Small intestine – food passes from the stomach through the pylorus into the first section of the small intestine, the duodenum, in portions small enough for the small intestine to digest.
The small intestine is a section of our digestive system about 5-7 metres long, where digestion and absorption take place. Two other organs also aid digestion in the small intestine: the liverand the pancreassupport it with the juices and enzymes they produce. The duodenum receives enzymes from the pancreas and bile from the liver and the gallbladder. Digestive enzymes and bile both play a major role in digestion, absorption and in creating the alkaline pH that the enzymes need in order to work. Through peristaltic movements, the small intestine moves the intestinal contents along and mixes them, and by adding intestinal juices it aids digestion and absorption.
Except for the first few centimetres of the duodenum (the bulb), the lining of the small intestine is covered with folds, small projections (villi) and even tinier projections (microvilli). The villi and microvilli increase the surface area of the small intestine (to about 400 m2), so nutrients can be absorbed over a larger surface. It is also the body's biggest immune organ.
The parts of the small intestine following the duodenum are the jejunum and the ileum. These parts are responsible for absorbing fats, minerals and other nutrients. The mixing movements of peristalsis promote absorption by bringing the chyme into contact with the folds, villi and microvilli over a larger surface. The wall of the small intestine has a rich blood supply; the vessels carry the absorbed nutrients to the liver via the portal vein (vena portae).
Protein digestion already begins in the stomach, but the main site of protein digestion is the small intestine. The pancreasand the small intestine produce the enzymes that break proteins down. Trypsin and chymotrypsin split proteins into ever smaller pieces, right down to their building blocks: amino acids.
Carbohydrate digestion begins in the mouth; in the small intestine, the breakdown of undigested starches is completed, together with that of the carbohydrates whose digestion has already begun. The end products of carbohydrate digestion are simple sugars: fructose, glucose and galactose.
Fats are digested in the small intestine. Here, fat-splitting enzymes produced by the pancreas, as well as by the bowel itself, do their work: these are the lipases. To digest fats, however, bile produced by the liver is needed, which breaks fat droplets up into smaller droplets (emulsifies them). Over this enlarged surface, lipase can break down fats more easily.
The bowel walls secrete mucus, which lubricates the intestinal contents, and water, which helps to dissolve the digested material. The consistency of the intestinal contents changes continuously as they pass through the small intestine. In the duodenum, food is mixed at an alkaline pH with pancreatic enzymes and with bile, which reduces acidity. Over the 7 metres travelled, most of the nutrients and fluid are reabsorbed, and about 1 litre of fluid reaches the colon, the large bowel.
