The small intestine consists of the duodenum, jejunum and ileum. The small intestine is subdivided into the duodenum, the jejunum and the ileum. The villi in the jejunum are much longer than in the duodenum or ileum. Iron is primarily absorbed in the small intestine, in particular the duodenum and jejunum. I cells secrete cholecystokinin (CCK), and are located in the duodenum and jejunum. It then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, and duodenum. It has three parts, the duodenum, jejunum and ileum. It is absorbed primarily in the duodenum of the small intestine. It begins with the duodenal bulb and ends at the suspensory muscle of duodenum. It starts at the duodenal bulb and ends at the suspensory muscle of duodenum. The pancreatic duct delivers substances such as bicarbonate and digestive enzymes to the duodenum. The pancreas develops from these buds on either side of the duodenum. The superior mesenteric artery and vein are anterior to the third part of duodenum. Amino acids absorbed by the duodenum stimulate acid secretion by unknown mechanisms.