The duodenum is the shortest part of the small intestine, only 10 to 12 inches (25 to 30 cm) long. It receives food from the stomach, and passes food on to the jejunum, the second section of the small intestine.
The pyloric sphincter separates the stomach from the duodenum. This valve keeps food and gastric juices in the stomach until liquefied enough to proceed into the duodenum. Peristaltic waves push the chyme (i.e., food stuff) through the pyloric sphincter, a small spurt at a time. This process cycles until all of the food is liquefied in the stomach and proceeds to the duodenum.
The partially digested food arriving from the stomach stimulates the duodenum to produce hormones. These hormones tell the pancreas to release digestive enzymes, and the gallbladder to release bile. Much of the food digestion takes place in the duodenum. Digestive enzymes from the pancreas breakdown fats, proteins, and carbohydrates. The bile helps in the digestion and absorption of fats.
Strong acid in the stomach, used in food digestion, could be harmful to the small intestine. In response to this acid entering the duodenum, secretin is secreted by the duodenum. Secretin stimulates the pancreas and bile ducts to release sodium bicarbonate. The sodium bicarbonate travels through the pancreatic duct to the duodenum to neutralize the acid.
The duodenum also plays a major role in controlling the rate of digestion. The Gastric Inhibitory Peptide (GIP) hormone, produced in the duodenum, decreases the rate of stomach contractions. This slows the emptying of food from the stomach into the duodenum. This provides the small intestine with more time to digest the fats, proteins, and carbohydrates.