ileum

Final and longest segment of the small intestine. It is the site of absorption of vitamin B (see vitamin B complex) and reabsorption of about 90% of conjugated bile salts. It extends about 13 ft (4 m), from the jejunum (middle section of the small intestine) to the ileocecal valve, where it joins the large intestine. Disorders produce vitamin B deficiency and extensive diarrhea (since bile salts in the large intestine interfere with water absorption).