The intestines work by absorbing nutrients and vitamins. They are part of the gastrointestinal (GI) tract. Together, the small and large intestines run from the end of the stomach to the anus.

In this article, we describe what the intestines are and what they do, as well as related health problems and their treatments.

Explore the interactive diagram below to learn more about the intestines.

The intestines sit in the abdomen and absorb nutrients and some water. Below, learn about the small and large intestines in more depth:

Small intestine

Food broken down by the stomach moves into the small intestine, which absorbs nutrients and sends them into the bloodstream.

Three major classes of nutrients pass through the small intestine:

The small intestine consists of three parts:

  • The duodenum: This connects to the stomach.
  • The jejunum: This, the middle part, absorbs nutrients and water.
  • The ileum: This further digests what was once food, called the digestive product, then passes it to the large intestine.

The small intestine also supports the immune system. As an older study, from 2011, suggests, the small intestine’s role in keeping bacteria under control is crucial and requires further investigation.

How long is the small intestine?

The small intestine is roughly 20–25 feet in length, making it the longer section. It has a very high surface area, which is amplified roughly 60–120 times.

The small intestine is not flat or smooth. Instead, the following characteristics help maximize its surface area and capacity for absorption:

  • mucosal folds — intricate folds on the intestine’s surface
  • villi — tiny, finger-like projections that line the inner wall of the intestine
  • microvilli — even smaller, hair-like projections on the villi

Large intestine

The large intestine absorbs water, salt, and other waste material from the digestive product and solidifies the waste into stool, which passes into the rectum.

The large intestine has several several parts, including:

  • The cecum: This section receives the digestive product from the small intestine and moves it to the colon.
  • The appendix: This is a finger-shaped pocket that joins with the cecum.
  • The colon: This is the longest part of the large intestine. It absorbs water and salt and solidifies liquid waste into stool.
  • The rectum: This stores stool until it passes through the anus and out of the body.

Various health issues can affect the intestines. For example, a person may develop:


Constipation involves the GI tract struggling to pass stool. It may result from a low fiber or fluid intake, hormonal imbalances, or a lack of mobility. Constipation can also be a side effect of medication.

It is more common in older people than younger people.

Eating more fiber, drinking plenty of fluids, and getting regular exercise may help ease constipation. A person might also try over-the-counter fiber supplements and laxatives.

Viral gastroenteritis

Viral gastroenteritis is also called the stomach flu. It results from an infection and can cause cramps, watery diarrhea, nausea, vomiting, and a fever. The symptoms may last from a few days to 10 days.

The stomach flu accounts for 19–21 million cases of diarrheal illness annually in the United States.

The key is to maintain hydration and replace electrolytes. To address specific symptoms, such as diarrhea, over-the-counter medications, such as loperamide (Imodium), can help.

Irritable bowel syndrome

This condition, usually called IBS, is characterized by abdominal pain, bloating, and changes in bowel habits. It affects around 25–45 million people in the U.S.

The exact cause is unknown, but the authors of one review observe that specific bacteria tend to be associated with it.

The right treatment depends on the person’s symptoms, but the results of a 2019 review suggest that a holistic approach, including personalized probiotic therapy and diet modifications, may be best.

Celiac disease

Over time, in a person with celiac disease, the gluten in wheat damages the small intestine so that it no longer absorbs nutrients as efficiently.

Symptoms vary, from diarrhea and abdominal pain to irritability and depression. Adopting a gluten-free diet is a key element of treatment.

Crohn’s disease

Crohn’s disease causes chronic inflammation in the GI tract. It most commonly affects the end of the small intestine and the juncture with the large intestine.

Because the symptoms of this inflammation also stem from many other health issues, a doctor may refer the person to a specialist, called a gastroenterologist, for further testing and a diagnosis.

The frequency and severity of symptoms can vary from person to person. Treatment involves taking medication, including biologic therapies.

Learn about the different types of Crohn’s disease here.

Ulcerative colitis

Ulcerative colitis causes inflammation, ulcers, and scarring of the large intestine. Symptoms include cramp-like abdominal pain and an urgent need to have a bowel movement.

To address flare-ups of symptoms, a doctor may prescribe medications: aminosalicylates, corticosteroids, or immunosuppressants. If the symptoms are severe and not responding well to medication, the doctor may recommend surgery.

Colorectal cancer

Depending on where the abnormal cells first form, the doctor may refer to colorectal cancer as bowel, colon, or rectal cancer. Taken as a whole, colorectal cancer is the third most common cancer diagnosed in adults in the U.S.

A variety of tests can detect colorectal cancer, including a colonoscopy, a CT scan, and a biopsy. Most early forms of the cancer cause no symptoms, so screening and early intervention are crucial.

Symptoms of colorectal cancer include:

  • diarrhea or constipation
  • blood in stool, making it look black
  • bleeding from the rectum
  • a feeling of fullness, pain, or bloating in the abdomen
  • fatigue
  • unexplained weight loss

The most common treatment is surgery, though the advisability of this depends on the tumor’s size and location and on the stage of the cancer.

Not all intestine-related health problems are preventable, but there are several ways to look after the gut, including:

  • eating a healthful diet that is rich in protein, fiber, and complex carbohydrates
  • staying hydrated
  • managing stress levels
  • quitting smoking and avoiding secondhand smoke

If stomach or gut pain lasts for more than a few days and does not ease with home care, contact a doctor.

Also, a person should receive medical attention if they have:

  • a flare-up of symptoms of a known health issue
  • a loss of appetite combined with tiredness and fatigue
  • blood in stool
  • intense, severe pain that lasts hours

Keeping the intestines healthy is crucial, and researchers continue to find new ways of treating related illnesses.

It is important to be aware of warning signs, such as bloody stool. Overall, anyone who has a concern about their gut health should consult a doctor.