802
UNIT FIVE
milliliters or more, the sensation of fullness becomes increas-
ingly uncomfortable.
As the bladder F lls with urine and its internal pressure
increases, contractions of its wall intensify. When these con-
tractions become strong enough to force the internal urethral
sphincter open, another refl ex signals the external urethral
sphincter to relax, and the bladder may empty. However,
because the external urethral sphincter is composed of skel-
etal muscle, it is under conscious control, and therefore usu-
ally remains contracted until a person decides to urinate.
Nerve centers in the brainstem and cerebral cortex that
inhibit the micturition refl
ex aid this control.
When a person decides to urinate, the external urethral
sphincter relaxes and inhibition of the micturition refl
ex lifts.
Nerve centers in the pons and the hypothalamus heighten
the micturition refl ex. The detrusor muscle contracts and
urine is excreted through the urethra. Within a few moments,
the neurons of the micturition refl ex tire, the detrusor mus-
cle relaxes, and the bladder begins to F
ll with urine again.
Table 20.5
outlines the micturition process, and Clinical
Application 20.4 discusses urinalysis and health.
Table 20.6
lists conditions that result from abnormal development of
part of the urinary system. About 1 in 500 newborns has a
birth defect affecting the urinary system.
Damage to the spinal cord above the sacral region may abolish vol-
untary control of urination. However, if the micturition reF
ex center
and its sensory and motor ±
bers are uninjured, micturition may con-
tinue to occur reF
exively. In this case, the bladder collects urine until
its walls stretch enough to trigger a micturition reF
ex, and the detru-
sor muscle contracts in response. This condition is called an
auto-
matic bladder.
The
membranous urethra
is about 2 centimeters long.
It originates just distal to the prostate gland, passes through
the urogenital diaphragm, and is surrounded by the F
bers of
the external urethral sphincter muscle.
The
penile urethra
is about 15 centimeters long and
passes through the corpus spongiosum of the penis, within
erectile tissue. This portion of the urethra terminates with
the
external urethral
oriF ce at the tip of the penis.
PRACTICE
33
Describe the structure of the urethra.
34
How does the urethra of a male di²
er from that of a female?
Micturition
Urine leaves the urinary bladder by the
micturition
(mik
tu-
rish
un) or urination refl
ex. The detrusor muscle contracts,
and contractions of muscles in the abdominal wall and pelvic
fl oor may help, as well as F
xation of the thoracic wall and
diaphragm. In micturition, the
external urethral sphincter
also relaxes. This muscle, part of the urogenital diaphragm
(see chapter 9, pp. 322–323), surrounds the urethra about 3
centimeters from the bladder and is composed of voluntary
skeletal muscle tissue.
Distension of the bladder wall as it F
lls with urine stim-
ulates the urge to urinate. The wall expands, stimulating
stretch receptors, which triggers the micturition refl
ex.
The
micturition refl
ex center
is in the sacral portion of
the spinal cord. When sensory impulses from the stretch
receptors signal the reflex center, parasympathetic motor
impulses travel out to the detrusor muscle, which contracts
rhythmically in response. A sensation of urgency accompa-
nies this action.
The urinary bladder may hold up to 600 milliliters of
urine. The desire to urinate usually begins when it contains
about 150 milliliters. Then, as urine volume increases to 300
TABLE
20.5
|
Major Events of Micturition
1. Urinary bladder distends as it ±
lls with urine.
2. Stretch receptors in the bladder wall are stimulated, and they signal
the micturition center in the sacral spinal cord.
3. Parasympathetic nerve impulses travel to the detrusor muscle, which
responds by contracting rhythmically.
4. The need to urinate is urgent.
5. Voluntary contraction of the external urethral sphincter and
inhibition of the micturition reF
ex by impulses from the brainstem
and the cerebral cortex prevent urination.
6. ³ollowing the decision to urinate, the external urethral sphincter is
relaxed, and impulses from the pons and the hypothalamus facilitate
the micturition reF
ex.
7. The detrusor muscle contracts, and urine is expelled through the
urethra.
8. Neurons of the micturition reF
ex center fatigue, the detrusor muscle
relaxes, and the bladder begins to ±
ll with urine again.
TABLE
20.6
|
Developmental Abnormalities
of the Urinary System
Condition
Description
Crossed fused ectopia
³used kidneys that lie on one side of the midline
Horseshoe kidney
³usion of kidneys at one pole, usually lower, with
most of each kidney on opposing side of midline
Nephrotic syndrome
Proteinuria (protein in urine) due to abnormal
glomeruli
Oligomeganephronia
Reduced number of nephrons that are
abnormally large
Polycystic kidney
disease
Cysts form in renal tubules and/or collecting
ducts
Renal agenesis
Absence of a kidney
Renal dysplasia
Abnormal kidney structure
Renal hypoplasia
Small kidney with fewer nephrons, but
development normal
Tubular dysgenesis
Abnormal formation of proximal tubules
Vesicoureteral reF
ux
Urine backs up from bladder to ureter or kidney
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