Large intestine

Function

The colon frames the small bowel: caecum, ascending, transverse, descending and sigmoid. Its sacculated haustra churn slowly (1–2 days of transit) while reclaiming most of the ~1.5 L of fluid it receives daily (about 9 L enters the gut each day; the small intestine absorbs most of it).

Its tens of trillions of bacteria — the gut microbiome — ferment the indigestible, synthesise vitamin K and short-chain fatty acids, and train the immune system. The caecum, ascending colon and proximal two-thirds of the transverse colon are midgut (SMA, vagus); the rest is hindgut (IMA, pelvic splanchnics) — the splenic-flexure watershed between them is ischaemic colitis's favourite site.

Blood supply

SMA branches to the transverse midpoint, then IMA branches; the marginal artery of Drummond links them.

Nerve supply

Vagus to the transverse colon (midgut), pelvic splanchnic S2–S4 beyond; sympathetic via SMA/IMA plexuses.

Clinical relevance

Appendicitis

Obstructed appendix inflames, then perforates — migration of pain from umbilicus to the right iliac fossa is the diagnostic tale.

Colorectal cancer

Adenoma-to-carcinoma progression over a decade — the rationale for screening colonoscopy and FIT testing.

Diverticulitis

Pulsion diverticula of the sigmoid, inflamed by entrapped bacteria — left lower quadrant pain with fever.

Related conditions

• Crohn's disease
• Ulcerative colitis
• Irritable bowel syndrome
• Colorectal cancer
• Diverticular disease and diverticulitis
• Bowel obstruction
• Appendicitis