Anatomy of the small intestine

The small intestine (small bowel) commences at the pyloroduodenal junction and ends at the ileocaecal junction. It comprises, successively, the duodenum, jejunum and ileum. The principal function of the small intestine is the digestion and absorption of ingested food, electrolytes and vitamins and this important functional specialization is reflected in the enormous surface area of the small intestinal mucosa which is the result of the extensive and wavy in folding of the mucosal lining along the entire length of the small intestine. As might be expected from its very active role as a secretory and absorptive viscus, the small intestine has a rich blood supply. This is derived chiefly from the superior mesenteric artery via its inferior pancreaticoduodenal, jejunal and ileal branches. Venous drainage is via corresponding tributaries that drain to the superior mesenteric vein and thence to the portal vein. These vessels are accompanied by lymphatic vessels and autonomic nerve fibres. Developmental anomalies, mechanical obstruction and inflammatory disease affect the small intestine far more commonly than do neoplastic conditions. A Meckel's diverticulum is a relatively common (2%) congenital anomaly of the distal ileum.In this article, an account of the topographical and surgical anatomy of the duodenum is followed by that of the jejunum and ileum. Various surgically-important features are highlighted.