The large intestine's function is much more complex than forming stool. Water, nutrients, and salts from food to prevent dehydration and electrolyte imbalances. The complex symbiotic relationships between bacteria in the large intestine produce vitamins the body needs. The organ is also an essential part of the immune system; its nerves communicate with the brain to influence hormone production, signaling hunger and satiety.

Basic Structure

The large intestine connects to the small intestine at the right side of the abdominal cavity. The last four regions of the intestinal tract -- the cecum, colon, rectum, and anus -- make up this organ. In total, it is five feet of length and a three-inch circumference. The valve between the small intestine and large intestine is called the ileocecal valve. Undigested food mass, known as chyme, passes through the ileocecal valve into the cecum.

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Functions of the Large Intestine

Longitudinal smooth muscle fibers cover the exterior of the large intestine in three bands. These muscle fibers produce the churning movements that move chyme along. The inner walls of the large intestine are smooth and lined with a layer of mucosa. This layer absorbs vitamins and electrolytes such as sodium, magnesium, and chloride. The cells of the intestinal lining also absorb water using sodium ions, through a process called osmosis. The water is then transferred to blood in capillaries next to the intestinal cells.


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Role in the Immune System

The layer of mucosa serves many purposes. It contains glands and specialized cells to secrete digestive fluids. Lymphoid tissue in the mucosa acts as a barrier to microbial infection and helps create antibodies that defend against disease. The large intestine is also a source of macrophages, eosinophils, and mast cells. All these fight invading viruses and harmful bacteria.

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Over 700 species of bacteria live in the large intestine. Most of these bacteria are anaerobes, which means they live in environments without oxygen. As bacteria break down food matter, gas is a byproduct. These commensal bacteria perform a vital role in digestion. Commensal bacteria produce B vitamins such as B12, thiamin, biotin, and riboflavin, and vitamin K. If a diet is low in these vitamins, the large intestine must produce them. Sometimes, antibiotics can kill too many of the good bacteria, leading to vitamin deficiencies. Probiotics in supplements or foods like yogurt and cheese can mitigate the negative effects of antibiotics on commensal bacteria.

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The process of removing nutrients from food and expelling undigested matter -- peristalsis -- takes an average of 36 hours. Fiber helps move food through the digestive tract and adds bulk to stool. Bacteria break down fiber to produce nutrients. Dietary fiber includes soluble fiber, which dissolves in water and is found in fruits, oats, legumes, and barley. Insoluble fiber, which does not dissolve in water, comes from wheat, vegetables, and seeds. Fiber is beneficial as long as the individual also consumes a sufficient amount of water. If water intake is inadequate, too much fiber can cause constipation and bloating, instead of preventing it.

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Inflammatory Disorders

Diverticulosis is a fairly common condition. When the inner lining of the colon protrudes through weak areas of the muscular outer wall, the protrusions form tiny pouches called diverticula. Inflamed or infected pouches can indicate diverticulosis. People with diverticulosis experience symptoms such as abdominal pain and constipation. Colitis is general inflammation of the colon. The most common causes are infections and inflammatory bowel diseases such as Crohn's. Bloody diarrhea and pain are the most common symptoms. Dietary modifications can treat inflammation, but severe cases may require medication or surgery.

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Colon Cancer

Colon cancer is life-threatening and is often fatal. Symptoms include blood in the stool, weight loss, stomach pain, constipation, and diarrhea. Unfortunately, symptoms often do not appear until more advanced stages of cancer. Routine screening starting at age 45 or 50 is the best method of catching colon cancer early. FOBT, high-sensitivity fecal occult blood testing, is an in-home stool test that people at risk should take annually. Medical professionals may employ flexible sigmoidoscopies and colonoscopies. Both procedures require the insertion of a thin tube into the rectum or colon to search for pre-cancerous polyps or other abnormal tissue. Doctors also take a biopsy for testing, when necessary.

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Warning Signs

The frequency, consistency, and appearance of stool can indicate problems in the large intestine. Diarrhea or constipation are common problems and not a cause for concern if they occur sporadically. Severe or ongoing diarrhea can be a sign of a bacterial or viral illness. Blood in the stool may indicate inflammation or ulcers. Hemorrhoids are also a common problem that cause symptoms such as pain and bright red blood during and after defecation. External hemorrhoids may be visible around the anus. Blood from ulcers or other sources in the intestines is generally very dark red or black. Long, thin stool and difficulty or pain while defecating can point to bowel obstruction. Such issues require medical evaluation immediately.

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Risk Factors

Obesity increases the risk of colon cancer -- it causes hormonal imbalances that lead to excess insulin production. Red and processed meats can contribute to colon cancer and inflammation. The likelihood of colon cancer increases with age. Those with a personal or family history of colon disease and polyps need frequent screening. Long-term or untreated inflammation of the large intestine may cause dysplasia, abnormal, potentially cancerous cells in the lining of the colon. Food allergies or intolerances that develop later in life may indicate bowel disease or irritation.

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Healthy Habits

A healthy large intestine is needed to perform essential functions on which the body depends. Eating foods rich in fiber, limiting red meat, and increasing consumption of fish can all contribute to good digestive health. Omega-3 fatty acids in fish, for instance, reduce inflammation, a major characteristic of colon disorders. Frequent exercise and weight maintenance also encourage internal wellness. Colon cleanses may be suitable for some. Laxatives, teas, supplements, enemas, or colon hydrotherapy can help clean the large intestine. People should always consult a doctor before taking any products or using any cleansing techniques.

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