|25: The Urinary System
1. Describe the gross anatomy of the kidney and its coverings.
2. Trace the blood supply through the kidney.
3. Describe the anatomy of a nephron.
Kidney Physiology: Mechanisms of Urine Formation
4. Describe the forces (pressures) that promote or counteract glomerular filtration.
5. Compare the intrinsic and extrinsic controls of the glomerular filtration rate.
6. Describe the mechanisms underlying water and solute reabsorption from the renal tubules into the peritubular capillaries.
7. Describe how sodium and water reabsorption is regulated in the distal tubule and collecting duct.
8. Describe the importance of tubular secretion and list several substances that are secreted.
9. Describe the mechanisms responsible for the medullary osmotic gradient.
10. Explain formation of dilute versus concentrated urine.
11. Define renal clearance and explain how this value summarizes the way a substance is handled by the kidney.
12. Describe the normal physical and chemical properties of urine.
13. List several abnormal urine components, and name the condition characterized by the presence of detectable amounts of each.
14. Describe the general location, structure, and function of the ureters.
15. Describe the general location, structure, and function of the urinary bladder.
16. Describe the general location, structure, and function of the urethra.
17. Compare the course, length, and functions of the male urethra with those of the female.
18. Define micturition and describe its neural control.
Developmental Aspects of the Urinary System
19. Trace the embryonic development of the urinary organs.
20. List several changes in urinary system anatomy and physiology that occur with age.
I. Kidney Anatomy (pp. 961–969; Figs. 25.1–25.8)
A. Location and External Anatomy (pp. 961–962; Figs. 25.1–25.2)
1. The kidneys are bean-shaped organs that lie retroperitoneal in the superior lumbar region.
2. The medial surface is concave and has a renal hilus that leads into a renal sinus, where the blood vessels, nerves, and lymphatics lie.
3. The kidneys are surrounded by an outer renal fascia that anchors the kidney and adrenal gland to surrounding structures, a perirenal fat pad that surrounds and cushions the kidney, and a fibrous capsule that prevents surrounding infections from reaching the kidney.
B. Internal Anatomy (pp. 962–963; Fig. 25.3)
1. There are three distinct regions of the kidney: the cortex, the medulla, and the renal pelvis.
2. Major and minor calyces collect urine and empty it into the renal pelvis.
C. Blood and Nerve Supply (pp. 963–964; Fig. 25.4)
1. Blood supply into and out of the kidneys progresses to the cortex through renal arteries to segmental, lobar, interlobar, arcuate, and cortical radiate arteries, and back to renal veins from cortical radiate, arcuate, and interlobar veins.
2. The renal plexus regulates renal blood flow by adjusting the diameter of renal arterioles and influencing the urine-forming role of the nephrons.
D. Nephrons are the structural and functional units of the kidneys that carry out processes that form urine (pp. 964–969; Figs. 25.4–25.8).
1. Each nephron consists of a renal corpuscle composed of a tuft of capillaries (the glomerulus) surrounded by a glomerular capsule (Bowman’s capsule).
2. The renal tubule begins at the glomerular capsule as the proximal convoluted tubule, continues through a hairpin loop, the loop of Henle, and turns into a distal convoluted tubule before emptying into a collecting duct.
3. The collecting ducts collect filtrate from many nephrons, and extend through the renal pyramid to the renal papilla, where they empty into a minor calyx.
4. There are two types of nephrons: 85% are cortical nephrons, which are located almost entirely within the cortex; 15% are juxtamedullary nephrons, located near the cortex-medulla junction.
5. The peritubular capillaries arise from efferent arterioles draining the glomerulus, and absorb solutes and water from the tubules.
6. Blood flow in the renal circulation is subject to high resistance in the afferent and efferent arterioles.
7. The juxtaglomerular apparatus is a structural arrangement between the afferent arteriole and the distal convoluted tubule that forms granular cells and macula densa cells.
8. The filtration membrane lies between the blood and the interior of the glomerular capsule, and allows free passage of water and solutes.
II. Kidney Physiology: Mechanisms of Urine Formation (pp. 969–984; Figs. 25.9–25.18; Table 25.1)
A. Step 1: Glomerular Filtration (pp. 969–974; Figs. 25.9–25.12)
1. Glomerular filtration is a passive, nonselective process in which hydrostatic pressure forces fluids through the glomerular membrane.
2. The net filtration pressure responsible for filtrate formation is given by the balance of glomerular hydrostatic pressure against the combined forces of colloid osmotic pressure of glomerular blood and capsular hydrostatic pressure exerted by the fluids in the glomerular capsule.
3. The glomerular filtration rate is the volume of filtrate formed each minute by all the glomeruli of the kidneys combined.
4. Maintenance of a relatively constant glomerular filtration rate is important because reabsorption of water and solutes depends on how quickly filtrate flows through the tubules.
5. Glomerular filtration rate is held relatively constant through intrinsic autoregulatory mechanisms, and extrinsic hormonal and neural mechanisms.
a. Renal autoregulation uses a myogenic control related to the degree of stretch of the afferent arteriole, and a tubuloglomerular feedback mechanism that responds to the rate of filtrate flow in the tubules.
b. Extrinsic neural mechanisms are stress-induced sympathetic responses that inhibit filtrate formation by constricting the afferent arterioles.
c. The renin-angiotensin mechanism causes an increase in systemic blood pressure and an increase in blood volume by increasing Na+ reabsorption.
B. Step 2: Tubular Reabsorption (pp. 974–978; Figs. 25.13–25.14, 25.18; Table 25.1)
1. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule, and involves near total reabsorption of organic nutrients, and the hormonally regulated reabsorption of water and ions.
2. The most abundant cation of the filtrate is Na+, and reabsorption is always active.
3. Passive tubular reabsorption is the passive reabsorption of negatively charged ions that travel along an electrical gradient created by the active reabsorption of Na+.
4. Obligatory water reabsorption occurs in water-permeable regions of the tubules in response to the osmotic gradients created by active transport of Na+.
5. Secondary active transport is responsible for absorption of glucose, amino acids, vitamins, and most cations, and occurs when solutes are cotransported with Na+ when it moves along its concentration gradient.
6. Substances that are not reabsorbed or incompletely reabsorbed remain in the filtrate due to a lack of carrier molecules, lipid insolubility, or large size (urea, creatinine, and uric acid).
7. Different areas of the tubules have different absorptive capabilities.
a. The proximal convoluted tubule is most active in reabsorption, with most selective reabsorption occurring there.
b. The descending limb of the loop of Henle is permeable to water, while the ascending limb is impermeable to water but permeable to electrolytes.
c. The distal convoluted tubule and collecting duct have Na+ and water permeability regulated by the hormones aldosterone, antidiuretic hormone, and atrial natriuretic peptide.
C. Step 3: Tubular Secretion (p. 978)
1. Tubular secretion disposes of unwanted solutes, eliminates solutes that were reabsorbed, rids the body of excess K+, and controls blood pH.
2. Tubular secretion is most active in the proximal convoluted tubule, but occurs in the collecting ducts and distal convoluted tubules as well.
D. Regulation of Urine Concentration and Volume (pp. 978–983; Figs. 25.15–25.18)
1. One of the critical functions of the kidney is to keep the solute load of body fluids constant by regulating urine concentration and volume.
2. The countercurrent mechanism involves interaction between filtrate flow through the loops of Henle (the countercurrent multiplier) of juxtamedullary nephrons and the flow of blood through the vasa recta (the countercurrent exchanger).
a. Because water is freely absorbed from the descending limb of the loop of Henle, filtrate concentration increases and water is reabsorbed.
b. The ascending limb is permeable to solutes, but not to water.
c. In the collecting duct, urea diffuses into the deep medullary tissue, contributing to the increasing osmotic gradient encountered by filtrate as it moves through the loop.
d. The vasa recta aids in maintaining the steep concentration gradient of the medulla by cycling salt into the blood as it descends into the medulla, and then out again as it ascends toward the cortex.
3. Because tubular filtrate is diluted as it travels through the ascending limb of the loop of Henle, production of a dilute urine is accomplished by simply allowing filtrate to pass on to the renal pelvis.
4. Formation of a concentrated urine occurs in response to the release of antidiuretic hormone, which makes the collecting ducts permeable to water and increases water uptake from the urine.
5. Diuretics act to increase urine output by either acting as an osmotic diuretic or by inhibiting Na+ and resulting obligatory water reabsorption.
E. Renal Clearance (p. 984)
1. Renal clearance refers to the volume of plasma that is cleared of a specific substance in a given time.
2. Inulin is used as a clearance standard to determine glomerular filtration rate because it is not reabsorbed, stored, or secreted.
3. If the clearance value for a substance is less than that for inulin, then some of the substance is being reabsorbed; if the clearance value is greater than the inulin clearance rate, then some of the substance is being secreted. A clearance value of zero indicates the substance is completely reabsorbed.
III. Urine (pp. 984–985; Table 25.2)
A. Physical Characteristics (pp. 984–985)
1. Freshly voided urine is clear and pale to deep yellow due to urochrome, a pigment resulting from the destruction of hemoglobin.
2. Fresh urine is slightly aromatic, but develops an ammonia odor if allowed to stand, due to bacterial metabolism of urea.
3. Urine is usually slightly acidic (around pH 6) but can vary from about 4.5–8.0 in response to changes in metabolism or diet.
4. Urine has a higher specific gravity than water, due to the presence of solutes.
B. Chemical Composition (p. 985; Table 25.2)
1. Urine volume is about 95% water and 5% solutes, the largest solute fraction devoted to the nitrogenous wastes urea, creatinine, and uric acid.
IV. Ureters (pp. 985–986; Figs. 25.19–25.20)
A. Ureters are tubes that actively convey urine from the kidneys to the bladder (pp. 985–986; Fig. 25.19).
B. The walls of the ureters consist of an inner mucosa continuous with the kidney pelvis and the bladder, a double-layered muscularis, and a connective tissue adventitia covering the external surface (p. 986; Fig. 25.20).
V. Urinary Bladder (pp. 986–987; Fig. 25.21)
A. The urinary bladder is a muscular sac that expands as urine is produced by the kidneys to allow storage of urine until voiding is convenient (p. 986; Fig. 25.21).
B. The wall of the bladder has three layers: an outer adventitia, a middle layer of detrusor muscle, and an inner mucosa that is highly folded to allow distention of the bladder without a large increase in internal pressure (pp. 986–987).
VI. Urethra (pp. 987–988; Fig. 25.21)
A. The urethra is a muscular tube that drains urine from the body; it is 3–4 cm long in females, but closer to 20 cm in males (p. 987; Fig. 25.21).
B. There are two sphincter muscles associated with the urethra: the internal urethral sphincter, which is involuntary and formed from detrusor muscle; and the external urethral sphincter, which is voluntary and formed by the skeletal muscle at the urogenital diaphragm (p. 987; Fig. 25.21).
C. The external urethral orifice lies between the clitoris and vaginal opening in females, or occurs at the tip of the penis in males (p. 987; Fig. 25.21).
VII. Micturition (p. 988; Fig. 25.22)
A. Micturition, or urination, is the act of emptying the bladder (p. 988; Fig. 25.22).
1. As urine accumulates, distention of the bladder activates stretch receptors, which trigger spinal reflexes, resulting in storage of urine.
2. Voluntary initiation of voiding reflexes results in activation of the micturition center of the pons, which signals parasympathetic motor neurons that stimulate contraction of the detrusor muscle and relaxation of the urinary sphincters.
VIII. Developmental Aspects of the Urinary System (pp. 988–991)
A. In the developing fetus, the mesoderm-derived urogenital ridges give rise to three sets of kidneys: the pronephros, mesonephros, and metanephros (pp. 988–989).
1. The pronephros forms and degenerates during the fourth through sixth weeks, but the pronephric duct persists, and connects later-developing kidneys to the cloaca.
2. The mesonephros develops from the pronephric duct, which then is named the mesonephric duct, and persists until development of the metanephros.
3. The metanephros develops at about five weeks, and forms ureteric buds that give rise to the ureters, renal pelvis, calyces, and collecting ducts.
4. The cloaca subdivides to form the future rectum, anal canal, and the urogenital sinus, which gives rise to the bladder and urethra.
B. Newborns void most frequently, because the bladder is small and the kidneys cannot concentrate urine until two months of age (p. 990).
C. From two months of age until adolescence, urine output increases until the adult output volume is achieved (p. 990).
D. Voluntary control of the urinary sphincters depends on nervous system development, and complete control of the bladder even during the night does not usually occur before 4 years of age (p. 991).
E. In old age, kidney function declines due to shrinking of the kidney as nephrons decrease in size and number; the bladder also shrinks and loses tone, resulting in frequent urination (p. 991).
Cross References From Chapters 1-15
Additional information on topics covered in Chapter 25 can be found in the chapters listed below.
1. Chapter 3: Hydrostatic pressure and membranes; membrane transport; microvilli
2. Chapter 4: Epithelial cells; dense connective tissue
3. Chapter 10: Levator ani
4. Chapter 14: Sympathetic control; parasympathetic pelvic splanchnic nerves; epinephrine
1. Marieb, E. N., and S. J. Mitchell. Human Anatomy & Physiology Laboratory Manual: Cat and Fetal Pig Versions. Ninth Edition Updates. Benjamin Cummings, 2009.
Exercise 40: Anatomy of the Urinary System
Exercise 41: Urinalysis
PhysioEx™ 8.0 Exercise 41B: Renal System Physiology: Computer Simulation
2. Marieb, E. N., and S. J. Mitchell. Human Anatomy & Physiology Laboratory Manual: Main Version. Eighth Edition Update. Benjamin Cummings, 2009.
Exercise 40: Anatomy of the Urinary System
Exercise 41: Urinalysis
PhysioEx™ 8.0 Exercise 41B: Renal System Physiology: Computer Simulation
Online Resources for Students
The following shows the organization of the Chapter Guide page in myA&P™. The Chapter Guide organizes all the chapter-specific online media resources for Chapter 25 in one convenient location, with e-book links to each section of the textbook. Students can also access A&P Flix animations, MP3 Tutor Sessions, Interactive Physiology® 10-System Suite, Practice Anatomy Lab™ 2.0, PhysioEx™ 8.0, and much more.
Section 25.1 Kidney Anatomy (pp. 961–969)
Interactive Physiology® 10-System Suite: Urinary System: Anatomy Review
Art Labeling: The Urinary System (Fig. 25.1, p. 961)
Art Labeling: Internal Anatomy of the Kidney (Fig. 25.3b, p. 963)
Art Labeling: Location and Structure of Nephrons (Fig. 25.5, p. 965)
Section 25.2 Kidney Physiology: Mechanisms of Urine Formation (pp. 969–984)
MP3 Tutor Session: Urine Production
Interactive Physiology® 10-System Suite: Glomerular Filtration
Interactive Physiology® 10-System Suite: Early Filtrate Processing
Interactive Physiology® 10-System Suite: Late Filtrate Processing
PhysioEx™ 8.0: Renal System Physiology
Case Study: Diabetic Nephropathy (Kidney Damage)
Case Study: Renal Failure
Section 25.3 Urine (pp. 984–985)
Section 25.4 Ureters (pp. 985–986)
Section 25.5 Urinary Bladder (pp. 986–987)
Section 25.6 Urethra (pp. 987–988)
Memory Game: The Urinary System In Situ
Section 25.7 Micturition (p. 988)
Case Study: Urinary Frequency (Polyuria) and Excessive Thirst (Polydipsia)
Section 25.8 Developmental Aspects of the Urinary System (pp. 988–991)
Crossword Puzzle 25.1
Crossword Puzzle 25.2
Art Labeling Quiz
Chapter Practice Test
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix G of the main text.
Short Answer Essay Questions
11. The perineal fat capsule helps to hold the kidney in place against the posterior trunk wall and cushions it against blows. (p. 962)
12. A creatine molecule travels the following route from a glomerulus to the urethra. It first passes through the glomerular filtration membrane, which is a porous membrane made up of a fenestrated capillary endothelium, a thin basement membrane, and the visceral membrane of the glomerular capsule formed by the podocytes. The creatine molecule then passes through the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, and into the collecting duct in which it travels into the medulla through the renal pyramids. From the medulla the molecule enters the renal pelvis, and leaves the kidney via the ureter. Then it travels to the urinary bladder and then to the urethra. (pp. 964–966; 985–987)
13. Renal filtrate is a solute-rich fluid without blood cells or plasma proteins because the filtration membrane is permeable to water and all solutes smaller than plasma proteins. The capillary endothelium restricts passage of formed elements, whereas the anion-rich basement membrane holds back most protein and some smaller anionic molecules. (p. 971)
14. The mechanisms that contribute to renal autoregulation are the myogenic mechanism and the tubuloglomerular feedback mechanism. The myogenic mechanism reflects the tendency of vascular smooth muscle to contract when it is stretched. An increase in systemic blood pressure causes afferent arterioles to constrict, which impedes blood flow into the glomerulus and prevents glomerular blood pressure from rising to damaging levels. Conversely, a decline in systemic blood pressure causes dilation of afferent arterioles and an increase in glomerular hydrostatic pressure. Both responses help maintain a normal GFR.
The tubuloglomerular mechanism reflects the activity of the macula densa cells in response to a slow filtration rate or low filtrate osmolarity. When so activated they release chemicals that cause vasodilation in the afferent arterioles.
Renal autoregulation maintains a relatively constant kidney perfusion over an arterial pressure range from about 80 to 180 mm Hg, preventing large changes in water and solute excretion. (p. 972)
15. Sympathetic nervous system controls protect the body during extreme stress by redirecting blood to more vital organs. Strong sympathetic stimulation causes release of norepinephrine to alpha-adrenergic receptors, causing strong vasoconstriction of kidney arterioles. This results in a drop in glomerular filtration, and indirectly stimulates another extrinsic mechanism, the renin-angiotensin mechanism. The renin-angiotensin mechanism involves the release of renin from the granular juxtaglomerular cells, which enzymatically converts the plasma globulin angiotensinogen to angiotensin I. Angiotensin I is further converted to angiotensin II by angiotensin converting enzyme (ACE) produced by capillary endothelium. Angiotensin II causes vasoconstriction of systemic arterioles, increased sodium reabsorption by promoting the release of aldosterone, decreases peritubular hydrostatic pressure, which encourages increased fluid and solute reabsorption, and acts on the glomerular mesangial cells, causing a decrease in glomerular filtration rate. In addition, angiotensin II results in stimulation of the hypothalamus, which activates the thirst mechanism and promotes the release of antidiuretic hormone, which causes increased water reabsorption in the distal nephron. Other factors that may trigger the renin-angiotensin mechanism are a drop in mean systemic blood pressure below 80 mm Hg, and activated macula densa cells responding to low plasma sodium. (pp. 972–973)
16. In active tubular reabsorption, substances are usually moving against electrical and/or chemical gradients. The substances usually move from the filtrate into the tubule cells by secondary active transport coupled to Na+ transport and move across the basolateral membrane of the tubule cell into the interstitial space by diffusion. Most such processes involve cotransport with sodium.
Passive tubular reabsorption encompasses diffusion, facilitated diffusion, and osmosis. Substances move along their electrochemical gradient without the use of metabolic energy. (pp. 975–976)
17. The peritubular capillaries are low-pressure, porous capillaries that readily absorb solutes and water from the tubule cells. They arise from the efferent arteriole draining the glomerulus. (p. 968)
18. Tubular secretion is important for the following reasons: (a) disposing of substances not already in the filtrate; (b) eliminating undesirable substances that have been reabsorbed by passive processes; (c) ridding the body of excessive potassium ions; and (d) controlling blood pH. Tubular secretion moves materials from the blood of the peritubular capillaries through the tubule cells or from the tubule cells into the filtrate. (p. 978)
19. Aldosterone modifies the chemical composition of urine by enhancing sodium ion reabsorption so that very little leaves the body in urine. (p. 978)
20. As it flows through the ascending limb of the loop of Henle, the filtrate becomes hypotonic because it is impermeable to water, and because sodium and chloride are being actively pumped into the interstitial fluid, thereby decreasing solute concentration in the tubule. The interstitial fluid at the tip of the loop of Henle and the deep portions of the medulla are hypertonic because: (1) the loop of Henle serves as a countercurrent multiplier to establish the osmotic gradient, a process that works due to the characteristics of tubule permeability to water in different areas of the tubule and ion transport to the interstitial areas; and (2) the vasa recta acts as a countercurrent exchanger to maintain the osmotic gradient by serving as a passive exchange mechanism that removes water from the medullary areas but leaves salts behind. The filtrate at the tip of the loop of Henle is hypertonic due to the passive diffusion of water from the descending limb to the interstitial areas. (pp. 979–981)
21. The bladder is very distensible. An empty bladder is collapsed and has rugae. Expansion of the bladder to accommodate increased volume is due to the ability of the transitional epithelial cells lining the interior of the bladder to slide across one another, thinning the mucosa, and the ability of the detrusor muscle to stretch. (p. 986)
22. Micturition is the act of emptying the bladder. The micturition reflex is activated when distension of the bladder wall activates stretch receptors. Afferent impulses are transmitted to the sacral region of the spinal cord and efferent impulses return to the bladder via the parasympathetic pelvic splanchnic nerves, causing the detrusor muscle to contract and the internal sphincter to relax. (p. 988)
23. In old age the kidneys become smaller, the nephrons decrease in size and number, and the tubules become less efficient. By age 70, the rate of filtrate formation is only about one half that of middle-aged adults. This slowing is believed to result from impaired renal circulation caused by arteriosclerosis. The bladder is shrunken, with less than half the capacity of a young adult. Problems of urine retention and incontinence occur. (p. 991)
Critical Thinking and Clinical Application Questions
1. Diuretics will remove water from the blood and eliminate it in the urine. Consequently, water will move from the peritoneal cavity into the bloodstream reducing her ascites.
(1) Osmotic diuretics are substances that are not reabsorbed or that exceed the ability of the tubule to reabsorb it, which increases osmolarity of the urine, and causes water to be drawn into the urine from the ISF. (2) Loop diuretics (Lasix) inhibit symporters in the loop of Henle by diminishing sodium chloride uptake. They reduce the normal hyperosmolality of the medullary interstitial fluid, reducing the effects of ADH, resulting in loss of NaCl and water. (3) Thiazides act on the distal convoluted tubule to inhibit water reabsorption.
Her diet is salt-restricted because if salt content in the blood is high, it will cause her to retain water rather than allowing her to eliminate it. (p. 982)
2. A fracture at the lumbar region will stop the impulses to the brain, so there will be no voluntary control of micturition and he will never again feel the urge to void. There will be no dribbling of urine between voidings as long as the internal sphincter is undamaged. Micturition will be triggered in response to bladder stretch by a reflex arc at the sacral region of the spinal cord as it is in an infant. (p. 988)
3. Cystitis is bladder inflammation. Women are more frequent cystitis sufferers than men because the female urethra is very short and its external orifice is closer to the anal opening. Improper toilet habits can carry fecal bacteria into the urethra. (pp. 987–988)
4. Hattie has a renal calculus, or kidney stone, in her ureter. Predisposing conditions are frequent bacterial infections of the urinary tract, urinary retention, high concentrations of calcium in the blood, and alkaline urine. Her pain comes in waves because waves of peristalsis pass along the ureter at intervals. The pain results when the ureter walls close in on the sharp kidney stone during this peristalsis. (p. 986)
5. The use of spermicides in females kills many helpful bacteria, allowing infectious fecal bacteria to colonize the vagina. Intercourse will drive bacteria from the vagina into the urethra, increasing the incidence of urinary tract infection in these females. (p. 988)
6. Renal failure patients accumulate both phosphorus and water between dialysis appointments. Increased levels of phosphorus can lead to leaching of calcium from the bones. Increased water can lead to relatively decreased red blood cell counts. Calcium/magnesium supplements can offset calcium loss from bones, but water intake should be carefully monitored to prevent accumulation in the plasma. (p. 984)