PowerLecture: Chapter 12 The Urinary System Learning 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. Impacts/Issues Truth in a Test Tube Truth in a Test Tube 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. Truth in a Test Tube 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. Useful References for Impacts/Issues The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. Science Daily: Early Prostate Cancer Detected in Molecular-Based Urine Test InfoTrac: NCAA Steps Up Steroid Tests: Organization Is Making a Far-Reaching Effort to Ensure That College Athletes Stay Clean. Kansas City Star, June 22, 2006. How Would You Vote? To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu. Do you think employers should be allowed to require a person to undergo urine testing before being hired? a. Yes, drug users are less safe and productive employees. Employers have a right to screen them out. b. No, unless the job involves public safety, it is a violation of an employee's privacy. Useful References for How Would You Vote? The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: Reasons to Consider: Drug Testing. Industrial Safety & Hygiene News, April 2002. InfoTrac: Urine – Or You're Out: Drug Testing Is Invasive, Insulting, and Generally Irrelevant to Job Performance. Reason, Nov. 2002. InfoTrac: Street Smarts: Just Say Yes. Inc., Nov. 1, 2004. Section 1 The Challenge: Shifts in Extracellular Fluid Shifts in Extracellular Fluid Extracellular fluid (ECF) is comprised of tissue fluid, blood plasma, and other fluids such as lymph that occurs 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. Shifts in Extracellular Fluid 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 byproduct. 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. Shifts in Extracellular Fluid The body exerts the most control over urinary excretion, the production of urine. The least amount of water is lost in feces. Shifts in Extracellular Fluid Solutes enter extracellular fluid from food, metabolism, and other ways. 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. food, water intake oxygen intake DIGESTIVE SYSTEM nutrients, water, salts RESPIRATORY SYSTEM oxygen carbon dioxide CARDIOVASCULAR SYSTEM elimination of food residues elimination of carbon dioxide URINARY SYSTEM water, solutes rapid transport to and from all living cells elimination of excess water, salts, wastes Fig. 12.1, p. 213 Shifts in Extracellular Fluid 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 crystallizes in joints, causing gout. 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. Shifts in Extracellular Fluid 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. Useful References for Section 1 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. Medicinenet.com: Electrolytes InfoTrac: Gouty Arthritis: A Primer on LateOnset Gout. Geriatrics, July 2005. Section 2 The Urinary System— Built for Filtering and Waste Disposal 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. kidney cortex kidney medulla renal artery renal vein renal pelvis renal capsule ureter Fig. 12.2c, p. 214 The Urinary System – Built for Filtering and Waste Disposal 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. heart POSTERIOR diaphragm adrenal gland kidney abdominal aorta ureter inferior vena cava ANTERIOR urinary bladder urethra Fig. 12.2ab, p. 214 POSTERIOR right kidney peritoneum vertebral left column kidney abdominal cavity ANTERIOR Fig. 12.2b, p. 214 The Urinary System – Built for Filtering and Waste Disposal 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. Bowman’s capsule (red) proximal tubule (orange) distal tubule (brown) KIDNEY CORTEX KIDNEY MEDULLA loop of Henle (yellow) The Urinary System – Built for Filtering and Waste Disposal collecting duct (tan) Some parts of the nephron allow absorption of water and solutes, other parts do not. Figure 12.3a The Urinary System – Built for Filtering and Waste Disposal 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. efferent arteriole afferent arteriole glomerular capillaries inside Bowman’s capsule peritubular capillaries threading around tubular nephron regions Fig. 12.3b, p. 215 Useful References for Section 2 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: Renal Anatomy and Overview of Nephron Function. Nephrology Nursing Journal, April 2003. Section 3 How Urine Forms: Filtration, Reabsorption, and Secretion 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. How Urine Forms: Filtration, Reabsorption, and Secretion 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. How Urine Forms: Filtration, Reabsorption, and Secretion Any solutes and water remaining in the tubules become part of urine. How Urine Forms: Filtration, Reabsorption, and Secretion 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. How Urine Forms: Filtration, Reabsorption, and Secretion 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. a Filtration. Occurs at glomerular capillaries in Bowman’s capsule. b Reabsorption. Occurs all along a nephron’s tubular parts. distal proximal tubule tubule c Secretion. Starts at proximal tubule and continues all along the nephron tubule. increasing solute concentration peritubular capillaries CORTEX MEDULLA loop of Henle d Urine is concentrated in loop of Henle and collecting loop of duct. Henle Urine flows from collecting duct into renal pelvis. Fig. 12.4, p. 216 efferent arteriole (to peritubular capillaries) glomerular capillaries enclosed by Bowman’s capsule Bowman’s capsule filtrate to proximal tubule afferent arteriole (from renal artery) Fig. 12.5a, p. 217 transport protein Na+ glucose Na+, glucose Cl– Cl– H2O H2O interior of tubule wall of tubule tissue fluid peritubular capillary Fig. 12.5b, p. 217 H+ H+ K+ K+ urea urea interior of tubule wall of tubule tissue fluid peritubular capillary Fig. 12.5c, p. 217 Useful References for Section 3 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: Renal Hemodynamics: An Overview. Nephrology Nursing Journal, Aug. 2003. InfoTrac: Glomerular Filtration: An Overview. Nephrology Nursing Journal, June 2003. Section 4 How Kidneys Help Manage Fluid Balance and Blood Pressure 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. How Kidneys Help Manage Fluid Balance and Blood Pressure 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. Figure 12.6 Na+ Cl– H2O kidney medulla saltiest near turn loop of Henle Fig. 12.6, p. 218 How Kidneys Help Manage Fluid Balance and Blood Pressure 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. How Kidneys Help Manage Fluid Balance and Blood Pressure 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. ADH targets aldosterone target KIDNEY CORTEX KIDNEY MEDULLA Fig. 12.7, p. 219 a Stimulus Water loss reduces blood volume. Sensors in the brain trigger release of ADH. e Response Receptors in brain detect the increase in blood volume. Signals for ADH secretion stop. b ADH makes distal tubules, collecting ducts more permeable to water. c Kidneys reabsorb more water, so less water leaves in urine. d The blood volume rises. Fig. 12.8, p. 219 a Stimulus Water loss reduces blood volume. Sensors in the brain trigger release of ADH. e Response Receptors in brain detect the increase in blood volume. Signals for ADH secretion stop. b ADH makes distal tubules, collecting ducts more permeable to water. c Kidneys reabsorb more water, so less water leaves in urine. d The blood volume rises. Stepped Art Fig. 12.8, p. 219 How Kidneys Help Manage Fluid Balance and Blood Pressure Decreases in the volume of extracellular fluid is 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. Fig. 12.9, p. 219 (efferent renin-secreting cells arteriole in juxtaglomerular leaving the apparatus glomerus) distal tubule © 2005 Bowman’s capsule (afferent arteriole leading into glomerus) proximal tubule Fig. 12.9a, p. 219 Receptors in kidneys, elsewhere detect falling sodium level Renin released from cells in the JGA Angiotensinogen converts to angiotensin I Angiotensin II Aldosterone secreted (from adrenal glands) Aldosterone acts on distal tubules Sodium (and water) reabsorbed Fig. 12.9b, p. 219 How Kidneys Help Manage Fluid Balance and Blood Pressure 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. Useful References for Section 4 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: Urinary Concentration and Dilution. Nephrology Nursing Journal, May–June 2004. Section 5 Removing Excess Acids and Other Substances in Urine Removing Excess Acids and Other Substances in Urine The body’s acid-base balance, the relative amounts of acidic and basic substances in extracellular fluid, is maintained in part by the kidneys. Kidneys maintain acid-base balance by controlling the levels of bicarbonate in the blood. Removing Excess Acids and Other Substances in Urine 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. cells of tubule wall tubule interior H2O peritubular capillary CO2 enzyme action (carbonic acid) H2CO3 tissue fluid H H ammonia HCO 3 bicarbonate enters bloodstream H phosphate excreted in urine Fig. 12.10, p. 220 Removing Excess Acids and Other Substances in Urine Many other substances end up in urine once filtered from the blood: traces of drugs; excess glucose, which is a sign of diabetes; pus, a sign of infection; and even blood, a sign of infection, cancer, or injury. Useful References for Section 5 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: Urine Albumin Considered Independent Marker for Kidney, Cardiovascular Disease. Heart Disease Weekly, July 4, 2004. Section 6 Kidney Disorders 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. Kidney Disorders Inflammation of the bladder (cystitis) or kidneys (pyelonephritis) is the result of infections to the urinary tract; nephritis is general inflammation of the kidneys and can be severe enough to limit function. Polycystic kidney disease is an inherited disorder in which cysts form in the kidneys and gradually destroy normal tissue. Glomerulonephritis describes a variety of disorders that disrupt the flow of blood through the glomeruli of the kidneys. Kidney Disorders 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. Figure 12.11 Useful References for Section 6 The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. National Institute of Diabetes and Digestive and Kidney Disease: National Kidney and Urologic Diseases Information Clearinghouse InfoTrac: Filter Fault Is Kidney Failure. The Economic Times, April 24, 2006. InfoTrac: Battle Well Fought: Miami Herald, July 13, 2006.