99 percent of the liquid in the filtrate is reabsorbed back into the blood. As a result, the quantity of urine we release is usually between one and two liters a day. The final stage of kidney function occurs in the last portion of the nephron tubule. Secretion of additional wastes from the blood, including urea, occurs here. This process of secretion is one of the final steps in maintaining homeostasis of the fluids in the body. Regulating the ph of the blood is one more way the kidneys maintain homeostasis. In this case, if the blood becomes too acidic, hydrogen ions are removed from the blood. The reverse occurs if the blood is too basic. The result of all this activity in the kidneys is that the blood returns through the renal vein to the body, newly cleansed of wastes and properly balanced with salts and water. In addition, urine that contains wastes, as well as any excess salts and water, is collected and removed from the body. REGULATION OF THE EXCRETORY SYSTEM The purpose of the excretory system is to maintain homeostasis by regulating the chemical composition of the blood and in turn of all the body s cells. At times, both the endocrine system and the nervous system help to carry out this extremely important job. Perhaps, on occasion, you have eaten an entire bag of salty potato chips. As we have said, the salt in that one bag, if it all got into your blood, could be dangerous. However, all the salt in the bag of potato chips does not get into your blood. Why? As soon as the sodium level in your blood starts to rise, the adrenal glands the endocrine gland located on top of each kidney respond by secreting less of the hormone aldosterone. This hormone, when it reaches the kidney, causes nephrons to reabsorb salt from the urine into the blood. Smaller amounts of the hormone result in less reabsorption of salt. Therefore, after you have eaten the salty chips, more salt is excreted in your urine. (See Figure 13-9 on page 286.) Regulation of the level of water in the body is very important. Consider when you have been exercising a great deal, sweating a lot, and drinking little to replace your water loss. Your body has been losing water. The result is that the volume of your blood decreases, since it consists

12 286 Maintaining a Dynamic Equilibrium Figure 13-9 The adrenal glands help regulate the amount of salt in our blood: Sodium level rises; adrenal glands secrete less aldosterone; kidneys reabsorb less salt from urine into blood; more salt is excreted in urine; homeostasis is maintained. Adrenal glands Kidney Kidney mostly of water. It is the blood and not your cells that loses the water. Our bodies know that it is absolutely necessary to keep the right amount of water inside our cells in the intracellular compartment. However, once again the lower volume of the blood acts as a stimulus to the endocrine and nervous systems. This time the hypothalamus in the brain detects the lower blood volume. It directs the posterior pituitary gland to secrete antidiuretic hormone (ADH). When ADH reaches the kidneys, it increases the reabsorption of water from the urine back into the blood. The blood volume returns to normal. Meanwhile, solutes, including urea and urobilin a yellow-colored waste become more concentrated in the urine. The urine becomes darker. The darker color of the urine shows that the body is automatically readjusting itself by not releasing as much water. Why do we feel thirsty? To be aware of this feeling, the cerebral cortex of the brain must be involved. It is the hypothalamus once again that does this job. In addition to releasing ADH when blood volume is low, the hypothalamus sends a message to the cerebral cortex. This message is the feeling you know as thirst. So you drink more water and, once again, homeostasis is maintained. HOMEOSTASIS AND THE SKIN The skin is the largest organ of the body. It is made up of a variety of different types of cells and tissues. (See Figure ) There are two separate layers in the skin: the outer layer, or epidermis, and the inner layer, or dermis. If we examine the structure of these layers, we can see how they function on behalf of homeostasis. It should be no surprise that the organ that surrounds the body has important roles in keeping the conditions that exist inside us fairly constant. The epidermis is a very thin layer. New skin cells are formed at the bottom of this layer by cell division. These cells eventually push their way toward the surface, replacing the dead skin cells there. This process occurs

13 Chapter 13 / Excretion and Water Balance 287 Hair Muscle Pore Dead cells Region of dividing cells Epidermis Sebaceous gland Hair pocket Hair root Nerve ending Capillary Tubule of sweat gland Fat Sweat gland Dermis Figure The skin, the largest organ of the body, helps us maintain homeostasis. all the time. We constantly leave behind a trail of dead skin cells wherever we have been. These epidermal cells are very tough. They form a protective layer that prevents bacteria and harmful chemicals from entering, and water from leaving, our body. Beneath this layer are cells that produce the pigments that give skin its color. The dermis is a thick, complex layer beneath the epidermis that contains a variety of structures. There are sensory nerve endings that can detect temperature and touch; hairs; and capillaries that help regulate body temperature (as discussed in Chapter 10). There also are muscles attached at the bottom ends of hairs. These muscles are able to cause the hairs to straighten, a response to fear in some animals. Some animals also conserve heat by causing their hairs to stand up. This increases the amount of air between the hairs, which insulates the body against heat loss. Early humans evolved in warm, tropical climates, where it was a disadvantage to be covered with hair. Thus, less body hair was a trait that was selected for, and humans lost the fur that most other mammals have. When these little muscles in our skin contract, the result is simply what we call goose bumps. In the dermis, there also are sebaceous glands, which keep skin and hair soft by releasing oils. Finally, there are sweat glands. Perspiration from sweat glands contains some urea, so they can be considered excretory structures. The main role of perspiration, however, is to assist in regulating temperature by cooling the body through evaporation.

14 288 Maintaining a Dynamic Equilibrium Beneath the dermis is a layer of fatty tissue that many people wish were missing. However, fatty tissue contributes to homeostasis by acting as an insulator. It prevents the loss of body heat to the environment. HOMEOSTASIS AND THE LIVER No theme on homeostasis and excretion would be complete without talking about the liver. The liver is the largest organ in the abdominal cavity. It is involved in homeostasis by assisting most of the important systems of the body. (See Figure ) The liver helps in excretion by removing nitrogen from waste amino acids and turning it into urea. The liver adds this urea to the blood. When the blood reaches the kidneys, this urea is removed in the nephrons. Red blood cells live for about 90 days. The liver breaks down old red blood cells. The components of these old cells are recycled to make new red blood cells. Chemical poisons are also made harmless by the liver. This is called detoxification. For example, alcohol considered a poison by the body is detoxified in the liver. Drinking excess alcohol makes the liver work very hard. Continued heavy drinking can, in time, cause liver disease. A clear example of homeostasis is the need to keep blood sugar levels constant. The liver stores excess glucose in the form of the polysaccharide glycogen. The liver releases stored glycogen into the blood when glucose is needed. Figure The liver turns amino acid wastes into urea, breaks down old red blood cells, and detoxifies chemical poisons such as alcohol. Liver Small intestine Stomach Large intestine

15 Chapter 13 / Excretion and Water Balance 289 The list of jobs of this 1.4-kilogram chemical factory, the liver, goes on. In Chapter 8, we saw how the liver helps the digestive process by making bile, the substance used by the body to digest fats. The liver packages fats for transport in the blood, controls the level of cholesterol in the blood, and stores vitamins A, D, and E. Finally, the liver helps maintain water balance between the blood and the intracellular fluid by making plasma proteins, important solutes in the blood. WHEN THINGS GO WRONG: DISEASES OF THE EXCRETORY SYSTEM One of the most common diseases of the kidney is nephritis, inflammation of the kidney. One way this disease can be detected is by a microscope examination of the urine. Cells and chemicals that normally should not be in the urine are indicators of the disease. Nephritis occurs most often in children and adolescents. Nephritis is caused by a bacterial infection and can usually be treated with antibiotics. Occasionally, however, nephritis lasts a long time and can lead to kidney failure. Kidney stones occur when a chemical compound that contains the mineral calcium builds up in the kidney. In some cases, an improper diet leads to kidney stones. However, in most cases, the cause of kidney stones is unknown. A kidney stone can produce a great deal of pain when it travels through the ureter. Once the kidney stone is in the urinary bladder, it often leaves the body unnoticed. If the stone is too large to be passed, surgery to remove it, or the use of shock waves to break it into smaller pieces, may be necessary. High levels of the nitrogenous waste uric acid in the blood causes gout, which occurs more often in men. The uric acid forms sharp crystals that produce severe pain in joints, usually in the big toe and sometimes in the anklebones. Treatment of gout includes drinking a lot of water, eating a special diet, and sometimes taking medication to lower the level of uric acid in the blood. As was mentioned, liver disease can occur when too much alcohol is consumed. The liver needs to work extra hard to metabolize alcohol. Some by-products of this breakdown of alcohol, such as fats, accumulate in the liver. This causes scarring of the liver cells, called cirrhosis. Cirrhosis causes more of the liver cells to stop working. As the liver shuts down, homeostasis can no longer be maintained. Cirrhosis of the liver is the ninth leading cause of death in the United States.

16 LABORATORY INVESTIGATION 13 How Can We Test for the Presence of Certain Substances in Urine? INTRODUCTION The wastes produced by our cells are filtered from the blood in the kidneys. Then these wastes are eliminated from the body in urine. The kidneys also reabsorb many materials that must be returned to the blood. The kidneys are excretory organs that carry out the process of homeostasis in the body. During medical examinations, physicians often test urine for the presence of substances that may indicate a disease. Urine is also tested to determine if a person has used an illegal or controlled substance. The chemical testing of urine is called urinalysis. In this investigation, you will analyze mystery substances to determine what they contain. MATERIALS Artificial urine samples, test tubes, test-tube holder, test-tube rack, heatresistant 500-mL beaker, tap water, hot plate, graduated cylinder, 10% acetic acid, Benedict solution, silver nitrate, biuret solution, safety goggles PROCEDURE CAUTION: Wear safety goggles during this laboratory investigation. 1. Your teacher will give you eight urine samples. Label eight clean test tubes the same way these samples are labeled. Place the labeled test tubes in your rack. 2. Prepare a hot-water bath by filling the beaker half full with tap water and placing it on the hot plate. Turn on the hot plate. 3. Test for phosphates: Place 10 ml of each of the two phosphate-urine samples into your two appropriately labeled test tubes. Look at the top of the liquid in the tube as you add five drops of acetic acid. Repeat for the other sample. Record your observations. 4. Test for glucose: Place 5 ml of each of the two glucose-urine samples into your appropriately labeled test tubes. Add five drops of Benedict solution to each test tube. Place both tubes in the hot-water bath for two minutes. Observe and record any color changes. 290 Maintaining a Dynamic Equilibrium

17 5. Test for chlorides: Place 5 ml of each of the two chloride-urine samples into your appropriately labeled test tubes. Carefully add three drops of silver nitrate to each test tube. Observe the surface of the solution in each tube and record any changes you observe. 6. Test for albumin (protein): Place 10 ml of each of the two albumin-urine samples into your appropriately labeled test tubes. Add five drops of biuret solution to each test tube. Observe and record the results. 7. Make a data table of your results. 8. Obtain an unknown urine sample from your teacher. The unknown samples will vary; some may test positive for one or more substances, some may test negative for one or more substances. Design and carry out an investigation to determine the substances present in your sample. INTERPRETIVE QUESTIONS 1. Human blood contains molecules of glucose and albumin, along with phosphates and chlorides. Urine normally contains phosphate and chloride ions. What does this information indicate about the functions of the kidneys? 2. Why may the presence of glucose in the urine be a cause of concern? 3. What may be indicated by the presence of albumin in the urine? Chapter 13 / Excretion and Water Balance 291

18 292 Maintaining a Dynamic Equilibrium CHAPTER 13 REVIEW Answer these questions on a separate sheet of paper. VOCABULARY The following list contains all of the boldfaced terms in this chapter. Define each of these terms in your own words. aldosterone, ammonia, antidiuretic hormone, Bowman s capsule, cirrhosis, contractile vacuole, dermis, detoxification, epidermis, excretion, filtrate, glomerulus, kidney, nephridium, nephrons, nitrogenous wastes, osmoregulation, sebaceous glands, sweat glands, urea, ureter, urethra, uric acid, urinary bladder, urine PART A MULTIPLE CHOICE Choose the response that best completes the sentence or answers the question. 1. The organ in vertebrates that is primarily responsible for regulating the chemical composition of plasma is the a. lymphatic system b. liver c. kidney d. hypothalamus. 2. Which of these is not a solute in blood? a. water b. salt c. urea d. carbon dioxide 3. Which of these is not one of the water compartments in the body? a. lymph b. intracellular fluid c. plasma d. urinary bladder 4. The process of maintaining water balance is called a. thermoregulation b. osmoregulation c. excretion d. metabolism. 5. Excretion is best described as the process of a. perspiring b. ridding the body of metabolic wastes c. regulating the water balance in the body d. producing urine. 6. An ameba produces nitrogenous wastes in the form of a. uric acid b. urea c. ammonia d. carbon dioxide. 7. The excretory structures in earthworms are a. nephridia b. kidneys c. urinary bladders d. contractile vacuoles. 8. The nitrogenous waste that is least soluble in water is a. uric acid b. urea c. ammonia d. carbon dioxide. 9. Gout occurs when a. too much alcohol is consumed, causing liver damage b. uric acid crystals form in the joints c. calcium compounds build up in the kidneys d. bacteria infect the kidneys, causing inflammation. 10. When a paramecium (a one-celled organism) is put into distilled water, structures at its front and tail ends start to pulsate

19 rapidly. These structures are probably a. lysosomes b. hearts c. sodium pumps d. contractile vacuoles. 11. The functional units of the human kidneys are the a. nephridia b. nephrons c. glomeruli d. renal medulla. 12. The thin, outermost layer of the skin is the a. endoderm b. dermis c. epidermis d. cortex. 13. Overactive sebaceous glands cause a. acne b. gout c. kidney failure d. gallstones. 14. Nephritis is a. an inflammation of the kidneys b. a scarring of liver cells c. an excretory organ in a crayfish d. a type of nitrogenous waste. 15. The dermis is involved with a. detecting temperature and pressure b. causing goose bumps in response to cold or fear c. regulating body temperature d. all of these. PART B CONSTRUCTED RESPONSE Use the information in the chapter to respond to these items. 16. Identify the structures labeled A through L in the diagram. 17. Where does the process of reabsorption take place? Why is this process important to maintaining homeostasis? 18. How do aldosterone and ADH contribute to homeostasis? B A Chapter 13 / Excretion and Water Balance 293 C 19. Compare the three types of nitrogenous wastes in animals. How might the excretion of uric acid be an adaptation to life on dry land? 20. Explain how the liver helps to maintain homeostasis. F G H D E J K L

20 294 Maintaining a Dynamic Equilibrium PART C READING COMPREHENSION Base your answers to questions 21 through 23 on the information below and on your knowledge of biology. Source: Science News (March 1, 2003): vol. 163, p Good Taste in Men Linked to Colon Risks Men with exceptionally good taste may pay for it in health risks. About 25 percent of people have extra taste buds on their tongues. They live in a neon taste world instead of a pastel one, as Yale University researcher Linda Bartoshuk puts it (SN: 7/5/97, p. 24). There may be a nasty consequence to the benefit. Among men over 65, intense tasters have significantly more colon polyps than other tasters, according to Bartoshuk and Marc Basson of Wayne State University of Detroit. Extra polyps suggest an extra risk of colon cancer. Bartoshuk speculates that sensitive tongues lead these men astray in food choices. Supertasters often cringe at intense vegetable flavors, and the supertasting seniors eat fewer vegetables than do their counterparts with normal taste sensitivity. The supertasters also tended to weigh more. Low-vegetable diets and extra weight both raise the risk of colon cancer. Ear infections may exacerbate this cancer risk. The nerves from the tongue pass through the ear, and ear infections distort neural mechanisms so that the tongue increases its sensitivity to fat. Bartoshuk has found that among men with a history of ear infections, supertasters are especially likely to be very overweight. 21. State the difference that has been discovered among men in their ability to taste foods. 22. State the health risk associated with the men that have increased taste sensitivity. 23. Describe two hypotheses that might explain the health risk faced by the men with sensitive tongues.