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PH-GI23

Narrowing, widening and diverticula of the gut

How the gut’s muscular tube can narrow (strictures, achalasia), bulge outwards (pulsion and traction diverticula, colonic diverticular disease) or widen and twist (megacolon, volvulus), and what each change does to the passage of contents.

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  • 5 steps · 10 questions
  • 3D anatomy, affected parts marked

By the end, you can

  • Describe how the layered gut wall, its muscle and the myenteric plexus keep the lumen open and its contents moving.
  • Explain how scarring, tumour and outside pressure narrow the lumen, and how achalasia narrows it functionally, relating each to its pattern of dysphagia.
  • Distinguish pulsion from traction diverticula and true from false diverticula, using Zenker, colonic and Meckel diverticula as examples.
  • Explain how a colonic diverticulum bleeds or becomes inflamed, and how diverticulitis leads to abscess, perforation, fistula or stricture.
  • Explain how lost muscle tone or nerve control widens the colon, and why a long, mobile loop can twist into a volvulus.
  • Recognise the classic imaging, manometry and pathology features of these changes, and explain how dilatation, myotomy, drainage, decompression and resection act on each mechanism.

What it is

The lumen of the gut can narrow (a stricture, a fixed narrowing from scar, tumour or outside pressure, or a functional narrowing when a sphincter fails to relax, as in achalasia), bulge outwards (a diverticulum, a blind pouch opening from the lumen) or widen (megaoesophagus, megacolon), and a long, mobile loop can twist on its mesentery (volvulus). A diverticulum is true when its wall contains every layer of the gut and false (a pseudodiverticulum) when only mucosa and submucosa push through the muscle.

5 types to tell apart
Fixed narrowing (stricture)
Scar after deep injury (reflux, caustics, radiation, ischaemia, Crohn’s disease, diverticulitis), a tumour, or compression from outside the wall.
Functional narrowing
A sphincter or segment that cannot relax because its inhibitory nerves are missing: achalasia in the oesophagus, an aganglionic segment in the colon.
Pulsion diverticulum
Pressure inside pushes mucosa and submucosa through a weak point in the muscle, giving a false diverticulum: Zenker, epiphrenic and colonic diverticula.
Traction diverticulum
Scarring outside the wall pulls it outwards, classically in the middle oesophagus beside inflamed lymph nodes.
Dilatation and twisting
The wall stops contracting (toxic megacolon, pseudo-obstruction, chronic megacolon) or a long loop twists on a narrow mesentery (volvulus).

US spelling: tumour → tumor, megaoesophagus → megaesophagus, oesophagus → esophagus, oesophageal → esophageal.

Normal function

A muscular tube that keeps its contents moving

From the oesophagus to the rectum the gut is a tube with the same plan: the mucosa, the , a muscle coat (the muscularis externa, or muscularis propria) and, on most of the intestine, the . The muscle coat has an inner , which narrows the tube, and an outer , which shortens it. In the colon the longitudinal muscle is gathered into three bands, the taeniae coli.

Between the two muscle layers lies the , the part of the enteric nervous system that runs peristalsis. When a bolus stretches the wall, excitatory neurons (releasing acetylcholine) contract the muscle behind it and inhibitory neurons (releasing nitric oxide and vasoactive intestinal peptide) relax the muscle ahead of it, so the contents move on. Sphincters, such as the upper and lower oesophageal sphincters, stay closed at rest and open only when inhibitory neurons relax them.

The wall has natural weak points: arteries and veins must pierce the muscle coat to reach the , leaving small gaps. Geometry matters too. By the law of Laplace, a narrow segment builds a higher pressure for the same muscle tension, while a wide segment carries the greatest tension in its wall.

Point to it on the model