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The Digestive Process: What Does the Small Intestine Do?

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

Parts of the small intestine

The upper part of your small intestine is the duodenum. It's the widest part of your small intestine and also the shortest. It's about 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, such as 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 part 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. Finger-shaped structures called villi line the entire small intestine. They 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. Water, minerals, and any nutrients are then absorbed from your food. The leftover waste is formed into a bowel movement.

Common disorders of the small intestine

Many conditions can damage or impair your small intestine. Among them are:

  • Irritable bowel syndrome (IBS).  This is a gastrointestinal (GI) disorder. It has many symptoms, including belly pain and cramps, diarrhea or constipation, and bloating. These symptoms generally occur without any visible signs of damage or disease to your digestive tract.

  • 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 won't be able to absorb nutrients correctly and you may become malnourished.

  • Crohn’s disease.  This is a chronic disease that causes inflammation and irritation in your digestive tract. This can cause ulcers and injury to the intestines. Crohn’s disease most often affects the ileum, a part of your small intestine but it can occur anywhere in the GI tract.

  • Small bowel obstruction. This is a narrowing of your intestine that prevents food from getting through. It most often affects the small intestine. Small bowel obstruction is often caused by hernias. It is also caused by bands of tissue (adhesions) that can twist or pull your intestine or tumors. A complete bowel obstruction is an emergency. It means that the intestine is completely blocked. It needs medical care right away.

Online Medical Reviewer: Jen Lehrer MD
Online Medical Reviewer: Rita Sather RN
Online Medical Reviewer: Tara Novick BSN MSN
Date Last Reviewed: 2/1/2023
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