Chapter 12: THE URINARY SYSTEM

12 THE URINARY SYSTEM Chapter Outline the challenge: Shifts in extracellular fluid The urinary system adjusts fluid that is outside cells The body gains water from food and metabolic processes The body loses water in urine, sweat, feces, and by evaporation Solutes enter extracellular fluid from food, respiration, and metabolism Solutes leave the ECF by urinary excretion, in sweat, and during breathing The Urinary System: Built for filtering and waste disposal Nephrons are the kidney filters Special vessels transport blood to, in, and away from nephrons how URINE forms: FILTRATION, REABSORPTION, AND SECRETION Filtration removes a large amount of fluid and solutes from the blood Next, reabsorption returns useful substances to the blood Secretion rids the body of excess hydrogen ions and some other substances Urination is a controllable reflex HOW KIDNEYS HELP MANAGE FLUID BALANCE AND BLOOD PRESSURE Water follows salt as urine forms Hormones control whether kidneys make urine that is concentrated or dilute A thirst center monitors sodium REMOVING EXCESS ACIDS AND OTHER SUBSTANCES IN URINE The kidneys play a key role in maintaining the balance of acids and bases in the blood Various factors may cause serious acid-base imbalances Kidney disorders: when kidneys fail Cancer, infections, and drugs in the urinary system Urinary system cancer is on the rise Urinary tract infections are common Painkillers and other drugs may harm the kidneys connections: The urinary system in homeostasis SUMMARY Review questions self-quiz critical thinking explore on your own your future Objectives Explain how the chemical composition of extracellular fluid is maintained by mammals. Describe the components of the human urinary system. Understand the processes of urine formation and excretion. Explain the controls that maintain fluid balance and blood pressure. Explain why kidney disorders can be so damaging to human health. Key Terms urinary excretion urine urea electrolytes kidney ureter urinary bladder urethra nephrons glomerulus Bowman’s (glomerular) capsule proximal tubule loop of Henle distal tubule peritubular capillaries filtration tubular reabsorption tubular secretion antidiuretic hormone (ADH) juxtaglomerular apparatus aldosterone acid-base balance metabolic acidosis metabolic alkalosis kidney stones glomerulonephritis polycystic kidney disease cystitis pyelonephritis nephritis Lecture Outline Urine can be used to determine much about the general health and habits of human beings. Changes to the composition of urine can indicate metabolic problems, infection, or even pregnancy. Urine can also show the presence of illegal performance-enhancing drugs in athletes. Urine is a useful indicator of health because each day the kidneys filter all of the blood in the body a total of 30 times, eliminating excess water and harmful solutes. The Challenge: Shifts in Extracellular Fluid The urinary system adjusts fluid that is outside cells. Extracellular fluid (ECF) is comprised of tissue fluid, blood plasma, and other fluids such as lymph that occur outside of cells; intracellular fluid is the fluid inside cells. There is a constant exchange of gases and other materials between intracellular and extracellular fluid. The volume and composition of the ECF must remain stable for these exchanges to occur. The urinary system is responsible for maintaining relatively stable conditions in the ECF. The body gains water from food and metabolic processes. Absorption of water from liquids and solid foods occurs in the gastrointestinal tract. Metabolism of nutrients yields water as a by-product. The body loses water in urine, sweat, feces, and by evaporation. Water leaves the body by excretion in urine, evaporation from the lungs and skin, sweating, and in feces. The body exerts the most control over urinary excretion, the production of urine. The least amount of water is lost in feces. Solutes enter extracellular fluid from food, respiration, and metabolism. Solutes enter the body when nutrients and mineral ions are absorbed from the GI tract. Living cells secrete substances into tissue fluid and blood. The respiratory system brings oxygen into the blood; respiring cells add carbon dioxide. Solutes leave the ECF by urinary excretion, in sweat, and during breathing. Respiratory exhalation rids the body of carbon dioxide; all other major wastes of metabolism leave in urine. Uric acid is formed in reactions that degrade nucleic acids; too much uric acid in the ECF is potentially toxic. Ammonia is formed when amino groups are removed from amino acids; it is turned into urea in the liver and either reabsorbed or excreted. Other products of protein degradation are also excreted. The kidneys filter a variety of substances from the blood, including nitrogen, sodium, potassium, and calcium. Sodium, potassium, and calcium are called electrolytes because a solution in which they are dissolved will carry an electric current. Only 1% of the water that enters the kidneys is excreted in urine; most is returned to the blood. The Urinary System: Built for Filtering and Waste Disposal Each kidney is a bean-shaped organ about the size of a rolled-up pair of socks. A kidney has several internal lobes; an outer cortex wraps around the central medulla. The whole kidney is wrapped in a coat of connective tissue called the renal capsule. The central cavity of the kidney is the renal pelvis. Kidneys have several functions: They produce erythropoietin, which stimulates production of red blood cells. They aid in calcium absorption from food. Kidneys make renin, an enzyme that helps regulate blood pressure. Their main function is to remove metabolic wastes and maintain fluid balance. The urinary system also consists of tubelike ureters that carry urine to the urinary bladder for storage until urination; urine leaves the bladder through the urethra. Nephrons are the kidney filters. Each lobe of the kidney contains blood vessels and over a million thin tubes called nephrons, which filter water and solutes from the blood. The wall of the nephron balloons around a cluster of blood capillaries called the glomerulus; the balloon is called the Bowman’s capsule; the rest of the nephron is a winding tubule. Filtrate from the Bowman’s capsule enters the proximal tubule, passes through the loop of Henle and into the distal tubule, and finally empties into a collecting duct. Some parts of the nephron allow absorption of water and solutes, other parts do not. Special vessels transport blood to, in, and away from nephrons. An afferent arteriole delivers blood to each nephron where it enters the glomerulus for filtration; the glomerular capillaries are much more permeable than other capillaries. Glomerular capillaries merge to form an efferent arteriole. The efferent arteriole splits to form the peritubular capillaries, which eventually carry filtered blood into venules and out of the kidneys. How Urine Forms: Filtration, Reabsorption, and Secretion Filtration removes a large amount of fluid and solutes from the blood. In filtration, blood pressure forces filtrate out of the glomerular capillaries into the Bowman’s capsule, then into the proximal tubule. Blood cells, proteins, and other large solutes cannot pass into the capsule; water, glucose, sodium, and urea, however, are forced out of the blood. Next, reabsorption returns useful substances to the blood. Reabsorption takes place across the walls of the proximal tubules. Water, glucose, and salt diffuse through the tubule wall; active transport then moves glucose and sodium ions into the tissue fluid. Negatively charged ions follow the sodium into the tissues; water also follows. Solutes are actively transported from the tissues to the peritubular capillaries, water follows, and reabsorption is complete. Any solutes and water remaining in the tubules become part of urine. Secretion rids the body of excess hydrogen ions and some other substances. During secretion, urea, excess hydrogen ions, and excess potassium ions are returned to the nephrons to add to forming urine. This process maintains the body’s acid-base balance and also rids the body of drugs, uric acid, hemoglobin breakdown products, and other wastes. Urination is a controllable reflex. The internal urethral sphincter (involuntary control) regulates urine flow from the bladder into the urethra during urination. The external urethral sphincter (voluntary control) opens to void urine from the body. How Kidneys Help Manage Fluid Balance and Blood Pressure The total volume of body fluids doesn’t vary much because the kidneys make adjustments to keep the volume of extracellular fluid, and blood in particular, in a normal range. Water follows salt as urine forms. The loop of Henle pulls more water and salts from the filtrate to return it to the body. The descending part of the loop sits in salty tissue fluid; water is drawn out of the tube to be reabsorbed. The salt concentration of the remaining fluid in the loop rises until it matches the concentration of the surrounding tissues. In the ascending limb of the loop, water is inhibited from passing through the wall of the loop, but sodium is actively transported out of the loop. Salt continues to be removed in the distal tubule, but not water; as salt leaves the filtrate, salt gradients become steep, driving reabsorption of solutes into the peritubular capillaries. Urea helps boost the gradient by diffusion out of the collecting duct, taking water with it. Hormones control whether kidneys make urine that is concentrated or dilute. Antidiuretic hormone (ADH) is secreted by the brain in response to a decrease in extracellular fluid; ADH causes the distal tubules and collecting ducts to become permeable to water, which moves back into the blood capillaries. Decreases in the volume of extracellular fluid are sensed by cells in the efferent arterioles; these cells, part of the juxtaglomerular apparatus, release renin. Renin stimulates production of angiotensin I, which is converted to angiotensin II. Angiotensin II stimulates the adrenal cortex of the kidney to make aldosterone, which causes cells of the distal tubules and collecting ducts to increase reabsorption of salts. Caffeine and alcohol are diuretics, substances that promote loss of water. A thirst center monitors sodium. When solute concentration in the extracellular fluid rises, the amount of saliva produced by the salivary glands drops; a dry mouth stimulates the thirst center of the brain. Stimulation of the thirst center and release of ADH cause liquid-seeking behavior. Removing Excess Acids and Other Substances in Urine The kidneys play a key role in maintaining the balance of acids and bases in the blood. The acid-base balance, the relative amounts of acidic and basic substances in extracellular fluid, is maintained in part by the kidneys. When the blood is too acid, water and carbon dioxide combine in cells in the wall of the nephron tubules to give rise to bicarbonate and H+. The bicarbonate enters the peritubular capillaries and from there it enters the blood to neutralize acid. The H+ in the tubules enters the filtrate to combine with phosphate, ammonia, or bicarbonate to be excreted. When the blood is too alkaline, less bicarbonate is reabsorbed into the blood. Various factors may cause serious acid-base imbalances. Metabolic acidosis is a decrease in blood pH and may lead to coma. Metabolic alkalosis is an increase in blood pH and may lead to convulsions. Kidney Disorders Kidney stones are deposits of uric acid, calcium salts, and other substances that have settled out of urine and collected in the renal pelvis. Small stones can pass out during urination, but larger stones can inhibit urination. Lithotripsy uses sound waves to fragment the stones so they can pass out in the urine. Glomerulonephritis describes a variety of disorders that disrupt the flow of blood through the glomeruli of the kidneys. Dialysis refers to the exchange of substances across a membrane between solutions of differing compositions; in hemodialysis, a machine is connected to an artery or vein, blood enters the tubes of the machine, and materials are removed from the blood before it is returned to the body. Polycystic kidney disease is an inherited disorder in which cysts form in the kidneys and gradually destroy normal tissue. Cancer, Infections, and Drugs in the Urinary System Urinary system cancer is on the rise. Carcinomas of the bladder and kidney are responsible for 100,000 cases per year. Urinary tract infections are common. Inflammation of the bladder (cystitis) or kidneys (pyelonephritis) is the result of infections of the urinary tract; nephritis is general inflammation of the kidneys and can be severe enough to limit function. Nephritis is an inflammation of the kidneys that makes filtering difficult. Painkillers and other drugs may harm the kidneys. Nonsteroidal anti-inflammatory drugs can damage the kidneys. Alcohol and many illegal drugs also damage the kidneys. Connections: The Urinary System in Homeostasis Urinary system functioning is essential to keep other body systems free of waste and toxins. Suggestions for Presenting the Material This chapter expands the concept of homeostasis introduced in Chapter 4 by focusing on regulation of solute-water balance by the kidneys. The “urinary system” section contains material that is less familiar to students, especially the intricacies of the kidney. Therefore, this section (sometimes referred to as the “excretory system”) will deserve some time and effort. The authors begin the chapter with an excellent prelude—water and solute gains and losses. These paragraphs make clear the necessity for the urinary system. If the word excretion is used in your lecture, clearly distinguish it from the word elimination, which is the voiding of undigested waste via the anus. Discussion of the nephron structure is meaningless without constant reference to Figure 12.4. Be certain to distinguish between tubes carrying blood and those carrying filtrate. Make use of the animation for this figure and others if possible. An analogy to the common drip-type coffee-brewing machine is helpful (see the “Classroom and Laboratory Enrichment” section). Formal discussion of the hormones ADH and aldosterone are found in Chapter 15. Observation of Table 12.2 will show that large quantities of fluid are filtered from the blood, but nearly all fluid is replaced. Use the “storeroom-cleaning” analogy (see the “Classroom and Laboratory Enrichment” section) as a possible clue to why this is the body’s method. Classroom and Laboratory Enrichment Use a mannequin to show the locations of the urinary organs. Exhibit a model of a kidney to illustrate its parts and the blood vessels associated with it or use an animation. Display a model or use an animation of a nephron. Use a model of a renal corpuscle to illustrate its structure. Figures 12.3, 12.4, and 12.5 are animated; show the animations if possible to help guide the students through the material. Constant reference should always be made to the structures so that students can visualize the flow of fluids. To aid the students’ conceptualization of the nephron function, the following comparison to a drip-type coffee maker can be made: Nephron Part Coffee Brewer Part Afferent arteriole carrying blood in…………………………. Hot water Blood with wastes……………………………………………... Coffee grounds Glomerulus…………………………………………………….. Filter paper Bowman’s capsule…………………………………………….. Filter holder Proximal and distal ducts, Henle’s loop, collecting ducts… Carafe The storeroom-cleaning analogy reveals a possible explanation for why the kidney removes from the blood much more than it will eventually excrete. Pose this question based on the two scenarios that follow: Which of the following methods, (a) or (b), results in a more efficient cleaning of the storeroom? Carefully removing and disposing of only those few selected items that are in plain view and identifiable as “no longer needed.” Removing all items from the storeroom; then sweeping, dusting, mopping; and finally replacing only those items selected as “still worth keeping.” The complete analysis of urine can reveal a wealth of information concerning the status of body metabolism. Ask a clinical lab technician to speak on modern analysis techniques. If possible, invite a kidney transplant or dialysis patient to discuss his or her condition with the class. Some kidney centers have a speaker’s bureau for just this purpose. They may also provide dialysis equipment for observation. Classroom Discussion Ideas Urine testing is a very effective way to assess many elements of body health. Should people be able to perform their own urine testing? Why or why not? Should it be legal for employers to monitor their workers for illegal drug use by urine testing? Who should, or should not, be subject to periodic drug testing? Of these three processes—filtration, reabsorption, secretion—which is (are) accomplished by a kidney dialysis machine? Explain any limitations of the device. Why do high-protein diet supplements for increasing muscle mass or losing weight include warnings saying that water intake must be increased when consuming the product? In the storeroom-cleaning analogy (see the “Classroom and Laboratory Enrichment” section), which scenario results in a better cleaning of the storeroom? Would the same apply to kidney function? When asked what the kidney does, most people would probably respond that it filters the blood. Why is this answer not a complete statement of kidney function? Obviously, humans can survive using only one kidney. Why then do we have two? Is it possible to overload the kidneys’ capacity by excessive intake of water? Why does thirst increase after eating salty foods? In addition to becoming thirsty, why would eating salty foods make you temporarily gain weight? Term Paper Topics, Library Activities, and Special Projects Investigate the workings of a kidney dialysis machine. Include historical perspectives and recent technological advances. Chronicle the developments in the treatment of kidney stones up to the present. Although Western cultures find the practice bizarre, the consumption of one’s own urine is practiced in Eastern cultures (India, for example). Report on the supposed benefit of such actions and the possible dangers. What are “kidney stones”? What are some of the factors responsible for their formation? Investigate the reasons why humans cannot meet their water needs by drinking seawater exclusively. Videos, Animations, and Websites VIDEOS Films for the Humanities and Sciences The Urinary Tract: Water! _http://ffh.films.com/id/10229/The_Urinary_Tract_Water.htm_ Medical University of South Carolina Short video on the process of dialysis _http://www.muschealth.com/video/Default.aspx?videoId=10098&cId=24&type=rel_ YouTube - Function of the Nephron Video on the nephron _http://www.youtube.com/watch?v=glu0dzK4dbU_ ANIMATIONS Biology Mad Interactive animation on the kidney. _http://www.biologymad.com/resources/kidney.swf_ WEBSITES National Kidney Foundation Website designed to educate the public about what the kidneys do, what happens when they do not work properly and who is at risk. _http://www.kidney.org/index.cfm_ Possible Responses to Review Questions The figure to be labeled matches Figures 12.3 and 12.4 of the text. Formation of urine helps maintain the body’s internal environment by regulating fluid volume inside and outside the cells; by controlling movement of solutes and water; by regulating pH and blood pressure; and, through the action of the kidney, by ridding the body of potentially harmful wastes. In filtration, blood pressure forces water and small solutes out of the glomerular capillaries into Bowman’s capsule; larger proteins and cells remain in the blood, but the rest is filtered before being allowed to be reabsorbed (most water and solutes are eventually reclaimed). Secretion takes unwanted materials and moves them from the peritubular capillaries back into the tubules so they can be excreted in urine. Water conservation is promoted by ADH, sodium conservation by aldosterone, and thirst behavior by lack of saliva coupled with the presence of ADH. Acid-base balance depends on the relative amounts of bicarbonate released into or removed from the blood; bicarbonate in the blood can be used to modify the hydrogen ions present in the tissues depending on the relative acidity of the given tissue. Excess ions move down their gradient out of the cell into the blood; there protons can join with bicarbonate produced in the nephrons, following enzymatic activity that combines carbon dioxide and water. Excess hydrogen ions will eventually find their way into the urine to be excreted. Possible Responses to Critical Thinking Questions Of the three items listed in the question, none is normally found in urine. However, one—glucose—is found in the glomerular filtrate before the process of reabsorption has occurred. If indeed glucose is found in the urine, it indicates that there was an elevated quantity in the blood and, therefore, more than could be reabsorbed. This might indicate a possible diabetic condition. Hemoglobin and pus in the blood indicates that something is amiss along the route from urine formation to the exit of the urethra. This could be an infection in the urinary tract (usually more common in females), an infection in the nephrons, or a kidney stone that is irritating the membranes of the renal pelvis and/or ureters. Any of these conditions requires the attention of a urologist and possibly a nephrologist. If the nephron loses its ability to concentrate urine (that is, reabsorb the proper quantities of water into the peritubular capillaries), this will result in excretion of a more dilute urine with a higher percent water content. This will increase the thirst sensation and, of course, lead to increased drinking of water and more frequent trips to the bathroom. Unfortunately, older persons do not take well to more frequent visits to the bathroom, so they often reduce their water intake—a bad idea. Best advice: follow your body’s directions; when it says “drink,” you drink!—water that is. If the flow of urine from the kidneys down the ureters to the bladder is blocked, then urine will back up like a stopped-up drain into the renal pelvis of the kidney. This will initially result in a distention of the kidney, but eventually could lead to kidney failure. Aldosterone is responsible for inducing the kidneys to increase reabsorption of sodium. Someone with high blood pressure already may have too much salt remaining in their system, and so if the licorice stimulates aldosterone production, this will only make a bad situation worse. Sodium balance is vital for promoting the movement of itself and other ions and solutes across membranes and ultimately into cells. Water will dilute the extracellular fluid, pulling more and more sodium and other ions out of the fluid into the blood, and ultimately interfering with filtration and reabsorption. Maintaining the proper gradients throughout the body is critical to homeostasis and continued functioning of the body. One of the methods used to facilitate reabsorption is active transport; active transport requires energy from ATP, which must be produced by the mitochondria. More mitochondria are needed to ensure adequate energy to filter all of the blood in the body 30 times a day. Possible Responses to Explore on Your Own Questions Heavy sweating is likely to concentrate the urine as water is drawn out of the body system to keep it cool. If you drink one quart of water in one hour, particularly if you are inactive, you should not only feel “water-logged,” but you should also have to urinate frequently; the urine should be fairly dilute. 142 Chapter Twelve The Urinary System 133 142 Chapter Twelve The Urinary System 133 142 Chapter Twelve The Urinary System 133 142 Chapter Twelve The Urinary System 141 142 Chapter Twelve The Urinary System 141 142 Chapter Twelve The Urinary System 141