What is the function of the small intestine?

The small intestine is 15 to 20 feet long and includes three sections:

• the duodenum (which makes up the first 10 inches of the small intestine)
• the jejunum (which makes up the next 5 feet) 
• the ileum (which makes up the rest).

Digestion and absorption of food take place primarily in the small intestine.

In the duodenum, bile and pancreatic juice are added to the food mixture. In the jejunum, fat, starch and proteins from the food material are broken down and absorbed by the intestinal lining. The lining is covered with millions of tiny villi, or finger-like projections, that greatly increase the surface area, allowing nutrients to be more efficiently absorbed into the bloodstream. In the ileum, which is the last part of the small intestine, water and nutrients (such as vitamin B12) are absorbed. Bile, which is released by the gallbladder into the duodenum in order to help emulsify (digest) ingested fats, is reabsorbed in the ileum to prevent its loss from the body.

The main work of digestion takes place in the small intestine, which is a remarkable 21 feet long. The small intestine breaks down fats, starches, and proteins into fatty acids, simple sugars, and amino acids, which it can then absorb.

The food you eat generally takes three to five hours to move through the small intestine. During this time, the food is bathed in digestive enzymes and juices that flow into the intestine through ducts from the liver and pancreas. Bile, produced by the liver and stored in the gallbladder, emulsifies fat, enabling its absorption. Enzymes secreted by the pancreas, such as trypsin, amylase, and lipase, help digest proteins, carbohydrates, and fats. Once reduced to products the body can manage, the nutrients from digested food are absorbed by the intestine's thin lining and sent to cells throughout the body by way of the bloodstream and lymphatic system.

Food moves through the small intestine as if on a conveyor belt. The first step, after the stomach empties food through the pyloric sphincter, is the foot-long duodenum, located a few inches above the navel. Many minerals, such as iron and calcium, are absorbed in the duodenum. This is also where bile and pancreatic juices join the mix.

After the duodenum, the next part of the small intestine is the jejunum, which measures eight feet in length. In the jejunum, fats, starches, and proteins are further broken down and absorbed.

The third and lowest portion of the small intestine, the ileum, is approximately 12 feet long. The ileum absorbs water, as well as vitamin B12 and bile salts.