Small intestine - from the stomach, food passes through the pylorus into the first part of the small intestine, the duodenum, in sufficient quantities to be digested by the small intestine.
The small intestine is the 5-7 metre long part of our digestive system where digestion and absorption take place. Digestion in the small intestine is supported by two other organs, the Mayand the pancreasand enzymes produced by the skin. The duodenum is supplied with enzymes from the pancreas, enzymes from the liver and the gall bladderfrom bile. Digestive enzymes and bile also play a major role in digestion, absorption and the creation of the alkaline pH necessary for enzymes to function. The small intestine transports and mixes the intestinal contents by peristaltic movements and aids digestion and absorption by adding intestinal fluids.
The mucosa of the small intestine, except for the first few centimetres of the duodenum (bulb), is covered with folds, small growths (papules) and very small growths (microbullae). The bulbils and microbulbils increase the surface area of the small intestine (about 400 m2 of surface area), so that nutrients can be absorbed over a larger surface area. It is also our body's largest immune organ.
The part of the small intestine following the duodenum is the jejunum and ileum. These parts of the small intestine are responsible for absorbing fats, minerals and other nutrients. The mixing movements of the peristalsis promote absorption by bringing the chyme into contact with folds, flocs and microflocs over a larger surface area. The walls of the small intestine have a rich vascular supply, with blood vessels carrying absorbed nutrients to the liver via the portal vein (the vena portae).
The digestion of proteins is already Stomachbut their main site of digestion is the small intestine. A pancreasand enzymes produced by the small intestine do the breakdown. Trypsin and chymotrypsin break down proteins into smaller and smaller fragments, down to the building blocks of proteins: amino acids.
The digestion of carbohydrates starts in the oral cavity and the breakdown of undigested starches is completed in the small intestine, along with the carbohydrates that have already started. The end products of carbohydrate digestion are simple sugars: fructose, glucose, galactose.
The place where fats are digested is the small intestine. Here the pancreasand fat-breaking enzymes (lipases) produced by the intestine. To digest them, however, they need bile produced by the liver, which breaks down (emulsifies) the fat droplets into smaller droplets. On the increased surface area, lipase can break down fats more easily.
The intestinal walls secrete mucus, which lubricates the intestinal contents, and water, which helps to dissolve digested material. The consistency of the intestinal contents changes continuously as they pass through the small intestine. In the duodenum, food is mixed with pancreatic enzymes at an alkaline pH and bile to reduce acidity. Over the 7 metres travelled, most of the nutrients and fluid are reabsorbed, and about 1 litre of fluid reaches the colonbe.