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CHAPTER
25
The Urinary System
Objectives
Kidney Anatomy
1. Describe the gross anatomy of the kidney and its coverings.
Kidney Physiology: Mechanisms of Urine Formation
4. Describe the forces (pressures) that promote or counteract glomerular filtration.
6. Describe the mechanisms underlying water and solute reabsorption from the renal tubules
into the peritubular capillaries.
8. Describe the importance of tubular secretion and list several substances that are secreted.
10. Explain formation of dilute versus concentrated urine.
Clinical Evaluation of Kidney Function
11. Define renal clearance and explain how this value summarizes the way a substance is
handled by the kidney.
Urine Transport, Storage, and Elimination
14. Describe the general location, structure, and function of the ureters.
15. Describe the general location, structure, and function of the urinary bladder.
Developmental Aspects of the Urinary System
19. Trace the embryonic development of the urinary organs.
Suggested Lecture Outline
I. Kidney Anatomy (pp. 955–963; Figs. 25.1–25.8)
A. Location and External Anatomy (pp. 955–956; 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 vertical cleft, the renal hilum, which leads
into a renal sinus, where the blood vessels, nerves, and lymphatics lie.
B. Internal Gross Anatomy (pp. 956–957; 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. 957–958; Fig. 25.4)
1. Blood supply into the kidneys progresses to the cortex through renal arteries to
segmental, lobar, interlobar, arcuate, and cortical radiate (interlobular) arteries.
2. Afferent arterioles branching away from the cortical radiate arteries give rise to the
microscopic vasculature that is the key element of kidney function.
D. Nephrons are the structural and functional units of the kidneys that carry out processes
that form urine (pp. 958–963; Figs. 25.5–25.8).
1. Each nephron consists of a renal corpuscle, composed of a tuft of capillaries
(the glomerulus) and surrounded by a glomerular capsule (Bowman’s capsule).
a. The glomerular capillaries are fenestrated to increase permeability, allowing the
formation of solute-rich, but protein-free, filtrate.
b. The nephron loop has a descending limb and an ascending limb that has both thick
and thin segments.
3. There are two classes of nephrons: 85% are cortical nephrons, which are located
almost entirely within the cortex; 15% are juxtamedullary nephrons located near the
cortex-medulla junction.
4. The renal tubule of each nephron is closely associated with two capillary beds: the
glomerulus; and the peritubular capillaries and vasa recta.
a. The glomerulus is specialized for filtration and is fed and drained by an afferent and
5. The juxtaglomerular complex is a structural arrangement between the afferent arteriole
and the distal convoluted tubule that forms granular cells and macula densa cells.
a. The macula densa are cells in the ascending limb that act as chemoreceptors that
monitor NaCl content of filtrate entering the distal convoluted tubule.
II. Kidney Physiology: Mechanisms of Urine Formation (pp. 963–977; Figs. 25.9–
25.17; Table 25.1)
A. Of the approximately 1200 ml of blood that passes through the glomeruli each minute,
roughly 650 ml is blood plasma, and one-fifth of this is filtered across the glomerulus
(p. 963).
B. Filtrate contains everything found in blood plasma except proteins, while urine contains
unneeded substances, such as excess salts and metabolic wastes (p. 965; Fig. 25.9).
3. The net filtration pressure responsible for filtrate formation is given by the balance of
hydrostatic pressure in the glomerulus against the combined forces of capsular
hydrostatic pressure, and colloid osmotic pressure of glomerular blood.
4. The glomerular filtration rate is the volume of filtrate formed each minute by all the
5. 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.
6. 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
E. Step 2: Tubular Reabsorption (pp. 968–972; Figs. 25.13–25.14; Table 25.1)
1. Tubular reabsorption is a selective transepithelial process that begins as soon as the
filtrate enters the proximal convoluted tubule.
2. In healthy kidneys, nearly all organic nutrients such as glucose and amino acids are
3. Active tubular reabsorption requires direct or indirect use of ATP, while passive
tubular reabsorption involves movement of molecules down their electrochemical
4. Passive tubular reabsorption of water occurs down osmotic gradients created by the
absorption of Na+ and other solutes.
a. Absorption of water in the collecting duct requires antidiuretic hormone (ADH).
5. Passive reabsorption of solutes such as lipid-soluble solutes, some ions, and urea
6. Transport maximums exist for most substances that are reabsorbed via transport
7. Different areas of the tubules have different absorptive capabilities:
a. The proximal convoluted tubule is most active in reabsorption; nearly all glucose,
amino acids, and vitamins are absorbed there, 65% of water and Na+.
1. Tubular secretion disposes of unwanted solutes, eliminates unwanted, reabsorbed
solutes, 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
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
nephron loops (the countercurrent multiplier) of juxtamedullary nephrons and the flow
of blood through the vasa recta (the countercurrent exchanger).
3. Formation of concentrated urine occurs in response to the release of ADH, which
makes the collecting ducts permeable to water and increases water uptake from the
urine.
4. Diuretics act to increase urine output by either acting as an osmotic diuretic or by
inhibiting Na+ and resulting obligatory water reabsorption.
III. Clinical Evaluation of Kidney Function (pp. 977–979; Table 25.2)
A. Renal clearance refers to the volume of plasma that is cleared of a specific substance in a
given time (p. 978; Table 25.2).
1. Inulin is used as a clearance standard to determine glomerular filtration rate because it
is not reabsorbed, stored, or secreted.
B. Physical Characteristics of Urine (pp. 978–979)
1. Freshly voided urine is clear and pale to deep yellow due to urochrome, a pigment
resulting from the destruction of hemoglobin.
C. Chemical Composition of Urine (p. 979; 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. Urine Transport, Storage, and Elimination (pp. 979–982; Figs. 25.18–25.21)
A. Ureters are tubes that actively convey urine from the kidneys to the bladder
(pp. 979–980; Figs. 25.18–25.19).
B. 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 (pp. 980–981; Fig. 25.20).
1. The bladder is a retroperitoneal organ on the pelvic floor, just posterior to the pubic
symphysis, and has openings in the interior for the ureters and urethra, which form a
triangular region called the trigone.
expands, the wall stretches and thins, and the folds, rugae, disappear.
C. 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 (pp. 981–982; Fig. 25.20).
1. There are two sphincter muscles associated with the urethra: the internal urethral
sphincter, which is involuntary and formed from detrusor smooth muscle; and the
external urethral sphincter, which is voluntary and formed by the skeletal muscle at
the urogenital diaphragm.
2. The external urethral orifice lies between the clitoris and vaginal opening in females,
and it is located at the tip of the penis in males.
3. There are two centers in the pons that participate in control of micturition: the pontine
storage center inhibits micturition, while the pontine micturition center promotes the
reflex.
V. Developmental Aspects of the Urinary System (pp. 982–985; Fig. 25.22)
A. In the developing fetus, the mesoderm-derived urogenital ridges give rise to three sets of
kidneys: the pronephros, mesonephros, and metanephros (pp. 983–984; Fig. 25.22).
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.
B. Newborns void most frequently because the bladder is small and the kidneys cannot
concentrate urine until two months of age (pp. 984–985).
C. From two months of age until adolescence, urine output increases until the adult output
volume is achieved (p. 985).
Cross References
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
7. Chapter 19: Fenestrated capillaries; arterioles; autoregulation of blood flow; vascular
resistance; fluid dynamics; renin-angiotensin-aldosterone mechanism
Lecture Hints
2. Use the analogy of a cone-shaped filter in a glass funnel to illustrate how a pyramid fits
into its calyx. This gives students a 3-D structure to relate to kidney anatomy.
4. Poke a finger into a partially inflated balloon to illustrate how the glomerular capsule
forms around the glomerulus.
5. Emphasize the unique microvasculature of the kidney arterioles and capillary beds.
8. State that unlike most capillary beds of the body in which blood flow is changed relative
to metabolic demand of the tissues they serve, the glomerular capillaries have a relatively
9. Establishing a point of reference is essential for student understanding of renal system
terminology. Students often have problems with reabsorption versus secretion: “Is the
tubule secreting into the blood?” Make sure the class establishes the epithelial cells of the
tubule (or blood) as a reference point when using the terms secretion and reabsorption.
Another possible source of confusion is secretion versus excretion.
10. Emphasize that ADH and aldosterone are hormones that, individually, address two
11. Emphasize that the backflow of urine into the ureters is prevented by means of a
physiological sphincter; the ureters enter the bladder wall at an angle so that volume and
12. Mention the similar function of the rugae in the bladder to the rugae in the stomach.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. If possible, arrange for someone from a local renal dialysis center to come to talk to the
class about how an artificial kidney works and other aspects of the dialysis process.
3. Display a hydrometer and other materials used to perform a urinalysis. Discuss the
importance of the urinalysis in routine physicals and in pathological diagnosis.
Critical Thinking/Discussion Topics
1. Discuss the link between emotions and kidney function.
2. Explore the effects of diuretics on kidney function.
3. Explain why physicians tell a sick individual to drink plenty of fluids and why fluid
intake and output is so carefully monitored in hospital settings.
Library Research Topics
1. Research the effects of common drugs such as penicillin, the myceins, etc., on kidney
function.
2. Study the effects of hypertensive drugs on kidney function.
3. Research the effect of circulatory shock on kidney function and explain why the kidneys
are affected.
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 25.1 The urinary system.
Figure 25.2 Position of the kidneys against the posterior body wall.
that adjust plasma composition.
Figure 25.10 The filtration membrane.
Figure 25.11 Forces determining net filtration pressure (NFP).
Figure 25.12 Physiological mechanisms regulating the glomerular filtration rate (GFR)
in the kidneys.
Figure 25.13 Transcellular and paracellular routes of tubular reabsorption.
Figure 25.14 Reabsorption by PCT cells.
Figure 25.15 Summary of tubular reabsorption and secretion.
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
11. The perirenal fat capsule helps to hold the kidney in place against the posterior trunk wall
and cushions it against blows. (p. 956)
12. A creatinine molecule travels the following route from a glomerulus to the urethra: first,
it passes through the glomerular filtration membrane, a porous membrane made up of a
fenestrated capillary endothelium, a thin basement membrane, and the visceral membrane
13. Glomerular filtrate is a solute-rich fluid without blood cells or plasma proteins due to the
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
15. Sympathetic nervous system controls protect the body during extreme stress by redirect-
ing blood to more vital organs. Strong sympathetic stimulation causes release of norepi-
nephrine to alpha-adrenergic receptors, causing strong vasoconstriction of kidney
arterioles. This results in a drop in glomerular filtration, and indirectly stimulates another
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
17. The peritubular capillaries are low-pressure, highly porous capillaries that readily absorb
solutes and water from the tubule cells. (p. 962)
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
19. Aldosterone modifies the chemical composition of urine by enhancing sodium ion
20. As it flows through the ascending limb of the nephron loop, the filtrate becomes hypo-
tonic because the loop is impermeable to water, and because sodium and chloride are
being actively pumped into the interstitial fluid, thereby decreasing solute concentration
21. The bladder is very distensible. An empty bladder is collapsed and has rugae, but expan-
sion of the bladder allows it to accommodate increased volume. This 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 to stretch. (p. 980)
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 transmit-
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
Critical Thinking and Clinical Application Questions
1. Diuretics will remove water from the blood and eliminate it in the urine. Consequently,
water will be osmotically drawn 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 osmolality of the urine, and causes water to be
drawn into the urine from the ISF. (2) Loop diuretics (Lasix) inhibit symporters in the
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
3. Cystitis is bladder inflammation. Women are more frequent cystitis sufferers than
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
5. The use of spermicides in females kills many helpful bacteria, allowing infectious fecal
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. 978)
Suggested Readings
Bader, Michael. “Tissue Renin-Angiotensin Systems.” Science and Medicine 8 (3) (May/June
2002): 128–137.
Depner, T. A. “‘Artificial’ Hemodialysis Versus ‘Natural’ Hemofiltration.” American Journal
of Kidney Diseases 52 (3) (Sept. 2008): 403–406.
Garty, Haim. “Complex Challenges—What Will the Collecting Duct Do When Both Na+ and
(2) (Aug. 2011): F344–F354.
Kurbel, S., K. Dodig, and R. Radić. “The Osmotic Gradient in Kidney Medulla: A Retold
Story.” Advances in Physiology Education 26 (4) (Dec. 2002): 278–281.
Medhora, Meetha, et al. “Radiation Damage to the Lung: Mitigation by Angiotensin-
Converting Enzyme (ACE) Inhibitors.” Respirology 17 (1) (Jan. 2012): 66–71.
Rastogi, A., and A. Nissenson. “The Future of Renal Replacement Therapy.” Advances in
Chronic Kidney Disease 14 (3) (July 2007): 249–255.
Ritz, E., and C. Wanner. “Statin Use Prolongs Patient Survival After Renal Transplantation.”
Journal of the American Society of Nephrology 19 (11) (Nov. 2008): 2037–2040.
