More information about the Small Intestine

The purpose of the small intestine is digestion and absorption. This is aided not only by the contraction of the intestine, but also by its large surface area which is increased by mucosal folds and the presence of millions of little finger like projections called villi (making the total area available in the adult about the size of a tennis court). Inside each villus there is a capillary network and some lymph drainage vessels called lacteals. As with many other structures in the abdomen, the small intestine is covered by a membrane called the peritoneum, which also lines the inner abdominal wall and which secretes a small amount of fluid called serous fluid to lubricate the movement of the bowel. The small bowel is connected to the back of the abdominal cavity by a double layer of peritoneum, called the mesentery, which carries the blood vessels and lymphatics. The blood vessels - called the splanchnic bed - come together and form the portal vein which drains into the liver.

Simple salts, glucose, water and water soluble vitamins are absorbed directly across the wall of the intestine - either passively by diffusing through gaps or crypts, or by active pumping processes. More complex sugars and proteins are broken down by enzymes contained in the cells of the villi before being transported by carrier mechanisms to the liver. Fats similarly are broken down and either absorbed as triglycerides, or combined with bile salts to make them water-soluble (in structures called micelles), which are absorbed into the lacteals and thence via the lymphatic system into the general circulation.

The contraction of the small bowel is controlled by pacemaker signals which set up both stationary contractions where the circular muscle contracts, and progressive contraction where the longitudinal muscle contracts. These two sorts of contraction usually occur together so that the contents are moved down the bowel. Contractions are increased in both strength and frequency by the presence of food in the stomach. They are also increased by the action of adrenaline - which is why we need to use the lavatory when we’re frightened.