Anatomy: Small and Large Intestine

Anatomy: Small and Large Intestine

Duodenum

The duodenum is C-shaped, surrounds the head of the pancreas at the level of L2, and has four parts: 1st (superior, intraperitoneal, adjacent to gall bladder), 2nd (descending, retroperitoneal — contains the major duodenal papilla where bile duct + main pancreatic duct open, and minor papilla for the accessory pancreatic duct), 3rd (horizontal, retroperitoneal, crosses aorta and IVC), 4th (ascending, retroperitoneal, ends at duodenojejunal flexure at L2 level, suspended by ligament of Treitz). The DJ flexure is the anatomical landmark for upper vs lower GI bleeding.

Jejunum and Ileum

The jejunum (upper 2/5 of small intestine, ~2.4 m) and ileum (lower 3/5, ~3.6 m) are suspended by the mesentery of the small intestine (attaches to posterior abdominal wall from duodenojejunal flexure to ileocaecal junction). Distinguishing features: jejunum has prominent circular folds (plicae circulares, felt through wall), thicker wall, more vascular arcades with longer vasa recta. Ileum has fewer and less prominent folds, Peyer's patches (lymphoid aggregates) on the antimesenteric border, and thinner wall.

Ileocaecal Junction

The ileum empties into the caecum via the ileocaecal valve (limits reflux from caecum). The appendix (vermiform) hangs from the posteromedial caecum at the confluence of taeniae coli; its base is at McBurney's point (one-third of the way from anterior superior iliac spine to umbilicus). Appendicitis: visceral pain (periumbilical, via T10) → parietal peritoneal irritation (RLQ, sharp, worse on movement).

Colon

Features distinguishing colon from small intestine: taeniae coli (three longitudinal muscle bands, cause haustration), haustra (sacculations), and appendices epiploicae (fat tags). Parts: caecum (intraperitoneal), ascending colon (retroperitoneal), transverse colon (intraperitoneal, with transverse mesocolon), descending colon (retroperitoneal), sigmoid colon (intraperitoneal, with sigmoid mesocolon), rectum (retroperitoneal, begins where sigmoid mesocolon is lost), anal canal. Rectal folds (Houston's valves) support faecal mass during defaecation. Anal canal: upper portion (visceral innervation, insensate; lined by columnar/transitional epithelium; lymph to internal iliac nodes) and lower portion (somatic innervation by inferior rectal nerve, pain-sensitive; lined by squamous epithelium; lymph to superficial inguinal nodes). Dentate (pectinate) line marks the boundary.