Urinary bladder

Function

The bladder is a collapsible muscular reservoir — the detrusor's criss-crossing smooth muscle lets it expand from 50 ml to 500 ml with barely a pressure rise (compliance), like a balloon of living cloth.

Stretch receptors signal fullness at 150–200 ml; the pontine micturition centre then choreographs a single event: detrusor contraction while the internal sphincter relaxes and the external sphincter's voluntary hold releases. In infants this is pure reflex; continence is a learned cortical veto over a spinal pattern. The trigone — between the two ureteric orifices and the internal meatus — is a fixed, smooth and highly sensitive triangle.

Blood supply

Superior and inferior vesical arteries from the internal iliac; vesical venous plexus drains to the internal iliac veins.

Nerve supply

Parasympathetic S2–S4 (pelvic splanchnic — motor to detrusor), sympathetic L1–L2 (relaxation/storage), pudendal nerve to the external sphincter.

Clinical relevance

Urinary tract infection

E. coli ascends the short female urethra — frequency, urgency, dysuria; dipstick nitrites and leukocytes confirm.

Overactive bladder

Detrusor overactivity fires before fullness — urgency and incontinence treated with bladder training and antimuscarinics.

Bladder cancer

Transitional cell carcinoma presents with painless haematuria; cystoscopy grades the invasion.

Related conditions

• Urinary tract infection
• Bladder cancer
• Urinary incontinence
• Enlarged prostate (BPH)