Download Physiology of Micturition

Dr. Eman El Eter
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Definition:
It is a complete autonomic spinal reflex to get
urine outside the body, that is facilitated or
inhibited by higher brain centers.
Processed tubular fluid is dumped by
the collecting system into the renal
pelvis where it enters the ureters.
Pelvis: Collects urine from collecting
ducts.
Ureters: conduits that propel urine by
peristaltic contractions toward the
bladder.
Bladder: a muscular “bag” that holds
urine and forces it by contraction.
Urethra: the conduit for urine from the
bladder to the outside .
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In the renal pelvis there are “electrical
pacemaker” cells that initiate peristaltic
waves in the smooth muscle sheaths
of the ureteral wall. (The pelvis to
ureter is a functional syncitium).
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The pacemaker cells seem to be
stimulated by the stretch of urine filling
the pelvis.
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The movement of the peristaltic wave
is about 2-6 cm/sec., traveling from its
origin at the pelvis down to the bladder
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The peristaltic waves propel the urine along the ureter,
generating a pressure head of which changes from a baseline
of 2-5 cm H2O up to 20-80 cm H2O.
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While peristalsis is independent of nerve input, the action of
symapthetic nerves innervating the ureter may modify the rate
or force of peristalsis.
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Interruption of the flow of urine by an obstruction (such as a
kidney stone) stops flow, increases pressure which can back
up through the ureter into the pelvis, and increase the nephron
and subcapsular hydrostatic pressure. This may result in the
condition hydronephrosis.
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There are autonomic pain fibers in the ureter which account
for the acute pain when a kidney stone is formed.
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Bladder tone is derived from the volume and pressure exerted
on the inside of the bladder (interavesical pressure).
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Increasing bladder volume by 50 ml increases pressure. As
volume increases further, the interavesical pressure increases,
but not much until you get above 300 ml. then the pressure
rises steeply with additional volume.
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This increase in volume and pressure increases bladder “tone”
triggering the mictiurition reflex.
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Trigone area is very sensitive to expansion and once stretched to a
certain degree, the urinary bladder signals the brain of its need to
empty. The signals become stronger as the bladder continues to fill.
Urinary bladder
distension
reaches the
conscious level at
urine volume of
150-200 ml
Cystometrogram
Next, the detrussor muscle of the bladder wall contracts in waves (see red
lines in previous figure) to expel the urine.
Contraction of the detrussor muscle is the mechanism responsible for the
micturition process.
Voluntary contraction of the abdominal muscles further contracts the
bladder, increasing the voiding.
Once the bladder is empty, we are back down to the “no tone” phase and
the sphincters can close again.
During micturition, the perineal muscles and external urethral sphincter are
relaxed, the detruser muscle contract and urine passes out.
After urination, the female urethra empties by gravity.
In the male urine remaining in the urethra is expelled by several
contractions of bulbocavernous muscle.
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Stimulus: distension of bladder stimulates stretch receptors in bladder wall.
Afferent: fibers in the pelvic nerves
Center: sacral segments S2,S3,S4
Efferent: Parasympathetic fibers to the bladder.
Response: relaxation of the sphincters and contraction of bladder wall.
In adults the volume of urine that initiates a reflex contraction is about 300-400 ml.
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Higher control: A facilitatory area in the pontine region and inhibitory area in the
midbrain. Efferent impulses from the brain suppress the reflex (a learned reflex) until
a decision is made to relax the external sphinctor using voluntary nerves.
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Voiding begins with relaxation of the external sphinctor, then the internal sphinctor.
Voluntary contraction of abdominal muscles helps the expulsion of urine by
increasing intra-abdominal pressure, but voiding can be initiated with straining.
Effect of spinal cord transection:
 -Spinal shock: bladder becomes flaccid and
unresponsive. It becomes overfilled and urine
dribbles through the sphincters (overflow
incontinence).
 - After spinal shock phase has passed, the
voiding reflex returns with no voluntary
control.