(See also Overview of the Digestive System.)
The duodenum is the first segment of the small intestine, and the stomach releases food into it. Food enters the duodenum through the pyloric sphincter in amounts that the small intestine can digest. When full, the duodenum signals the stomach to stop emptying.
The duodenum receives pancreatic enzymes from the pancreas and bile from the liver and gallbladder. These fluids, which enter the duodenum through an opening called the sphincter of Oddi, are important in aiding digestion and absorption. Waves of rhythmic muscular contractions (called peristalsis) also aid digestion and absorption by churning up food and mixing it with intestinal secretions.
The first few inches of the duodenal lining are smooth, but the rest of the lining has folds, small projections (villi), and even smaller projections (microvilli). These villi and microvilli increase the surface area of the duodenal lining, allowing for greater absorption of nutrients.
The jejunum and ileum make up the rest of the small intestine and are located below the duodenum. These parts of the small intestine are largely responsible for the absorption of fats and other nutrients. Churning movements facilitate absorption. Absorption is also enhanced by the vast surface area made up of folds, villi, and microvilli. The intestinal wall is richly supplied with blood vessels that carry the absorbed nutrients to the liver through the portal vein. The intestinal wall releases mucus, which lubricates the intestinal contents, and water, which helps dissolve the digested fragments. Small amounts of enzymes that digest proteins, sugars, and fats are also released.
The consistency of the intestinal contents changes gradually as the contents travel through the small intestine. In the duodenum, food is diluted with pancreatic enzymes and bile, which decrease stomach acidity. The contents continue to travel through the lower small intestine, becoming more liquid as they mix with water, mucus, bile, and pancreatic enzymes. Ultimately, the small intestine absorbs most of the nutrients and all but about 1 liter of fluid before emptying into the large intestine.