Rectum

Function

The rectum is the 12–15 cm terminal reservoir, its ampulla accommodating stool until the defecation reflex — stretch receptors firing the urge at 150 ml. Three transverse folds (Houston's valves) slow descent and support continence's social grace.

Below, the puborectalis sling kinks the anorectal angle shut, and the anal canal's sphincters — internal (smooth, involuntary) and external (striated, voluntary) — hold the gate until a suitable moment. Haemorrhoids are enlarged vascular cushions of the anal canal — distinct from the rectal varices of portal hypertension.

Blood supply

Superior rectal (from the IMA), middle and inferior rectal arteries; the superior rectal vein drains to the portal system, the middle and inferior to the systemic (iliac) veins.

Nerve supply

Involuntary: pelvic splanchnic S2–S4. Voluntary sphincter: pudendal nerve.

Clinical relevance

Colorectal cancer of rectum

Rectal bleeding and tenesmus; low tumours test the surgeon's craft to preserve the sphincter.

Haemorrhoids

Engorged anal cushions bleed bright red and prolapse; they are not caused by portal hypertension.

Related conditions

• Ulcerative colitis
• Colorectal cancer
• Haemorrhoids (piles)