5.14 Urinary System

 

Structure (342-4)

Empty = entirely pelvic; Distended = comes up into abdominal cavity.

Detrusor muscle = spirals of interlacing smooth muscle in various directions, trabeculated (exaggeratedly so in hypertrophy of bladder) which contracts rather than peristalses.

-           well supplied by PNS (cholinergic)

TrigoneÕs superficial trigonal muscle is different; extends into proximal urethra

Mucous membrane: thick, lax, glands absent, no muscularis mucosae.

 

Undistended bladder is 3-sided pyramid with apex ¨ symphysis pubis, base facing backwards, the two inferolateral surfaces cradled by levator ani / pelvic floor.

Apex

Attached to urachus within median umbilical ligament (runs up medial umbilical fold).

-           cf the medial umbilical ligaments which are remnants of obliterated umbilical arteries

-           top surface is covered with peritoneum (most of bladder is not)

Base (posterior surface)

Lies below rectovesical pouch; only uppermost portion is covered in peritoneum

-           ductus deferens and SVÕs are applied to each side, ureters enter each superolateral corner (see pl 338 for relations: ductus on top, ureter inferior and SVÕs most inferior (ie posteriorly)

In females the base is firmly applied to the anterior wall of the vagina.

Trigone is lowest part of base. 

Inferolateral surfaces

Slope forwards & medially to meet in midline.  Lie on pelvic diaphragm and obturator internus. 

-           anteriorly there is the retropubic space of Retzius: contains loose fatty tissue and fibromuscular pubovesical ligaments (from bladder neck to inferior pubic bones)

Neck

Lowest part, pierced by urethra at internal urethral orifice (358)

Lies against base (upper surface) of prostate in males. 

Superior surface (337)

Covered in peritoneum which is stripped from behind rectus as the bladder fills (337)

-           only the transversalis fascia then lies between bladder and back of muscle

-           can puncture here in midline above pubic symphysis without entering peritoneum.

Posteriorly in females is vesicouterine pouch; body of uterus rides up on filling bladder

-           cf in males = rectovesical pouch. 

Trigone (343)

Triangular area with internal urethral orifice inferiorly, ureteric orifices laterally (2.5cm apart in non-distended bladder, 5cm if very-distended, with interureteric bar between: the continuation of ureteric longitudinal muscle coats across bladder wall).

-           oblique path of ureter through bladder wall ¨ flap-valve action.

-           Closed normally, except open rhythmically in response to ureteric peristalsis

Trigone overlies median lobe of prostate; or c.t. of anterior vagina \ ) least mobile part of bladder.

-           prostate may project into bottom of bladder after middle age ¨ ÔuvulaÕ of bladder

-           appearance or trigone varies little, cf rest of bladder undersurface, which looks smooth and thin when distended, but thrown into trabeculated folds when collapsed.

Blood supply

Mainly superior & inferior vesical arteries (369)

-           small contributions from obturator, inferior gluteal, uterine & vaginal arteries.

Superior vesical vessels can raise a mesentery from pelvic side wall to bladder.

Veins donÕt follow arteries; form a plexus which converges on bladder neck and drains across pelvic floor (374) ¨ vesicoprostatic plexus ¨ internal iliac veins.

Or in females ¨ base of broad ligament.

Lymphatics

Follow arteries to internal and external iliac nodes.

Innervation (322)

Parasympathetic are motor to detrusor; Sympathetic fibres vasomotor & inhibitory to detrusor.

Detrusor muscle receives mainly parasympathetic fibres via pelvic splanchnic nerves from S3.

Trigone receives mainly sympathetic adrenergic fibres from L1 & L2 segments via superior & inferior hypogastric plexuses.

-           sympathetic fibres paradoxically motor to trigone and muscle of bladder neck in males, may help close the uretal orifices. (P - Pass urine, S - Stop)

-           Internal urethral sphincter exists only in males, thought to prevent retrograde ejaculation rather than to have a role in continence (a circular smooth muscle coalescence around neck)

Sensation of distension: passes via parasympathetics to cord then to gracile tract.

Pain: travels with both pns and sns.

Control of micturition

Coordinated contraction of detrusor & relaxation of pelvic floor ¨ bladder emptying. 

Initial filling ¨ øtone ¨ no change in intravesical P.

Further ­vol ¨ ­wall tension ¨ stretch receptors stimulate parasympathetic sacral cord segments (travel via pelvic splanchnics) ¨ detrusor contraction (efferent via pelvic splanchnics).

-           this stretch reflex exists in infants; higher centres take over later:

-           Control inhibitory centre in middle frontal gyrus, passes via detrusor motor centre in pons.

-           ¨ External sphincter via fibres OnufÕs nucleus (anterior horn cells of S2) and perineal branch of pudendal nerve.

Cord transection above S2 ¨ loss of cortical control of sacral reflex, & external sphincter relaxation canÕt be prevented ¨ automatic emptying of distended bladder.

-           destruction of sacral segments ¨ paralysis of detrusor ¨ overflow incontinence. 

Development

Endodermal Urogenital sinus ¨ bladder mucosa;

Surrounding mesochyme ¨ muscle and connective tissue.

Lower ends of mesonephric ducts incorporate into bladder as ureters and trigone.

-           at a lower level the ducts form ejaculatory ducts entering urethra.

Allantois regresses into fibrous urachus.

Ureters in pelvis (319-20)

1/2 of ureter lies in pelvis.

Ureter runs over EIA and vein just at its origin (the bifurcation of CIA)

¨ down side wall of pelvis anterior to IIA & its branches. (behind the ovary if female)

Crosses:  obturator nerve, obliterated umbilical, superior vesicle, artery, obturator vessels.  

-           a pelvic appendix may lie adjacent to it

In males

At level of ischial spine ureter turns medially; crossed by ductus deferens here ¨ bladder base just above where the SVÕs lie against the bladder.

In females (344)

Lies on base of broad ligament; crossed by uterine artery.

Pierces lateral cervical ligament, crosses lateral vaginal fornix 1-2cm from cervix and enters bladder in front of fornix.  Endangered in hysterectomy. 

Only one structure lies anterior to ureter in pelvis: ductus deferens (males) or  uterine artery (females).