How the Large Intestine Functions and Keeps You Healthy

The large intestine, also known as the large bowel, is the last part of the gastrointestinal (GI) tract which plays a central role in digestion. It is where undigested food received from the small intestine is processed and any water and remaining nutrients are absorbed before the indigestible matter is excreted from the body as feces (stool).

Although the large intestine is sometimes referred to as the colon, the colon is only a part—albeit the largest part—of the digestive organ.

The large intestine also plays a central role in your health by influencing the enteric nervous system which not only directs digestion and gut motility (movement) but is also involved in stimulating hormones and the immune response.

The large intestine is vulnerable to diseases like hemorrhoids, inflammatory bowel disease (IBD), diverticulitis, and cancer, the risk of which can be reduced with diet, smoking cessation, and other healthy lifestyle choices.

This article describes the anatomy and function of the large intestine, including the diseases and disorders that can affect this important organ. It also offers tips on how to keep your large intestine healthy to avoid disease.

Location and Size of the Large Intestine

The large intestine is housed in the abdominal cavity along with the stomach, liver, pancreas, spleen, kidneys, and other organs. The cavity is lined with a membrane called the peritoneum that secretes fluids that keep the organs moist and help them glide over each other.

The large intestine is so named because of its width. It is approximately 3 inches in diameter compared to the small intestine which is closer to 1 inch. In total, the large intestine is around 6 feet long, while the small intestine is approximately 21 feet long.

Both the large intestine is flexible and can expand somewhat to accommodate larger amounts of partially digested food or stool.

Parts of the Large Intestine

The large intestine consists of the cecum, colon, rectum, and anal canal. Each plays a specific role in digestion and your digestive health.

Cecum

The first part of the large intestine is a small pouch called the cecum which connects to the last part of the small intestine, called the ileum. The section of the large intestine, measuring only about 2.5 inches long, receives and holds partially digested food (chyme) so that gut bacteria can break down an indigestible fiber called cellulose.

The cecum also absorbs salts and electrolytes from chyme and secretes mucus that lubricates the solid waste so that it passes more easily into the colon.

Colon

The colon is the largest part of the large intestine. It is tasked with extracting key nutrients and any remaining water from the indigestible material to form stool. From start to finish, the colon is between 60 and 65 inches long.

The colon consists of four sections:

• Ascending colon: This is the first part of the colon that starts in the lower right abdomen and runs upward toward the diaphragm.
• Transverse colon: This is the second part which runs horizontally across the top of the abdomen from the right to the left,
• Descending colon: This is the third part which moves downward along the left side of the abdomen.
• Sigmoid colon: This is the final S-shaped section situated in the lower left abdomen that connects to the rectum.

Rectum

The rectum is the final straight portion of the large intestine measuring about 5 inches long. It acts as a temporary storage site for feces until it is ready to be passed (defecated).

At the top and bottom of the rectum are two circular muscles, called the internal and external sphincters, that open and close to move feces in and out of the rectum.

The internal sphincter next to the sigmoid colon is involuntary and opens and closes when needed. The external sphincter next to the anus is voluntary and can be held shut to keep stool from leaking out (referred to as fecal incontinence).

As the walls of the rectum expand with feces, nerve signals are sent to the brain that trigger contractions that initiate defecation.

Anus

The anus is the external passageway through which feces exit the body. Running about two inches, the anus is where mucosal membranes in the digestive tract change to normal skin. When you are ready to defecate, the internal sphincter helps push stool out of the body.

The anal canal is composed of inner grooves, called anal sinuses, that secrete mucus to aid with the smooth passage of stool.

The final grooved section (where mucosal tissues turn to skin) is called the anal valve. This is the area populated with nerves that can sense pain, itch, heat, and cold. The rest of the large intestine lacks sensory nerves.

Function of the Large Intestine

As part of the gastrointestinal tract, the large intestine has four key functions that enable digestion:

• It breaks down indigestible fiber so that it can pass more easily through the intestine.
• It absorbs water from chyme to form healthy stools.
• It secretes mucus to make the passage of stool smoother.
  
• It absorbs any remaining nutrients in chyme, including minerals and vitamins like thiamine and riboflavin.

Structure of the Large Intestine

The colon is comprised of four layers of tissues, similar to other regions of the digestive tract. These include:

• Mucosa: This innermost layer secretes mucus to lubricate stools, protect against abrasive particles, and prevent parasites, viruses, or bacteria from penetrating intestinal tissues.
• Submucosa: This underlying layer contains blood vessels, nerves, and connective tissue that supports the other tissue layers. 
• Muscularis: This is the layer of smooth muscles that contract involuntarily to move food through the digestive tract (otherwise known as peristalsis).
• Serosa: This is a thin layer on the outside of the intestine that secretes a lubricating fluid that helps it glide against other internal organs.
  

Conditions Associated With the Large Intestine

Many medical conditions can affect the large intestine, including infections, inflammatory diseases, functional disorders, and cancer.

Some of the more common include:

• Constipation: These are hardened stools often caused by dehydration or eating too little fiber.
• Diarrhea: These are watery stools often caused by fatty diets, intestinal infections, motility disorders, and inflammatory bowel disease.
• Hemorrhoids: This is the abnormal bulging of veins inside the rectum or anal canal that can cause pain, itching, and bleeding.
• Inflammatory bowel disease (IBD): This is an immune-mediated disorder that causes ulcerative colitis and Crohn's disease.
• Irritable bowel syndrome (IBS): This is a non-inflammatory functional disorder of uncertain origin that causes abdominal pain, bloating, gas, diarrhea, or constipation.
• Dysmotility disorders: These are functional conditions that affect peristalsis, either making the intestines move too quickly or not quickly enough.
  
• Diverticular disease: This is the formation of abnormal pouches (diverticulosis) due to stress placed on the intestine which can become infected (diverticulitis)
• Bowel obstruction: This is the partial or complete blockage of the intestine caused by scar tissues from surgery or disease, tumors, or dysmotility disorder that affects peristalsis.
• Rectal ulcer syndrome: This is a rare disorder that can affect people with chronic constipation, causing open sores in the rectum due to straining.
• Intestinal malrotation: This can occur early in pregnancy when a baby's intestines don't form normally, causing twists that can cause blood flow restriction or bowel perforation.
• Neurogenic bowel: This is the loss of normal bowel function caused by a nerve-related problem like a stroke, multiple sclerosis, or a spinal cord injury.
• Toxic megacolon: This is a potentially life-threatening complication of IBD in which the colon is severely distended or inflated, reducing blood flow to intestinal tissues.
• Necrotizing enterocolitis (NEC): This is an intestinal infection mainly affecting premature babies that causes tissue death and bowel perforation.
• Colorectal polyps: These are benign (noncancerous) growths in the lining of the colon or rectum that can become cancerous, particularly when they become large.
• Colorectal cancer: This is the third leading cause of cancer-related deaths in the United States.

Signs of Problems With the Large Intestine

A gastroenterologist is a specialist trained in the diagnosis, treatment, and prevention of diseases of the gastrointestinal tract. It is important to see a gastroenterologist if you develop potentially serious signs and symptoms that affect large intestine function like:

• Persistent abdominal pain or cramping
• Chronic constipation or diarrhea
• Changes in bowel habits, including pencil stools or intermittent diarrhea and constipation
• Hemorrhoids that don't improve with self-care
• Rectal bleeding
• Bloody or tarry stools
  
• Loss of bowel control
  

Tips for Keeping Your Intestine Healthy

Some conditions affecting the large intestine cannot be avoided, influenced by things that are beyond your control (like family history or sex).

However, many conditions like cancer, IBS, IBD, and colon cancer are influenced by what you do. This includes the food you eat, how much you weigh, how active or inactive you are, and whether or not you smoke.

To keep your large intestine healthy over the long term:

• Stay well hydrated: Drink six to eight 8-ounce glasses of water or other fluids per day to reduce the risk of constipation. Limit caffeine and alcohol which have diuretic effects.
• Increase your fiber intake: Eating whole grains, nuts, seeds, fruits, and vegetables can keep you regular and even reduce your risk of colon cancer.
• Cut back on red meat: Eating lots of red meat, especially fatty meats and processed meats, increases the risk of IBS, IBD, and colon cancer.
• Lose weight: Being overweight or having obesity places you at increased risk of IBD, diarrhea, constipation, diverticular disease, and colon cancer.
• Exercise regularly: Exercise helps keep the bowels moving, reducing the risk of constipation. Even taking a walk around the block can help.
• Stop smoking: Cigarette smoke reduces blood circulation in the intestine, increasing the risk of IBD, diverticulitis, intestinal stenosis (narrowing), and bowel obstruction.
  

Summary

The large intestine is the last part of the gastrointestinal tract comprised of the cecum, colon, rectum, and anus. Its role is to break down indigestible fiber, absorb water and nutrients, and form stools to pass out of the body. It also houses gut bacteria that influence your overall health by interacting with the central nervous system.

There are many conditions that can affect the large intestines, from hemorrhoids and irritable bowel syndrome (IBS) to inflammatory bowel disease (IBD) and colorectal cancer. A gastroenterologist can help diagnose, treat, and prevent these and other conditions affecting the large intestine.