Small Intestine

About the Small Intestine

The small intestine is approximately 6 to 7 meters long in vivo shorter post mortem due to muscle relaxation and has three segments: duodenum 25 cm, jejunum 2.5 m, and ileum 3.5 m. It is the primary site of digestion and nutrient absorption. The surface area is expanded 600 fold by circular folds plicae circulares 3x, villi 10x, and microvilli 20x, totaling approximately 30 square meters. Each villus contains a capillary network for amino acid and sugar absorption, and a lacteal for fat absorption into the lymphatic system. Digestive enzymes from the pancreas trypsin, chymotrypsin, amylase, lipase and the intestinal brush border complete digestion. Bile from the gallbladder emulsifies fats into micelles for absorption. Carbohydrates are absorbed as monosaccharides via SGLT1 and GLUT2 transporters. Proteins are absorbed as amino acids and di tripeptides via various transporters. Fats are absorbed as free fatty acids and monoglycerides, reassembled into triglycerides, packaged into chylomicrons, and transported via lacteals to the bloodstream. B12 and bile salts are absorbed specifically in the terminal ileum. The migrating motor complex MMC sweeps the intestine clean between meals.