The duodenum is the first segment of the small intestine, which is located immediately after the stomach in the digestive tract. The duodenum receives food that is has already begun to be digested by the stomach. The duodenum helps to digest all categories of food; carbohydrates, fats, and proteins.
The duodenum secretes a number of enzymes which contribute to the digestion process, as well as several hormones which help regulate such things as the speed of the stomach's emptying and the release of bile from the gall bladder; bile helps to break down fats into small, easily absorbed globules. Additionally, the pancreatic duct opens into the duodenum. The pancreas makes the enzymes tripsin and chymotripsin, which break down proteins into smaller units. Carboxypeptidase, aminopeptidase, and dipeptidase then reduce the protein parts into single amino acids. Amylase, another enzyme, acts to split long-chain carbohydrates into smaller sugars. Lipase works to break complex fat molecules into monoglycerides and fatty acids.
Brunner's glands in the duodenum secrete an alkaline mucus. While mucus is not strictly digestive in nature, it does help to lubricate the food's passage, and its bicarbonate content helps to neutralize the acids added by the stomach.