Chapter 25
The Urinary System
985
Te bladder of an aged person is shrunken, with less than
half the capacity of a young adult (250 ml versus 600 ml). Loss
of bladder tone causes an annoying increase in frequency of
micturition.
Nocturia
(nok-tu
9
re-ah), the need to get up during
the night to urinate, plagues almost two-thirds of this popula-
tion. Many people eventually experience incontinence, which
can usually be treated with exercise, medications, or surgery.
Check Your Understanding
22.
Name the three sets of embryonic kidneys in the order that
they develop.
23.
List two factors that might contribute to urinary retention in
elderly men.
For answers, see Appendix H.
Te ureters, urinary bladder, and urethra play important
roles in transporting, storing, and eliminating urine from the
body, but when the term “urinary system” is used, it is the
kidneys that capture center stage. As summarized in
System
Connections
in Chapter 26, other organ systems of the body
contribute to the well-being of the urinary system in many
ways. In turn, without continuous kidney function, the elec-
trolyte and fluid balance of the blood is dangerously disturbed,
and internal body fluids quickly become contaminated with
nitrogenous wastes. No body cell can escape the harmful ef-
fects of such imbalances.
Now that we have described renal mechanisms, we are ready
to integrate kidney function into the larger topic of fluid and
electrolyte balance in the body—the focus of Chapter 26.
voids 5 to 40 times daily, depending on fluid intake. By 2 months,
infants void approximately 400 ml/day, and the amount stead-
ily increases until adolescence, when adult urine output (about
1500 ml/day) is achieved.
Incontinence, the inability to control micturition, is nor-
mal in infants because their nervous systems have not matured
enough to control the external urethral sphincter. Reflex void-
ing occurs each time a baby’s bladder fills enough to activate
the stretch receptors. Control of the voluntary urethral sphinc-
ter goes hand in hand with nervous system development. By
15 months, most toddlers know when they have voided. By 24
months, some children are ready to begin toilet training. Day-
time control usually is achieved first. It is unrealistic to expect
complete nighttime control before age 4.
From childhood through late middle age, most urinary sys-
tem problems are infectious conditions.
Escherichia coli
(esh
0
ĕ-
rik
9
e-ah ko
9
li) bacteria are normal residents of the digestive
tract and generally cause no problems there, but these bacteria
account for 80% of all urinary tract infections.
Sexually trans-
mitted infections
can also inflame the urinary tract and clog
some of its ducts. Childhood streptococcal infections such as
strep throat and scarlet fever, if not treated promptly, may cause
long-term inflammatory renal damage.
Only about 3% of elderly people have histologically normal
kidneys, and kidney function declines with advancing age. Te
kidneys shrink as the nephrons decrease in size and number,
and the tubule cells become less efficient. By age 80, the GFR
is only half that of young adults, possibly due to atherosclerotic
narrowing of the renal arteries. Diabetics are particularly at risk
for renal disease, accounting for almost half of new cases.
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Kidney Anatomy
(pp. 955–963)
Location and External Anatomy
(pp. 955–956)
1.
Te paired kidneys are retroperitoneal in the superior lumbar
region.
2.
A fibrous capsule, a perirenal fat capsule, and renal fascia
surround each kidney. Te perirenal fat capsule helps hold the
kidneys in position.
Internal Gross Anatomy
(pp. 956–957)
3.
A kidney has a superficial cortex, a deeper medulla consisting
mainly of medullary pyramids, and a medial pelvis. Extensions of
the pelvis (calyces) surround and collect urine draining from the
apices of the medullary pyramids.
Blood and Nerve Supply
(pp. 957–958)
4.
Te kidneys receive 25% of the total cardiac output per minute.
5.
Te vascular pathway through a kidney is as follows: renal artery
→ segmental arteries → interlobar arteries → arcuate arteries
→ cortical radiate arteries → afferent arterioles → glomeruli →
efferent arterioles → peritubular capillary beds → cortical radiate
veins → arcuate veins → interlobar veins → renal vein.
6.
Te nerve supply of the kidneys is derived from the renal plexus.
Nephrons
(pp. 958–963)
7.
Nephrons are the structural and functional units of the kidneys.
8.
Each nephron consists of a glomerulus (a high-pressure capillary
bed), a glomerular capsule, and a renal tubule that is continuous
with the capsule. Subdivisions of the renal tubule (from the
glomerular capsule) are the proximal convoluted tubule, nephron
loop, and distal convoluted tubule. A second capillary bed, the
low-pressure peritubular capillary bed, is closely associated with
the renal tubule of each nephron.
9.
Te more numerous cortical nephrons are located almost
entirely in the cortex; only a small part of their nephron loop
penetrates into the medulla. Glomeruli of juxtamedullary
Chapter Summary
25
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