Ileum

The ileum is the final portion of the small intestine, extending from the end of the jejunum to the cecum, where it meets the large intestine at the ileocecal valve. In adults it typically measures about 2 to 4 meters in length. It resides mainly in the lower right abdomen and pelvis, with its position varying with body habitus and organ arrangement.

Anatomically, the ileum contains mucosal villi and microvilli that increase surface area, though fewer and less pronounced than in the jejunum. It harbors abundant lymphoid tissue, especially Peyer's patches, which are important for immune surveillance of intestinal contents. The distal ileum contains the majority of these lymphoid aggregates.

Arterial supply comes from branches of the superior mesenteric artery; venous drainage mirrors the arterial flow into the portal system via the superior mesenteric vein. Lymphatics drain to mesenteric nodes. Innervation is autonomic, with parasympathetic input from the vagus nerve and sympathetic input modulating peristalsis and secretory activity.

The ileum has several key functions. It continues nutrient absorption, including fats, electrolytes, and water, but is particularly important for reabsorbing bile acids and vitamin B12 (cobalamin), which are recycled back to the liver through enterohepatic circulation. The distal ileum contains specialized enterocytes and transport mechanisms suited to these tasks.

Clinically, the terminal ileum is a common site of Crohn’s disease and is often involved in ileal inflammation. Resection of the ileum can cause bile acid malabsorption and B12 deficiency, and may necessitate surgical planning such as ileal pouch procedures. The ileocecal valve regulates flow into the cecum and helps prevent reflux between the small and large intestines.