805
CHAPTER TWENTY
Urinary System
muscle tone in the bladder, urethra and ureters. Incontinence
affects 15% to 20% of women over sixty-F ve and half of all
men. In women, incontinence refl
ects the stresses of child-
birth and the effects of less estrogen during menopause.
Bladder sphincter muscles atrophy, muscles in the pelvic
floor weaken, and muscle tone of the urethra wanes. In
males, incontinence usually is a response to an enlarged
prostate gland pressing on the bladder.
PRACTICE
37
How do the kidneys change in appearance with advancing years?
38
What happens to glomeruli as a person ages?
39
How does kidney function change with age?
40
How do aging-related changes in the cardiovascular system aF
ect
the kidneys?
41
How do the urinary bladder, ureters, and urethra change with age?
slower to dilate or constrict in response to body conditions.
The kidneys’ release of renin declines, hampering control
of osmotic pressure, blood pressure, and sodium and potas-
sium ion concentrations in the blood. The kidneys are also
less able to activate vitamin D, which may contribute to the
higher prevalence of osteoporosis among the elderly.
The urinary bladder, ureters, and urethra lose elasticity
and recoil with age. In the later years, the bladder holds less
than half of what it did in young adulthood, and may retain
more urine after urination. In the elderly, the urge to urinate
may become delayed, so when it does happen, it is sudden.
Older individuals have to urinate at night more than younger
people.
Controlling bladder function is a challenge at the begin-
ning of life and much later too. A child usually learns to con-
trol urination by about age two or three years. Incontinence
becomes more common in advanced years, although it is
not considered a normal part of aging. It results from loss of
5. Nephrons
a. Structure of a nephron
(1) A nephron is the functional unit of the kidney.
(2) It consists of a renal corpuscle and a renal
tubule.
(a) The corpuscle consists of a glomerulus
and a glomerular capsule.
(b) Parts of the renal tubule include the
proximal convoluted tubule, the nephron
loop (ascending and descending limbs),
and the distal convoluted tubule.
(3) The nephron joins a collecting duct, which
empties into a minor calyx.
b. Juxtaglomerular apparatus
(1) The juxtaglomerular apparatus is between the
ascending limb of the nephron loop and the
afferent and efferent arterioles.
(2) It consists of the macula densa and the
juxtaglomerular cells.
c. Cortical and juxtamedullary nephrons
(1) Cortical nephrons are the most numerous and
have corpuscles near the surface of the kidney.
(2) Juxtamedullary nephrons have corpuscles near
the medulla.
d. Blood supply of a nephron
(1) The glomerular capillary receives blood from
the afferent arteriole and passes it to the
efferent arteriole.
(2) The efferent arteriole gives rise to the
peritubular capillary system, which surrounds
the renal tubule.
(3) Capillary loops, called vasa recta, dip down
into the medulla.
CHAPTER SUMMARY
20.1
INTRODUCTION (PAGE 775)
The urinary system maintains homeostasis by regulating
the composition, pH, and volume of body fl
uids. It does
this by producing urine. The urinary system includes the
kidneys, ureters, urinary bladder, and urethra.
20.2
KIDNEYS (PAGE 776)
1. Location of the kidneys
a. The kidneys are bean-shaped organs on either side
of the vertebral column, high on the posterior wall
of the abdominal cavity.
b. They are posterior to the parietal peritoneum and
anchored by adipose and connective tissues.
2. Kidney structure
a. A kidney has a hollow renal sinus.
b. The ureter expands into the renal pelvis, which, in
turn, leads to the major and minor calyces.
c. Renal papillae project into the minor calyces.
d. Kidney tissue is divided into a medulla and a
cortex.
3. ±unctions of the kidneys
a. The kidneys remove metabolic wastes from the
blood and excrete them.
b. They also help regulate red blood cell production,
blood pressure, calcium ion absorption, and the
volume, composition, and pH of the blood.
4. Renal blood vessels
a. Arterial blood fl
ows through the renal artery,
interlobar arteries, arcuate arteries, cortical radiate
arteries, afferent arterioles, glomerular capillaries,
efferent arterioles, and peritubular capillaries.
b. Venous blood returns through a series of vessels
that correspond to those of the arterial pathways.
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