ileums

The ileum is the final section of the small intestine in humans and many mammals, located between the jejunum and the cecum. In adults, it measures roughly 2–4 meters and contains mucosa with villi and microvilli, plicae circulares, goblet cells, and crypts of Lieberkühn. A distinguishing feature is the presence of aggregated lymphoid tissue called Peyer's patches, especially prominent in the distal ileum, contributing to gut-associated immune defense. The ileum plays a key role in absorption, including bile acids and vitamin B12, as well as other nutrients and fats via chylomicrons. Bile acids are reabsorbed via the enterohepatic circulation primarily in the ileum. Vitamin B12 absorption requires intrinsic factor; the ileum houses receptors for the intrinsic factor–B12 complex.

Anatomy: The ileum begins at the terminal jejunal loop and ends at the ileocecal valve, which regulates flow into the large intestine and prevents backflow. Its blood supply arises from the ileal branches of the superior mesenteric artery, with venous drainage to the portal system. Innervation is via parasympathetic fibers from the vagus nerve and sympathetic fibers from the superior mesenteric plexus.

Clinical relevance: The terminal ileum is frequently involved in Crohn’s disease. Resection or severe inflammation can impair B12 absorption and bile acid reclamation, causing anemia and diarrhea. The ileocecal valve can be involved in disease, and appendiceal inflammation is adjacent to this region.