Small Intestine Anatomy
Overview
The small intestine spans approximately 6-7 metres, extending from the pyloric sphincter to the ileocaecal valve. It is the primary site of digestion and absorption, divided anatomically into the duodenum, jejunum, and ileum.
The Duodenum
C-shaped and almost entirely retroperitoneal (except the 'duodenal cap', D1), it curves around the head of the pancreas.
- D1 (Superior/Cap): Intraperitoneal. Most common site of peptic ulcers. Triangular opacity on barium meal.
- D2 (Descending): The Ampulla of Vater opens on the posteromedial wall, receiving bile and pancreatic secretions.
- D3 (Horizontal): Crossed anteriorly by the superior mesenteric vessels: the anatomical basis of SMA syndrome.
- D4 (Ascending): Joins the jejunum at the ligament of Treitz.
Jejunum vs Ileum
- Jejunum: Thicker walls, wider lumen, prominent plicae circulares, fewer vascular arcades, less mesenteric fat.
- Ileum: Thinner walls, narrower lumen, less prominent plicae, multiple complex vascular arcades, abundant Peyer's patches (lymphoid tissue).
WarningTerminal Ileum: The Selective Absorber
Vitamin B12 (bound to Intrinsic Factor) and bile salts are specifically absorbed ONLY in the terminal ileum. Crohn's disease targets this region. After terminal ileal resection: B12 deficiency (megaloblastic anaemia) and bile salt malabsorption (secretory diarrhoea and fat malabsorption).
High Yield Facts
LightbulbFRCR / MD Prep Pearl
On barium follow-through or CT enterography: jejunum = left upper quadrant, feathery folds (valvulae conniventes), wide lumen. Ileum = right lower quadrant (converging on RIF), smooth walls, narrow lumen. Peyer's patches create lymphoid nodular pattern in the distal ileum on CT enterography.