The Digestive Process: What Does the Small Intestine Do?

Your small intestine is the longest part of the human digestive system. It is 20 feet long. After food leaves your stomach, it passes into your small intestine, where most of the digestive process takes place.

Parts of the small intestine

The upper part of your small intestine is the duodenum. It is the widest part of your small intestine and also the shortest. It is almost 10 inches long. When food moves into your duodenum, it mixes with digestive enzymes that your pancreas secretes. These enzymes break down the largest molecules of food, like proteins and starches. They also neutralize stomach acid. Bile is a substance that breaks down the fats in foods. It also empties into your duodenum by the common bile duct. Some minerals are absorbed here, such as iron and folate.

The middle portion of your small intestine is the jejunum. The jejunum absorbs most of your nutrients: carbohydrates, fats, minerals, proteins, and vitamins.

The lowest part of your small intestine is the ileum. This is where the final parts of digestive absorption take place. The ileum absorbs bile acids, fluid, and vitamin B-12. Villi line the entire small intestine. They are finger-shaped structures that help absorb nutrients.

Moving on through

Contractions move food through your small intestine. After you eat a meal, your small intestine contracts in a random, unsynchronized manner. Food moves back and forth and mixes with digestive juices. Then, stronger, wave-like contractions push the food farther down your digestive system. These movements are known as peristalsis. Your enteric nervous system controls the movements in your small intestine. This is a network of nerves that runs from your esophagus to your anus.

After food leaves your small intestine, contractions push any food that remains in your digestive tract into your large intestine, so that your body can eliminate it. 

Common disorders of the small intestine

A variety of conditions can damage or impair your small intestine. Among them are:

Irritable bowel syndome (IBS). This is the most common gastrointestinal condition. It consists of a variety of symptoms including abdominal pain and cramps, diarrhea and/or constipation, and bloating, among others.

Celiac disease. This is an allergy to gluten, a protein found in wheat, barley, and rye. When your body digests gluten, your immune system attacks the villi lining your small intestine. Without treatment, your body will not be able to absorb nutrients properly, and you may become malnourished.

Crohn’s disease. This is a disorder in which your body’s immune system attacks harmless substances in your digestive tract, like beneficial bacteria. This causes white blood cells to build up in your small intestine and results in ulcers and injury to the intestines. Crohn’s disease most commonly affects the ileum, a portion of your small intestine.

Small bowel obstruction. This is a narrowing of your intestine (most commonly the small intestine) that prevents food from getting through. If your intestine is completely blocked, tissue may die, so surgery is often necessary. Hernias and adhesions — bands of tissue that can twist or pull your intestine — are common causes of small bowel obstruction.