982
UNIT 4
Maintenance of the Body
25
Te male urethra has a double function: It carries semen as well
as urine out of the body. We discuss its reproductive function in
Chapter 27.
Homeostatic Imbalance 25.6
Because the female’s urethra is very short and its external orifice
is close to the anal opening, improper toilet habits (wiping back
to front aFer defecation) can easily carry fecal bacteria into the
urethra. Actually, most
urinary tract infections
occur in sexually
active women, because intercourse drives bacteria from the va-
gina and external genital region toward the bladder. Te use of
spermicides magnifies this problem, because the spermicide kills
helpful bacteria, allowing infectious fecal bacteria to colonize the
vagina. Overall, 40% of all women get urinary tract infections.
Te urethral mucosa is continuous with that of the rest of the
urinary tract, and an inflammation of the urethra (
urethritis
)
can ascend the tract to cause bladder inflammation (
cystitis
) or
even renal inflammations (
pyelitis
or
pyelonephritis
). Symptoms
of urinary tract infection include dysuria (painful urination),
urinary
urgency
and
frequency
, fever, and sometimes cloudy or
blood-tinged urine. When the kidneys are involved, back pain
and a severe headache oFen occur. Antibiotics easily cure most
urinary tract infections.
Micturition
Define micturition and describe its neural control.
Micturition
(mik
0
tu-rish
9
un;
mictur
5
urinate), also called
uri-
nation
or
voiding
, is the act of emptying the urinary bladder. ±or
micturition to occur, three things must happen simultaneously:
(1) the detrusor must contract, (2) the internal urethral sphincter
must open, and (3) the external urethral sphincter must open.
Te detrusor and its internal urethral sphincter are composed
of smooth muscle and are innervated by both the parasympathetic
and sympathetic nervous systems, which have opposing actions.
Te external urethral sphincter, in contrast, is skeletal muscle,
and therefore is innervated by the somatic nervous system.
How are the three events required for micturition coordi-
nated? Micturition is most easily understood in infants where
a spinal reflex coordinates the process. As urine accumulates,
distension of the bladder activates stretch receptors in its walls.
Impulses from the activated receptors travel via visceral afferent
fibers to the sacral region of the spinal cord. Visceral afferent im-
pulses, relayed by sets of interneurons, excite parasympathetic
neurons and inhibit sympathetic neurons
(Figure 25.21)
. As a
result, the detrusor contracts and the internal sphincter opens.
Visceral afferent impulses also decrease the firing rate of so-
matic efferents that normally keep the external urethral sphinc-
ter closed. Tis allows the sphincter to relax so urine can flow.
Between ages 2 and 3, descending circuits from the brain
have matured enough to begin to override reflexive urination.
Te pons has two centers that participate in control of micturi-
tion. Te
pontine storage center
inhibits micturition, whereas
the
pontine micturition center
promotes this reflex. Afferent im-
pulses from bladder stretch receptors are relayed to the pons, as
well as to higher brain centers that provide the conscious aware-
ness of bladder fullness.
Lower bladder volumes primarily activate the pontine storage
center, which inhibits urination by suppressing parasympathetic
and enhancing sympathetic output to the bladder. When a person
chooses not to void, reflex bladder contractions subside within a
minute or so and urine continues to accumulate. Because the ex-
ternal sphincter is voluntarily controlled, we can choose to keep
it closed and postpone bladder emptying temporarily. AFer addi-
tional urine has collected, the micturition reflex occurs again and,
if urination is delayed again, is damped once more.
Te urge to void gradually becomes greater and greater, and
micturition usually occurs before urine volume exceeds 400 ml.
AFer normal micturition, only about 10 ml of urine remains in
the bladder.
Homeostatic Imbalance
25.7
AFer the toddler years,
incontinence
is usually a result of weak-
ened pelvic muscles following childbirth or surgery, physical
pressure during pregnancy, or nervous system problems. In
stress incontinence
, a sudden increase in intra-abdominal pres-
sure (during laughing and coughing) forces urine through the
external sphincter. Tis condition is common during pregnancy
when the heavy uterus stretches the muscles of the pelvic floor
and the urogenital diaphragm that support the external sphinc-
ter. In
overflow incontinence
, urine dribbles from the urethra
whenever the bladder overfills.
In
urinary retention
, the bladder is unable to expel its
contained urine. Urinary retention is common aFer general
anesthesia (it takes a little time for the detrusor to regain its
activity). Urinary retention in men oFen reflects hypertrophy
of the prostate, which narrows the urethra, making it difficult
to void. When urinary retention is prolonged, a slender drain-
age tube called a
catheter
(kath
9
ĕ-ter) must be inserted through
the urethra to drain the urine and prevent bladder trauma from
excessive stretching.
Check Your Understanding
18.
A kidney stone blocking a ureter would interfere with urine
flow to which organ? Why would the pain occur in waves?
19.
What is the trigone of the bladder, and which landmarks
define its borders?
20.
Name the three regions of the male urethra.
21.
How does the detrusor respond to increased firing of the
parasympathetic fibers that innervate it? How does this
affect the internal urethral sphincter?
For answers, see Appendix H.
Developmental Aspects
of the Urinary System
Trace the embryonic development of the urinary organs.
List several changes in urinary system anatomy and
physiology that occur with age.
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