Ileum

The ileum is the final section of the small intestine, extending from the jejunum to the large intestine. It absorbs bile salts and vitamin B12 and contains prominent immune tissue.

Bile acid: A steroid acid made from cholesterol that aids fat digestion and is recycled between the intestine and liver. The ileum actively reabsorbs most bile acids, returning them to the liver.
Ileocecal valve: A muscular valve at the junction of the ileum and cecum that regulates passage into the large intestine. It marks the ileum's outlet and helps limit reflux from the colon.
Crohn's disease: A chronic inflammatory bowel disease that can affect any part of the gastrointestinal tract, often involving the terminal ileum. Ileal inflammation is a common presentation and can impair absorption.
Duodenum: The first section of the small intestine, receiving stomach contents and digestive secretions from the pancreas and liver. Unlike the ileum, the duodenum is the main site for mixing chyme with digestive secretions.
Vitamin B12: An essential vitamin required for red blood cell formation and normal nervous-system function. Vitamin B12 bound to intrinsic factor is absorbed in the terminal ileum.
Cecum: The pouch-like first part of the large intestine, receiving contents from the ileum. The ileum opens into the cecum through the ileocecal valve.
Ileostomy: A surgically created opening that brings the ileum through the abdominal wall to divert intestinal contents. It uses the ileum when stool must bypass the colon or rectum.
Colon: The largest part of the large intestine, where water and electrolytes are absorbed and feces are formed. The colon follows the ileum and has distinct functions and mucosal structure.
Intrinsic factor: A protein secreted by stomach parietal cells that binds vitamin B12 and enables its absorption. The ileum recognizes and absorbs the intrinsic factor–B12 complex.
Mesentery: A fold of peritoneum that suspends much of the intestine and carries its blood vessels, nerves, and lymphatics. The ileum is attached to the posterior abdominal wall by mesentery.

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