Parts of the Small Intestine

This Organ Is Where Most Vitamins and Minerals Are Absorbed From Food

Small Intestine

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The small intestine is a 20-foot-long tube that is part of the digestive tract and connects the stomach and the large intestine. Because it is so long, it must twist and turn through the abdomen. The small intestine is where most digestion takes place: most vitamins and minerals, as well as fats and some water, are absorbed in the small intestine.

Muscle contractions, called peristalsis, move food through the small intestine as it is digested. Enzymes in the small intestine work to break down food to the nutrient level. These enzymes are created by other organs and then moved into the small intestine, or are secreted by the intestinal wall. Once the food is broken down, it can be absorbed and enter the bloodstream.

Sections of Small Intestine

The small intestine is divided into three main sections, and different processes take place in each one. The parts of the small intestine are the:

  • Duodenum: The first and shortest section, which is roughly shaped like a "C." Food passes from the stomach to duodenum through a muscle called the pyloric sphincter. Iron is absorbed in the duodenum.
  • Jejunum: Sugars, amino acids, and fatty acids are absorbed in this part of the small intestine.
  • Ileum: This last part of the small intestine is where vitamin B12, bile acids, and other nutrients are absorbed.

How Food Is Absorbed by the Small Intestine

The pancreas is where digestive enzymes are created, and they are passed through a duct into the small intestine. These enzymes break down food so that they can be absorbed and used by the body. Proteins, fats, and carbohydrates are degraded into their component parts and taken up by the villi located in the jejunum and the ileum. The villi are structures that protrude from the inner wall of the small intestine like fingers or hair and take up nutrients.

Crohn's Disease of the Small Intestine

When Crohn's disease (one form of inflammatory bowel disease, or IBD) affects the small intestine, the absorption of vitamins and minerals can be affected. 

Ileocolitis is the most common form of Crohn's disease and is when the last part of the small intestine and the large intestine are involved. Ileitis is the type of Crohn's disease that affects the ileum and is the second-most-common form of Crohn's disease. People who have ileocolitis or ileitis may be deficient in vitamin B12 because inflammation may prevent its absorption in the ileum. These types of Crohn's disease may also result in a deficiency of the fat-soluble vitamins (A, D, E, and K), because the bile salts that facilitate the uptake of those vitamins are also absorbed in the ileum.

Jejunoileitis is a less-common type of Crohn's disease that affects the jejunum. Because most of the absorption of vitamins, minerals, proteins, fat, and carbohydrates takes place in the jejunum, inflammation in this section of the small intestine could lead to several nutritional deficiencies.

Gastroduodenal Crohn's disease is another more uncommon form of the disease that affects the duodenum (as well as the stomach). The minerals that could be affected include iron, calcium, and magnesium because they are all absorbed in the duodenum.

The Small Intestine and The J-Pouch

J-pouch surgery (also called ileal pouch or anal anastomosis (IPAA) surgery) is done to treat ulcerative colitis or familial adenomatous polyposis (FAP). In this surgery, the colon is removed and the ileum is used to create a pouch that's shaped like a "J" (sometimes an "S" or a "W" may also be created). Because the j-pouch is created from the ileum, if it later becomes inflamed (such as due to pouchitis), vitamin and mineral deficiencies might occur.

Other Diseases and Conditions of the Small Intestine

Besides Crohn's disease, several diseases, and conditions that can affect the small intestine, including:

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Article Sources
  • American Gastroenterological Association. "Patient Center: Inflammatory Bowel Disease." 2014. 17 Nov 2014.
  • MUSC Health. "Small Bowel." MUSC Digestive Disease Center. 2014. 17 Nov 2014.