In humans, the ileum, which is the last and longest part of the small intestine, is where vitamin B12 is absorbed. Also, bile salts are reabsorbed here and sent back to the liver.
The entire small intestine is about 20 feet long, and the ileum accounts for about 13 feet. It runs from the middle section of the small intestine, the jejunum, to the ileocecal valve, which leads to the large intestine. The ileum is connected to the abdominal wall by a membrane. The walls of the ileum are relatively thin, smooth muscle. The contractions, or squeezing motions, of this muscle are slower than in other intestinal sections. The ileum's lining is also less porous than in the rest of the organ.