Small Intestine

The lining of the small intestine is characterized by numerous circular folds called plicae circulares. The plicae are lined with fingerlike villi. From a cross-sectional view, the villus contains a network of capillaries which surround a specialized lymphatic vessel known as a lacteal. The epithelium of an intestinal villus consists of columnar cells which are covered with microvilli. This succession of folds and projections increases the surface of the intestinal lining for efficient absorption. Carbohydrate digestion is completed by enzymes in the small intestine. Carbohydrates are absorbed by the villi and then enter the capillary. Fat digestion occurs primarily in the small intestine. Fat molecules are digested and absorbed into the epithelial cells of the villus. The fats are formed into clusters called chylomicrons which pass into the lacteal. Lymph carries chylomicrons away from the villus. Protein digestion is completed in the small intestine. Proteins are broken down first into peptides, then into amino acids. These are absorbed into the villi, then into the capillary.