Urinary bladder

At a glance

English

Urinary bladder

Latin (TA2)

Vesica urinaria

System

Urinary

Region

Pelvis

Side

Single, on the midline

About the urinary bladder

The urinary bladder stores urine delivered by the two ureters and releases it through the urethra. When empty it lies wholly within the pelvis, just behind the pubic bones and the pubic symphysis, and is shaped like a three-sided pyramid. As it fills it becomes rounded and rises above the pubis into the lower abdomen, peeling the peritoneum off the front abdominal wall; this is why a full bladder can be reached through a needle just above the pubis without entering the peritoneal cavity. The urge to empty usually comes at a few hundred millilitres, though it can hold more.

Four regions are described. The apex points forwards towards the top of the symphysis and is continued up to the umbilicus by the median umbilical ligament, a remnant of the fetal urachus. The fundus, or base, faces backwards and downwards towards the rectum and receives the ureters. The body lies between apex and fundus, and the neck is the lowest, most fixed part, surrounding the internal urethral orifice and anchored by pubovesical ligaments and the pelvic floor. Only the upper surface is covered by peritoneum.

Inside, most of the lining forms folds when the bladder is empty, but a smooth triangle on the back wall, the trigone, stays flat. Its corners are the two ureteric orifices above and the internal urethral orifice below, and a ridge, the interureteric fold, joins the ureteric openings. The wall consists of urothelium, a lamina propria and the detrusor, a thick mesh of interlacing smooth muscle bundles that contracts as a unit to empty the bladder.

Control is neural. Parasympathetic fibres from the pelvic splanchnic nerves (S2 to S4) make the detrusor contract and also carry the sensation of fullness; sympathetic fibres relax it during filling. Infection of the bladder, cystitis, is very common. Failure to empty (retention) and leakage (incontinence) follow damage to these nerves or to the pelvic floor. Bladder cancer usually arises in the urothelium, often presenting with blood in the urine, and is investigated by cystoscopy.

Function and supply

Function

Stores urine at low pressure and expels it during urination through contraction of the detrusor muscle.

Nerve supply

Parasympathetic fibres from the pelvic splanchnic nerves (S2-S4) to the detrusor, which also carry sensation of filling; sympathetic fibres reaching it through the hypogastric plexuses; together forming the vesical plexus.

Blood supply

Superior and inferior vesical arteries from the internal iliac artery; a vesical venous plexus empties into the internal iliac veins of each side.