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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 is 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 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, 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 changed 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 the most common gastrointestinal (GI) condition. It has many symptoms, including belly pain and cramps, diarrhea or constipation, and bloating.

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. You may become malnourished.

Crohn’s disease. With this disorder, your body’s immune system attacks harmless substances in your digestive tract. These can include helpful bacteria. White blood cells then build up in your small intestine. This results in 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. If your intestine is completely blocked, tissue may die. So surgery is often needed. Small bowel obstruction is often caused by hernias. It is also caused by bands of tissue (adhesions) that can twist or pull your intestine. A complete bowel obstruction is an emergency. It needs medical care right away.

Online Medical Reviewer: Jen Lehrer MD
Online Medical Reviewer: L Renee Watson MSN RN
Online Medical Reviewer: Marianne Fraser MSN RN
Date Last Reviewed: 8/1/2018
© 2000-2020 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.
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