The Urinary System
Objectives
Kidney Anatomy
1. Describe the anatomy of the kidney and its placement in the body.
2. List the regions of the kidney and the structures found within each region.
3. Trace the vascular pathway through the kidney.
4. Name the structures and functions of the nephron and its elements.
Kidney Physiology: Mechanisms of Urine Formation
5. List the steps of urine formation.
6. Explain glomerular filtration and the mechanisms that control its pressure and
rate.
7. Define tubular reabsorption; list the solutes that are reabsorbed and the
mechanisms used to reclaim them from the filtrate.
8. Discuss the differences in solute reabsorption in each portion of the nephron
tubules.
9. Explain tubular secretion, and list the solutes that are secreted.
10. Describe the countercurrent mechanism regulating urine concentration and
volume.
11. Identify the roles of antidiuretic hormone and aldosterone in water and
sodium reabsorption.
Urine
12. List the physical characteristics of urine and indicate its chemical
composition.
Ureters
13. Describe the anatomy of the ureters.
Urinary Bladder
14. Explain the structure, location, and capacity of the urinary bladder.
Urethra
15. Identify the general location, structure, and function of the urethra and
compare the male and female urethras.
Micturition
16. Define micturition and the events controlling it.
Developmental Aspects of the Urinary System
17. Explain the developmental events of the fetal urinary system.
18. Discuss the changes in control of micturition that occur during childhood.
19. List the age-related changes that occur in the urinary system.
Lecture Outline
I. Kidney Anatomy (pp. 998–1006; Figs. 25.1–25.7)
A. Location and External Anatomy (pp. 998–999; 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 a fibrous, transparent renal capsule; a
fatty adipose capsule that cushions the organ; and an outer fibrous
renal fascia that anchors the kidney to surrounding structures.
B. Internal Anatomy (pp. 999–1001; 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 (p. 1001; Fig. 25.3)
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. 1001–1006; Figs. 25.4–25.7).
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. 1007–1021;
Figs. 25.8–25.16; Tables 25.1–25.2)
A. Step 1: Glomerular Filtration (pp. 1007–1011; Figs. 25.9–25.10)
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. 1011–1015; Figs. 25.11–25.12; 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. 1015)
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. 1015–1021; Figs.
25.13–25.16)
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. Since 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. Since 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. 1021)
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 since 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. 1021–1023; Table 25.2)
A. Physical Characteristics (p. 1021)
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 (pp. 1022–1023; 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. 1023–1024; Fig. 25.17)
A. Ureters are tubes that actively convey urine from the kidneys to the
bladder.
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.
V. Urinary Bladder (pp. 1024–1026; Figs. 25.18–25.19)
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.
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.
VI. Urethra (p. 1026; Fig. 25.18)
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.
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.
C. The external urethral orifice lies between the clitoris and vaginal opening
in females, or occurs at the tip of the penis in males.
VII.
Micturition (p. 1027; Fig. 25.20)
A. Micturition, or urination, is the act of emptying the bladder.
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. 1027–1030; Fig.
25.21)
A. In the developing fetus, the mesoderm-derived urogenital ridges give rise
to three sets of kidneys: the pronephros, mesonephros, and metanephros.
(pp. 1027–1028; Fig. 25.21)
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 persist until development of the
metanephros.
3. The metanephros develops at about five weeks, and forms ureteric
buds that give rise to the ureters, renal pelvises, 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. 1030)
C. From two months of age until adolescence, urine output increases until
the adult output volume is achieved. (p. 1030)
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. 1030)
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. 1030)