Absorption in the Small Intestine

The small intestine, mainly the jejunum, is where most nutrients are absorbed into the blood.
As shown in Figure, the mucous membrane lining the small intestine is covered with very small, fingerlike projections called villi (singular: villus). Epithelial cells of each villus has thousands of microscopic projections called microvilli (singular: microvillus). Because there are millions of these tiny projections, they greatly increase the surface area for absorption. In fact, villi increase the absorptive surface of the small intestine to the size of a tennis court! This increases the amount of surface area available for the absorption of nutrients.

Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to the liver and different organs of the body. The food that remains undigested and unabsorbed passes into the large intestine.