The Kidneys and Regulation of Water and Inorganic Ions
499
contract. As noted previously, this contraction facilitates the
opening of the internal urethral sphincter. Simultaneously,
the afferent input from the stretch receptors refl exly inhib-
its the sympathetic neurons to the internal urethral sphinc-
ter, which further contributes to its opening. In addition, the
afferent input also refl exly inhibits the somatic motor neurons
to the external urethral sphincter, causing it to relax. Both
sphincters are now open, and the contraction of the detrusor
muscle can produce urination.
We have thus far described micturition as a local spinal
refl ex, but descending pathways from the brain can also pro-
foundly infl uence this refl ex, determining the ability to prevent
or initiate micturition voluntarily. Loss of these descending
pathways as a result of spinal cord damage eliminates one’s abil-
ity to voluntarily control micturition. Prevention of micturition,
learned during childhood, operates in the following way. As the
bladder distends, the input from the bladder stretch receptors
causes, via ascending pathways to the brain, a sense of bladder
fullness and the urge to urinate. But in response to this, urina-
tion can be voluntarily prevented by activating descending path-
ways that stimulate both the sympathetic nerves to the internal
urethral sphincter and the somatic motor nerves to the external
urethral sphincter. In contrast, urination can be voluntarily ini-
tiated via the descending pathways to the appropriate neurons.
Incontinence
Incontinence
is the involuntary release of urine, which can
be a disturbing problem both socially and hygienically. The
most common types are
stress incontinence
(due to sneezing,
coughing, or exercise) and
urge incontinence
(associated
with the desire to urinate). Incontinence is more common
in women and may occur one to two times per week in more
than 25 percent of women older than 60. It is very common
in older women in nursing homes and assisted living facilities.
In women, stress incontinence is usually due to a loss of
urethral support provided by the anterior vagina (see Figure
ADDITIONAL CLINICAL EXAMPLES
17–13a). Medications (such as estrogen replacement therapy
to improve vaginal tone) can often relieve stress incontinence.
Severe cases may require surgery to improve vaginal support of
the bladder and urethra. The cause of urge incontinence is often
unknown in individual patients. However, any irritation to the
bladder or urethra (e.g., with a bacterial infection) can cause
urge incontinence. Urge incontinence can be treated with drugs
such as tolterodine or oxybutin, which antagonize the effects
of the parasympathetic nerves on the detrusor muscle. Because
these drugs are anticholinergic, they can have side effects such
as blurred vision, constipation, and increased heart rate.
SECTION A SUMMARY
Renal Function
I. The kidneys regulate the water and ionic composition of
the body, excrete waste products, excrete foreign chemicals,
produce glucose during prolonged fasting, and release factors
and hormones into the blood (renin, 1,25-dihydroxyvitamin D,
and erythropoietin). The fi rst three functions are accomplished
by continuous processing of the plasma.
Structure of the Kidneys and the Urinary System
I. Each nephron in the kidneys consists of a renal corpuscle and a
tubule.
a. Each renal corpuscle comprises a capillary tuft, termed
a glomerulus, and a Bowman’s capsule that the tuft
protrudes into.
b. The tubule extends from Bowman’s capsule and is
subdivided into the proximal tubule, loop of Henle, distal
convoluted tubule, and collecting-duct system. At the level
of the collecting ducts, multiple tubules join and empty
into the renal pelvis, from which urine fl
ows through the
ureters to the bladder.
c. Each glomerulus is supplied by an afferent arteriole, and
an efferent arteriole leaves the glomerulus to branch into
peritubular capillaries, which supply the tubule.
Basic Renal Processes
I. The three basic renal processes are glomerular fi ltration,
tubular reabsorption, and tubular secretion. In addition, the
kidneys synthesize and/or catabolize certain substances. The
excretion of a substance is equal to the amount fi ltered plus the
amount secreted, minus the amount reabsorbed.
II. Urine formation begins with glomerular fi ltration—
approximately 180 L/day—of essentially protein-free plasma
into Bowman’s space.
a. Glomerular fi ltrate contains all plasma substances other
than proteins (and substances bound to proteins) in
virtually the same concentrations as in plasma.
b. Glomerular fi ltration is driven by the hydrostatic pressure
in the glomerular capillaries and is opposed by both the
hydrostatic pressure in Bowman’s space and the osmotic
force due to the proteins in the glomerular capillary plasma.
III. As the fi ltrate moves through the tubules, certain substances
are reabsorbed either by diffusion or by mediated transport.
a. Substances to which the tubular epithelium is permeable are
reabsorbed by diffusion because water reabsorption creates
tubule-interstitium concentration gradients for them.
b. Active reabsorption of a substance requires the participation
of transporters in the luminal or basolateral membrane.
c. Tubular reabsorption rates are very high for nutrients, ions,
and water, but lower for waste products.
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