Function: Acid chyme enters the duodenum from the stomach, and also bile from the liver via the gallbladder and pancreatic juice from the pancreas. Bile salts emulsify fats; bile and pancreatic bicarbonate juice neutralize the acidity of the chyme. Pancreatic enzymes are lipase, which digests emulsified fats to fatty acids and glycerol; amylase, which digests starch to maltose; and trypsin, chymotrypsin, and carboxypeptidase, which continue the protein digestion begun in the stomach by pepsin. Intestinal enzymes are peptidases, which complete protein digestion to amino acids, and sucrase, maltase, and lactase, which digest disaccharides to monosaccharides. Some of these enzymes are in the brush border of the intestinal epithelium and are not secreted into the lumen. Three hormones are secreted by the duodenum when chyme enters. Gastric inhibitory peptide decreases gastric motility and secretions. Secretin stimulates the pancreas to secrete sodium bicarbonate and the liver to produce bile. Cholecystokinin stimulates secretion of enzymes from the pancreas and contraction of the gallbladder to propel bile into the common bile duct.

The end products of digestion (amino acids, monosaccharides, fatty acids, glycerol, vitamins, minerals, and water) are absorbed into the capillaries or lacteals within the villi. Blood from the small intestine passes through the liver by way of the portal vein before returning to the heart.