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Micturition

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8
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1 Bladder
The bladder is a hollow muscular sac that sits on the pelvic floor and varies in
size according to the volume of urine it contains (k Fig
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8)
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The bladder wall contains three involuntary smooth muscles:
• trigone – a triangular area on the floor of the bladder which is very
sensitive to stretch; when the bladder is full, nerve endings detect the
stretch and transmit the information to the spinal cord
• detrusor muscle contracts and expels urine
• internal urethral sphincter must relax in order to allow urine to be
expelled
...
One is
voluntary, the other involuntary, and both function as sphincters (rings of
muscle tissue)
...
It is
composed of skeletal muscle and therefore under voluntary control, and
must relax to allow voiding of urine
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URINARY
BLADDER
rugae
detrusor muscle

left ureter
ureteral openings
trigone

bladder neck

internal urethral sphincter

pelvic floor muscle

external urethral sphincter
urethra

Fig
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8 Section through the bladder of a female
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4
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8)
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8
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3 Voluntary control of voiding
The urge to empty the bladder is perceived as an uncomfortable sensation that
is correlated with the level of filling of the bladder
...
Impulses from spinal neurons stimulate the contraction of the
detrusor muscle until the pressure of urine is high enough to empty the
bladder
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8
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Box 8
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Box 8
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Box 8
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Symptoms include an urgent
feeling to urinate or urinating
frequently
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However, if the bladder has become too full, or if a person is very frightened,
then sphincters can relax involuntarily, allowing urine to be voided
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4
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14)
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g
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This is usually due to weakening
of the muscles used to prevent urination such as the pelvic floor muscles
and urethral sphincter
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15)
• urge incontinence is a sudden, intense urge to pass urine
...
It
is possible to have a mixture of both stress and urge incontinence (k Box
8
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g
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EMPTYING
THE BLADDER

URINE
STORAGE
control centre

recognition of appropriate
time and place
(from the age of 2–3 years)

inhibition
of voiding

cerebral cortex (brain)

integrating centre

coordination
of the voiding
response

coordination
of the storage
response

brainstem (pons)

somatic parasympathetic
neurons
neurons
inhibited
stimulate

+

–

spinal
centres

sympathetic
neurons

motor
neurons

sensory neurons
send impulses
about fullness of
the bladder
contraction of
detrusor muscle

R
internal
sphincter
pelvic floor
muscle

external
sphincter
relaxes to
allow flow

relaxation of
detrusor muscle
which lines the
bladder

external sphincter
sensory (afferent) neurons
interneurons
motor (efferent) neurons

external
sphincter
contracts and
stops flow

Fig
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9 The urinary bladder stores urine, allowing it to be emptied at infrequent intervals

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