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