The Urinary System
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
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
Name the three sets of embryonic kidneys in the order that
List two factors that might contribute to urinary retention in
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
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.
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.
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.
For more chapter study tools, go to the Study Area of
There you will ﬁnd:
Practice Anatomy Lab
Videos, Practice Quizzes and Tests, MP3 Tutor Sessions,
Case Studies, and much more!
Location and External Anatomy
Te paired kidneys are retroperitoneal in the superior lumbar
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
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
Te kidneys receive 25% of the total cardiac output per minute.
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.
Te nerve supply of the kidneys is derived from the renal plexus.
Nephrons are the structural and functional units of the kidneys.
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.
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