URINARY SYSTEM
Physiology 2
Presented by: Dr. Shaimaa Nasr Amin
Lecturer of Medical Physiology
General Education Program
Micturition
Micturation is the process by which the
urinary bladder empties after it become
filled.
*It involves 2 steps:
1- Filling of the bladder.
2- Micturation reflex.
Physiologic anatomy of the urinary bladder
Urinary bladder composed of 2 parts:
Neck Funnel shaped
extension of the
body
Surrounded by
the internal
urethral
sphincter.
Body Its smooth
muscles called
detrusor muscle
Act as functional
syncitium .
Function of internal sphincter
a)Its natural tone keep the posterior
urethra empty of urine.
b)Prevents reflux of semen into the
bladder during ejaculation.
External urethral sphincter
*It is voluntary
skeletal muscle used
for conscious
control of
micturation.
Innervation of the bladder and sphincters
Parasympathetic Sympathetic Somatic
Origin S2 and S3 L2 S2 and S3
Supplies Detrusor
muscle&internal
uretheral sphuncter.
Internal urethral
sphincter.
External urethral sphincter.
Afferent 1-Detect degree of
stretch in the bladder.
2-from posterior
urethra initiates reflex
bladder emptying.
1-Sensation of
fullness.
2-Pain sensation due
to overstretch or
infection.
From strech receptos in rhe
posterioor urethra transmits
sensation of urine flow to
the urethra.
Efferent Contraction of
detrusor muscle in
the bladder wall and
relaxation of internal
urethral sphincter.
Contraction of
internal urethral
sphincter to prevent
reflux of semen into
the bladder during
ejaculation.
Control the external urethral
sphincter.
Bladder filling
-Peristaltic contraction of the ureter force
the urine from the renal pelvis towards
the bladder.
-The ureter pass obliquely for several
cms through the bladder wall then pass
for 1-2 cm beneath the bladder mucosa
before emptying into the bladder.
*Normal detrusor tone compress the
ureters and prevent reflux of urine into
the ureters with↑intravesical pressure.
*Urine enters the bladder without
producing much ↑in intravesical
pressure explained by:
Low of Laplace: P=2T÷r
Cystometrogram
A plot of the intravesical pressure against the volume of urine in the bladder
*At intravesical zero there is no urine in the bladder
*Segment Ia : Pressure↑to 5-10cm water by 50 ml of urine .
*Segment Ib : Small ↑in pressure with↑volume to 200-300ml .
*Segment II : Sharp ↑ in pressure at volume of 400ml .
The 1st urge to void is
felt at bladder volume of
about 150 ml and
marked sensation of
fullness at about 400 ml.
Micturation reflex
In adult the volume of urine in the bladder that initiats
micturation reflex is about 300-400ml
Component of the micturation reflex:
1- Receptors: Strech receptors in the bladder wall and in the
posterior urethra.
2-Afferent: Pelvic parasympathetic.
3-Center: S2 and S3.
4-Efferent:Pelvic parasympathetic.
5-Effector and response: Detrusor moscle→Contraction.
internal urethral
sphincter→Relaxation.
Micturation reflex is a self regenerative,
Once it begins; contraction of the
bladder further activate more stretch
receptors→↑sensory impulse from the
bladder and posterior urethra→↑reflex
contraction of the bladder
Once the micturation reflex becomes
powerful enough Another reflex
through the pudendal nerve
inhibition of external urethral sphincter .
Higher control of micturation
Facilitatory centres a)Pontine centers b)Posterior hypothalamus
Inhibitory centre Mid-brain
The cortical micturation
centre (CMC) *Is located in the superior
frontal gyrus .
*It may facilitate or inhibit
the spinal micturation
reflex.
`
How these higher centres control micturation
1-They keep micturation reflex partially
inhibited except when micturation is desired.
2-Can prevent micturation by contraction of
external urethral sphincter.
3-When it is time to urinate the cortical areas
facilitate the sacral centers to initiates
micturation reflex and to inhibit the external
urethral sphincter.
Mechanism of Voluntary urination
1-Voluntry contraction of the pelvic floor
muscles→downward tug on detrusor to
initiate its contraction.
2-Voluntry contraction of the abdominal
muscles→↑intravesical pressure→entry of
urine in the bladder neck→strech of the
bladder neck→stimulate strch
receptors→excite micturation reflex.
3-Simultaneous relaxation of the external
urethral sphincter .
After urination the female
urethra empties by gravity.
However male urethra is
emptied by contraction of the
bulbocavernous muscle.
Abnormalities of micturation
Deaffrenation Denervation Sp.cord damage
Cause Damage of the
dorsal root as in
Tabes dorsalis.
Destruction of the
afferent and efferent
nerves.
Transection of the
spinal cord leaving
sacral segment intact
Micturation reflex Abolished Abolished 1-Spinal shock: Lost
2-Recovery stage:
return
Voluntary control Lost Lost Lost
Bladder Thin walled
,distended ,hypotonic
Thick wall ,shrunken
,hyperactive.
Shock stage: Flaccid
Recovery
Hypertrophied with
reduced capacity
Urination Retention with
overflow
Hyperactive bladder
expelsdribbles of
urine
Shock stage:
retention with
overflow
Recovery stage:
Automatic bladder