Colorectal Surgery

A linear tear in the sensate anal mucosa that triggers reflex internal sphincter spasm, and that spasm both causes the pain and starves the wound of blood, locking the fissure into a self-perpetuating cycle that pharmacological sphincter relaxation is designed to break.

A malignancy that arises through a stepwise accumulation of mutations turning benign adenomatous polyps into invasive carcinoma, so screening and symptoms both follow directly from where in the bowel that clone happens to grow.

A structural consequence of high intraluminal pressure forcing mucosa out through weak points in the colonic wall, so a low-fibre Western diet, the sigmoid colon's narrow calibre, and the complications of diverticulitis all follow from one mechanical principle.

Engorgement and downward displacement of the normal vascular anal cushions that help maintain continence, so bleeding is bright and painless because the cushions themselves have no somatic sensory innervation above the dentate line.

A fall in colonic perfusion below what the mucosa needs, striking hardest at the watershed zones where two arterial territories barely overlap, so the splenic flexure suffers first and the resulting mucosal injury produces the classic pairing of left-sided pain and bloody diarrhoea.

Blockage of a mucus-secreting anal gland lets bacteria multiply into a walled-off abscess, and if that pus tracks a path to the skin before it is drained, the epithelialised tunnel it leaves behind becomes a persistent fistula-in-ano.