Small bowel

Last revised by Raymond Chieng on 3 Feb 2023

The small bowel (or small intestine) is the section of bowel between the stomach and the colon. It has distinctive mucosal folds, valvulae conniventes, and is made up of three functional units:

Although anatomically it is stated that the duodenum forms the first part of the small bowel/intestine, clinically it is often used to refer to the jejunum and ileum only 1.

The total length of the jejunum and ileum varies from 3 to 10 m with an average length of 6 m. Both jejunum and ileum are attached to mesentery. The root of mesentery extends from the left of L2 vertebrae to the right sacroiliac joint, measuring 15 cm. Jejunum occupies proximal two-fifths while the ileum occupies the distal three-fifths of the small intestine. The mucosal folds in small intestine are known as valvulae conniventes (or plica semilunaris). Lymphoid follicles in the small intestine become progressively more numerous as they go into the distal ileum. In the distal ileum, these lymphoid follicles clump together to form Peyer's patches and are located at the antimesenteric border of the small intestine 2.

The small intestine is supplied by superior mesenteric artery (arise from abdominal aorta at the level of L1 vertabra) from middle portion of the second part of the duodenum until terminal ileum. Jejunal and ileum branches arise from the left side of the main trunk of the superior mesenteric artery. These branches are linked to each other through a series of arcades (single arcade in jejunum and multiple arcades in ileum). They terminated as vasa recta (end arteries) within the intestinal wall 2.

Veins from the small intestine drains into superior mesenteric vein which in turns drain into portal vein 2.

Lymph from small intestine drains into the superior mesenteric group of pre-aortic nodes 2.

See main article: barium studies of small bowel

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