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Structure

Large intestine

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The large intestine extends from the ileocaecal valve to the anus, about 1.5 m long. It comprises the caecum with appendix, colon, rectum and anal canal and is responsible for water absorption and faecal formation.

Terminologia Anatomica (Latin): Intestinum crassum · Open in the 3D atlas

Location

The colon frames the small intestine: the ascending colon (retroperitoneal) rises on the right to the hepatic flexure, the transverse colon (intraperitoneal on its mesocolon) crosses to the splenic flexure, and the descending colon (retroperitoneal) descends on the left to the sigmoid colon, which is intraperitoneal and leads to the rectum.

Structure

Distinctive features are the three taeniae coli, haustra and omental appendices. The wall has crypts with abundant goblet cells and no villi. Embryologically the midgut gives the caecum, appendix, ascending colon and proximal two thirds of the transverse colon, and the hindgut the distal third of the transverse colon, descending colon, sigmoid, rectum and upper anal canal.

Blood supply

The superior mesenteric artery supplies the midgut part (ileocolic, right colic and middle colic arteries) and the inferior mesenteric artery the hindgut part (left colic, sigmoid and superior rectal arteries); the marginal artery links them, with a watershed near the splenic flexure. The rectum also receives middle (internal iliac) and inferior (internal pudendal) rectal arteries. Veins drain to the superior and inferior mesenteric veins, with portosystemic anastomoses in the rectum.

Innervation

The enteric nervous system controls local activity. The midgut part receives vagal parasympathetic and sympathetic fibres via the superior mesenteric plexus; the hindgut part receives parasympathetic fibres from pelvic splanchnic nerves (S2 to S4) and sympathetic fibres from lumbar splanchnic nerves via the inferior mesenteric plexus. The external anal sphincter is somatic, supplied by the pudendal nerve.

Function

The large intestine absorbs water and electrolytes, ferments undigested carbohydrate with the resident microbiota (producing short-chain fatty acids and some vitamin K), stores faeces and takes part in defecation. Mass movements propel contents toward the rectum.

Clinical correlations

Colorectal cancer is a leading cancer worldwide, and screening programmes vary by country. Diverticular disease, inflammatory bowel disease (ulcerative colitis, Crohn disease), appendicitis (pain migrating to the right iliac fossa), sigmoid volvulus and ischaemic colitis at the watershed areas are other key conditions. Rectal venous anastomoses explain haemorrhoids.

Common exam points

  • Features: taeniae coli (3), haustra, omental appendices; no villi.
  • Retroperitoneal: ascending and descending colon; intraperitoneal: transverse and sigmoid colon.
  • Midgut (SMA) up to the proximal two thirds of the transverse colon; hindgut (IMA) from the distal third.
  • Marginal artery (Drummond); watershed near the splenic flexure; rectal arteries: superior (IMA), middle, inferior.
  • Parasympathetic: vagus to the midgut part, pelvic splanchnic nerves (S2 to S4) to the hindgut part.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Sleisenger and Fordtran's Gastrointestinal and Liver Disease. Elsevier, 11th edition (2020) — Tier 3 (textbook)

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