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PH-GI07

Bowel obstruction

How a blockage stops the gut’s contents moving on, why the bowel above it fills with gas and fluid, how that drains the body’s circulating volume and can cut off the bowel’s own blood supply, and how small and large bowel blockages differ.

By the end, you can

  • Describe how the small and large bowel move and reabsorb the large volumes of fluid secreted into them.
  • Distinguish mechanical obstruction from ileus and name the main causes in the small and large bowel.
  • Explain why the bowel above an obstruction dilates and why dehydration and electrolyte loss follow.
  • Explain how distension and twisting lead to strangulation and perforation.
  • Relate colicky pain, vomiting, distension and constipation to the level of the obstruction.
  • Recognise the gross and microscopic changes, classifications and key imaging and blood tests of obstruction, and explain how fluids, decompression, surgery and other treatments act on its mechanism.

What it is

Bowel obstruction is a blockage of the onward passage of intestinal contents. In mechanical obstruction something physically blocks the lumen, compresses it or twists the bowel; in functional obstruction (ileus) the bowel stops moving without a blockage. Strangulation is obstruction with loss of the bowel’s blood supply: a surgical emergency.

3 types to tell apart
Small bowel obstruction
Most often from adhesions after previous surgery, or from hernias; vomiting is early and prominent.
Large bowel obstruction
Most often from colorectal cancer, volvulus or diverticular disease; distension and constipation are prominent.
Ileus
Paralysis of the bowel after abdominal surgery, with peritonitis, low potassium or some drugs.

US spelling: litres → liters.

Normal function

Moving contents and recycling fluid

The , several metres long, leads to the large intestine at the , which continues as the , , and . Waves of peristalsis move the contents along.

Every day, saliva, gastric juice, bile, pancreatic juice and intestinal secretions pour roughly seven to nine litres of fluid into the gut, together with what we drink. Almost all of it is reabsorbed, mostly in the small intestine, so only a little leaves in the stool.

Gas in the bowel comes mostly from swallowed air, with some from bacterial fermentation in the colon. The ileocaecal valve normally lets contents pass from small to large bowel but limits flow back.

Point to it on the model