The absorptive area of this section of the digestive passage is increased an estimated 600 fold by the villi, which are fingerlike projections in the lining of the small intestine. Each villi contains a lymph vessel surrounded by a network of capillaries. The nutrients absorbed into the lymph vessel pass into the lymphatic system, and those absorbed by the capillaries empty into the portal vein and are carried direction to the liver. The stomach produces gastric juice and mixes it with food. Gastric juice contains enzymes that begin the digestion of proteins.
Proteins are the only substances digested in the stomach.
However, they are only partially digested there. The food is churned and mixed with stomach fluids until a thick paste called chyme is produced. The chyme passes through the stomach into the small intestine. Enzymes from the pancreas and intestinal glands continue the breakdown of proteins. They also change starch into simple sugars and fats into fatty acids.