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Structure

Small intestine

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

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

The small intestine is the longest part of the digestive tube and the main site of digestion and absorption. It comprises the duodenum, jejunum and ileum.

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

Location

It extends from the pylorus to the ileocaecal valve and measures several metres in life (variable, often quoted as about 6 m in cadavers). The duodenum (about 25 cm, C-shaped) is mostly retroperitoneal around the head of the pancreas, whereas the jejunum and ileum are intraperitoneal and suspended by the mesentery.

Structure

The duodenum has four parts; the major duodenal papilla in the second part receives the common bile duct and main pancreatic duct at the hepatopancreatic ampulla (ampulla of Vater), regulated by the sphincter of Oddi. The mucosa is amplified by plicae circulares, villi and microvilli, with crypts of Lieberkühn, Brunner glands in the duodenal submucosa and Peyer patches in the ileum. The jejunum is wider with more prominent folds, and the ileum has more lymphoid tissue.

Blood supply

The proximal duodenum (foregut) is supplied by the gastroduodenal artery and the superior pancreaticoduodenal artery, a branch of it. The distal duodenum, jejunum and ileum (midgut) are supplied by the superior mesenteric artery (inferior pancreaticoduodenal, jejunal and ileal branches); veins drain through the superior mesenteric vein to the portal vein.

Innervation

The enteric nervous system (submucosal Meissner and myenteric Auerbach plexuses) controls motility and secretion locally. Extrinsic parasympathetic supply is from the vagus nerve and sympathetic supply from thoracic splanchnic nerves via the coeliac and superior mesenteric ganglia; visceral pain from the midgut is felt in the periumbilical region.

Function

The small intestine completes digestion with pancreatic enzymes, bile and brush-border enzymes and absorbs most nutrients, water and electrolytes. Iron and calcium are absorbed mainly in the duodenum and proximal jejunum, whereas vitamin B12 and bile salts are absorbed in the terminal ileum.

Clinical correlations

Small bowel obstruction (adhesions, hernias), Crohn disease (often terminal ileum), coeliac disease (villous atrophy) and mesenteric ischaemia are important conditions. Meckel diverticulum is a persistent vitelline duct remnant on the ileum, and duodenal ulcers typically involve the first part.

Common exam points

  • Duodenum (about 25 cm, four parts, mostly retroperitoneal), jejunum, ileum (intraperitoneal on the mesentery).
  • Major duodenal papilla in the second part: ampulla of Vater, controlled by the sphincter of Oddi; the foregut ends here and the midgut begins.
  • Proximal duodenum from the coeliac trunk; distal duodenum, jejunum and ileum from the superior mesenteric artery.
  • Enteric nervous system: Meissner (submucosal) and Auerbach (myenteric) plexuses, modulated by vagus and sympathetic fibres.
  • Absorption: iron in the duodenum, vitamin B12 and bile salts in the terminal ileum.

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)
  4. Pawlina W. Histology: A Text and Atlas. Wolters Kluwer, 8th edition (2020) — Tier 3 (textbook)

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