What is the function of the small intestine?

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The small intestine is a hollow, muscular tube about 20 feet long that comprises the longest part of your digestive system. Your small intestine connects your stomach above it to your large intestine (or colon) below it. Your small intestine has three parts.
  • The duodenum is a short section at the top of the small intestine that takes semi-digested food from your stomach and digests it further, using bile from your gallbladder and liver and enzymes from the pancreas.
  • The jejunum is the middle portion of the small intestine. It uses wave-like contractions to push the food through to the lower part of your small intestine. As the food moves through the jejunum it continues being digested and takes on a more fluid state.
  • The ileum is the longest part of the small intestine and the part from which most nutrients and liquid are absorbed from food, before the remainder of the food is deposited into the large intestine.
Dr. Lawrence S. Friedman, MD
Gastroenterologist
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.
Dr. William B. Salt, MD
Gastroenterologist

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.

Irritable Bowel Syndrome & the MindBodySpirit Connection: 7 Steps for Living a Healthy Life with a Functional Bowel Disorder, Crohn's Disease, or Colitis (Mind-Body-Spirit Connection Series.)

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Irritable Bowel Syndrome & the MindBodySpirit Connection: 7 Steps for Living a Healthy Life with a Functional Bowel Disorder, Crohn's Disease, or Colitis (Mind-Body-Spirit Connection Series.)

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The small bowel, which is about 20 to 25 feet long, performs a variety of functions. The first part, the duodenum, is about a foot long and is responsible for things such as iron absorption. It is also connected to the pancreas, the bile ducts and the liver, and the digestive enzymes released by these organs help process proteins, fats and carbohydrates.

The jejunum is the next part of the small bowel. It's about 10 to 15 feet long and leads to the ileum, which is about 10 feet long. The last part of the ileum is called the terminal ileum, which is very important in Crohn's disease and is responsible for B12 absorption.  Patients who have had resections of the terminal ileum may be at a greater risk of B12 deficiency.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.