Ileum

Ileum is the distal and longest part of the small intestine, extending from the jejunum to the ileocaecal junction, where it joins the caecum of the large intestine. It forms the terminal three-fifths of the mobile small intestine and is predominantly located in the lower abdomen and pelvis.

Mesentery

Like the jejunum, the ileum is suspended from the posterior abdominal wall by the mesentery, a fan-shaped double layer of peritoneum that provides mobility while carrying the intestinal blood vessels, lymphatics, nerves, lymph nodes, and variable amounts of fat.

Differentiating features: ileum Vs. jejunum

Compared with the jejunum, the ileum has a thinner wall, a smaller lumen, fewer and less prominent plicae circulares, shorter villi, and greater mesenteric fat extending to the intestinal border. Its arterial supply is characterised by numerous arterial arcades and short vasa recta.

Payer's patches

The mucosa and submucosa of ileum contain abundant aggregated lymphoid nodules (Peyer's patches), particularly in the terminal ileum, which contribute to mucosal immunity as part of the gut-associated lymphoid tissue (GALT).

B12 and bile salts' reabsorption

The ileum is responsible for the absorption of nutrients not absorbed proximally and is the principal site for the absorption of vitamin B12 and the reabsorption of bile salts.

Clinical correlates

The ileum is commonly affected by conditions such as Crohns disease, typhoid ulceration, intestinal tuberculosis, and Meckel's diverticulum.