Your small intestines break down consumed food and absorb the nutrients and water that your body needs to function. Spanning more than 26 feet in adults, the small intestines are divided into three sections: the duodenum, the jejunum and the ileum. Each of these sections secrete a different combination of enzymes to break down food and transport a different combination of nutrients into the body.
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Function of the Small Intestines
The small intestines lie in your gastrointestinal track between the stomach and large intestines. The cells lining the intestines secrete enzymes -- proteins produced by your body that perform a specific biochemical reaction -- that break down the complex food structure of your diet releasing individual nutrients. Other cells lining your small intestines select particular nutrients, including amino acids, fats, carbohydrates, minerals, vitamins and water, to transport from the intestines to the blood stream.
The duodenum is the first and shortest section of the small intestine. It receives roughly digested and mixed food from the stomach and neutralizes the stomach acid. The duodenum also receives bile from the galbladder and digestive enzymes from the pancreas. The combination of food, bile and enzymes are mixed with mucus and passed into the jejunum. The intermixed structure of protein, fats and carbohydrates of food is broken apart as it passes through the duodenum.
The combination of food, bile, enzymes and mucus passes into the jejunum after exiting the duodenum. The inner lining of the jejunum and the later section the ileum are lined with villi, small fingers containing capillaries that increase the surface area that can absorb nutrients. The jejunum absorbs nutrients such as carbohydrates that have been broken down into simple sugars, proteins that have been broken down into amino acids and many vitamins. Much of this transport is accomplished by an array of transporter proteins that shuttle the nutrients from the intestines to the capillaries.
The ileum is the final and longest section of the small intestines. The differentiation between the jejunum and ileum is indistinct and gradual, although they have different functions. Water, minerals and salts as well as fats and remaining nutrients are absorbed by the ileum. Where much of the initial transport of nutrients was specific, the ileum takes small amounts of water as well as the minerals and vitamins floating in it and shuttles them into the capillaries. Fats are able to pass directly from the intestines to the bloodstream.