Final and longest segment of the small intestine .
It is the site of absorption of vitamin B 12 (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 12 deficiency and extensive diarrhea (since bile salts in the large intestine interfere with water absorption).