The duodenum is a short section of smooth muscle that connects the stomach with the jejuneum section of the small intestine . Although it is actually considered part of the small intestine, the duodenum begins at the stomach, loops around the head of the pancreas, and ends at the jejuneum. Attached to the duodenum is the common bile duct from which the liver and the gallbladder secrete bile and the pancreas secretes pancreatic juice. The submucosa layer of the duodenum contains glands called Brunner glands. These glands secrete mucus that is rich in bicarbonate. This bicarbonate neutralizes the acid from the stomach in the chyme and protects the duodenum from the acid.
When the chyme enters the duodenum, cells in the duodenum release the hormones intestinal gastrin, secretin, cholecystokinin, and gastric inhibitory peptide. Intestinal gastrin initially stimulates the stomach. However, secretin, cholecystokinin, and gastric inhibitory peptide inhibit the stomach. Similarly, the acids and partially digested fats in the chyme cause the enteric nervous system to send a reflex to the stomach, which decreases the activity of the stomach. These measures help to inhibit the stomach until the duodenum can process the chyme.
These hormones have other functions in addition to inhibiting the stomach. Secretin causes sodium bicarbonate to be released from the pancreatic duct and the common bile duct. Cholecystokinin causes the gallbladder to contract, which causes bile to flow into the common bile duct. Cholecystokinin also causes the pancreas to secrete pancreatic juice and the sphincter of Oddi to relax. Through the action of these hormones, pancreatic juice and bile are released through the duct into the duodenum. The pancreatic juice and bile then mix with the chyme.
A number of factors cause the stomach to secrete more chyme in the duodenum. The presence of chyme in the duodenum causes the duodenum to stretch. This stretching triggers a reflex that stimulates the stomach. The presence of peptides and amino acids in the chyme also cause cells in the duodenum to secrete gastrin, which causes the stomach to release more chyme into the duodenum.Last Updated: Saturday, July 16, 2011