The Anatomy of the Colon (Large Intestine)

The colon, which is another name for the large intestine, is an important part of the digestive system. Many people think of the large intestine as simply a storage organ, a conduit for carrying indigestible nutrients from the small intestine to the anus to be discharged, yet this organ has many very important functions in the gastrointestinal (GI) tract, including:

• Reabsorbing water and maintaining the body's balance of fluids
• Absorbing some vitamins
• Processing undigested food material such as fiber
• Storing waste before elimination (converting food remnants to feces)

Anatomy

Location

The colon extends from the cecum (where the small intestine meets the large intestine) to the anus (where waste exits the body), and comprises four main regions that are labeled in the image above:

• Ascending colon: The ascending colon is the first part of the large intestine. It begins just beyond the cecum (a pouch-like structure at the end the ileum – the part of the small intestine furthest from the stomach) on the bottom right side of the abdomen and ascends (goes upwards) to the area of the abdomen just below the diaphragm.
• Transverse colon: The transverse colon runs horizontally across the abdomen from the right to the left.
• Descending colon: The descending colon descends (goes downward) along the left side of the abdomen from just beneath the diaphragm at the top of the abdomen to the left pelvic region.
• Sigmoid colon: The S-shaped connection between the last part of the colon and the rectum, located on the bottom left side of the abdomen is called the sigmoid colon.

Structure

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

• Mucosa: This is the innermost layer and is made of simple columnar epithelial tissue, making it smooth (compared to the small intestine, which contains villi, small fingerlike protrusions). Many glands secrete mucus into the interior lumen of the large intestine, which lubricates its surface and protects it from abrasive food particles.
• Submucosa: The mucosa is surrounded by the submucosa, which is a layer of blood vessels, nerves and connective tissue that supports the other layers of the large intestine. 
• Muscularis: The submucosa is surrounded by the muscularis, which contains many layers of visceral muscle cells that contract and move waste product through the large intestine in a process known as peristalsis.
• Serosa: The outermost layer, known as the serosa, is a thin layer of simple squamous epithelial tissue. The serosa secretes a watery fluid that provides lubrication for the colon's surface that protects it from damage due to contact with other abdominal organs as well as the muscles and bones of the lower torso that surround it. 

Function

A slurry of digested food (called chyme) passes from the small intestine into the colon through the ileocecal valve and the cecum, where it mixes with beneficial bacteria from the colon. It then moves through the four regions of the colon (haustra) over the course of several hours as a result of peristalsis. In some cases, this process can become much faster by stronger waves of peristalsis that follow a large meal.

Vitamin Absorption

You may think of vitamins as nutrients that are absorbed higher in the digestive tract, but the colon plays a very important role in the absorbing vitamins necessary for good health. These vitamins are actually produced by healthy bacteria in the colon through fermentation and include:

• Vitamin K
• Biotin (vitamin B7)

Associated Conditions

There are a number of medical conditions that can affect the colon. Some of these include:

• Colorectal cancer: Colon cancer can affect each part of the colon discussed above and is the 3rd leading cause of cancer-related deaths in both men and women in the United States.
• Inflammatory bowel disease: Conditions such as ulcerative colitis and Crohn's disease can affect the colon, and also increase the risk of developing colon cancer.
• Diverticulosis and diverticulitis: Tiny sacs (out pouches) called diverticula can develop along the colon. When these become inflamed it may result in a very uncomfortable condition known as diverticulitis, sometimes referred to as "left-sided appendicitis."
• Dehydration: When the colon does not perform effectively to reabsorb water from the colon, dehydration may result.
• Bowel obstruction: Sometimes the colon becomes kinked or wrapped in adhesions or scar tissue. This can result in either a partial or complete bowel obstruction, and if left untreated, can result in perforation of the bowel. The most common causes are conditions that result in scar tissue in the abdomen, such as previous abdominal surgery, inflammatory bowel disease, and pelvic inflammatory disease.
• Vitamin deficiencies: When the colon does not function properly, vitamins such as biotin and vitamin K are not adequately absorbed, leading to symptoms related to a particular deficiency.
• Constipation and diarrhea.

A Word From Verywell

As noted earlier, though many people picture the colon primarily being a storage unit, it has many important functions. It can also be affected by conditions such as colitis and cancer, which remain far too common in the United States.