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PH-GI05

Ulcerative colitis

How inflammation confined to the colonic mucosa spreads continuously up from the rectum, why it causes bloody diarrhoea and urgency, and how it differs from Crohn’s disease.

By the end, you can

  • Describe the normal colonic mucosa and its roles in absorbing water and salt and keeping bacteria at bay.
  • Explain how mucosal inflammation produces cryptitis, crypt abscesses and goblet-cell depletion.
  • Explain the continuous spread from the rectum and relate the extent of disease to symptoms and risk.
  • Compare ulcerative colitis with Crohn’s disease by site, pattern, depth and complications.
  • Recognise the classic gross and microscopic features, the extent and severity classifications and the key tests of ulcerative colitis, and explain how each main treatment acts on its mechanism.

What it is

Ulcerative colitis is a chronic, relapsing inflammatory bowel disease of the colon in which inflammation is confined mainly to the mucosa and extends continuously from the rectum upwards for a variable distance. It is classified by how far it reaches.

3 types to tell apart
Ulcerative proctitis
Inflammation limited to the rectum. Urgency, tenesmus and blood are prominent; systemic illness is unusual.
Left-sided colitis
Continuous inflammation from the rectum up to, but not beyond, the left colic (splenic) flexure.
Extensive colitis (pancolitis)
Inflammation beyond the splenic flexure, sometimes the whole colon. Attacks are more often severe, and cancer risk is higher with long duration.

US spelling: litres → liters, fibre → fiber.

Normal function

The colon and its lining

The large intestine runs from the through the , , and to the . Each day it receives about 1–1.5 litres of fluid from the ileum and absorbs most of the water and sodium, so only a little leaves in the stool.

Unlike the small intestine, the colonic mucosa has no villi. Its surface is flat, with straight, closely packed crypts lined by absorptive cells and many goblet cells. Their mucus forms two layers: an outer loose layer where bacteria live, and an inner dense layer that bacteria normally barely penetrate.

Colonic bacteria ferment fibre into short-chain fatty acids such as butyrate, the main fuel of the colonic epithelium. The immune cells of the mucosa normally tolerate this vast population.

Point to it on the model