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What is the colon?

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Structure and Functions

In the average adult man, the large intestine is about 1.5 to 1.8 meters long. It is divided into the cecum; the ascending, transverse, descending, and sigmoid colon; and the rectum, ending in the anus. Its wall contains both circular and longitudinal layers of smooth muscle and innervation that controls its motility. The longitudinal musculature runs along the outside of the colon in three separate bands, called teniae coli, which converge around the sigmoid colon and the rectum. The inner layer of the large intestine consists of mucosa with sparse or no villi but with numerous invaginations (glands); it is lined with simple columnar epithelium. The glands contain goblet cells, endocrine cells, and absorptive cells. There are no digestive enzymes linked to the inner surface of the colon.

The main function of the colon is to absorb water from the chyme and to process it into feces for elimination. Most nutrients and about 90 percent of water are absorbed in the small intestine. When it reaches the large intestine, chyme still contains some electrolytes (sodium, magnesium, and chloride) and indigestible food components, such as fiber.

An abundant and varied bacterial population colonizes the human colon shortly after birth and resides in the large intestine for life. Bacteria digest fiber and produce short-chain fatty acids (acetate, propionate, and butyrate). Short-chain fatty acids promote the integrity of the colonic epithelial cells, prevent inflammation, and provide some protection against potential pathogens.

The large intestine absorbs some vitamins (mainly vitamin K) and electrolytes but mainly water—up to five liters of water per day. Water moves passively with sodium, which is mostly absorbed in the distal colon. Water absorption is in part regulated by aldosterone, a hormone that increases the absorption of sodium in response to volume depletion. Water absorption solidifies the chyme into stools.

The motility of the colon allows for mixing the contents and retaining them for prolonged periods. Periodically, the colon is swept with propulsive contractions (peristalsis) that move its contents toward the rectum. The gastrocolic reflex causes mass peristalsis after a meal. The sigmoid colon and rectum serve as a reservoir and participate in defecation.

Disorders and Diseases

A number of disorders are associated with the colon. Appendicitis is the inflammation of the vermiform appendix. It requires surgery. Constipation is the failure to empty the bowels regularly and easily. It can be linked to diet, stress, and a variety of conditions and medications. It is treated with dietary fiber and laxatives. Diarrhea involves frequent loose or liquid bowel movements. It may have many different causes and is treated mainly with loperamide or bismuth salicilate.

Diverticulitis refers to the development of outpouchings in the colon. A low-fiber diet and age are risk factors. Symptoms are linked to inflammation (diverticulosis) and are mostly treated with antibiotics. Complications may require surgical removal of the outpouchings. Hirschsprung disease (congenital aganglionic megacolon) involves the complete absence of neuronal ganglion cells (which make the intestinal muscles contract, so the stool is pushed forward) from a segment of the intestine, usually the distal colon. It requires surgery.

Inflammatory bowel disease (IBD) is a general name for diseases that cause intestinal swelling. They include ulcerative colitis (inflammation and ulcers in the top layer of the lining of the large intestine) and Crohn’s disease (all layers of the intestine may be involved; healthy bowel segments alternate with affected segments). Treatment varies widely, but the condition will recur. Irritable bowel syndrome (IBS) is a functional disorder of the colon of unknown cause. Its symptoms are abdominal pain, abnormal bowel habit, bloating, and either constipation, diarrhea, or both alternating. It is worsened by stress. IBS may be linked to hypersensitivity of intestinal muscles and nerves. Its treatment varies according to symptoms.

Colorectal polyps are growths on the lining of the colon or rectum. In time, they can develop into colorectal cancer. They are removed with endoscopic microsurgery. Colorectal cancer refers to cancerous growths in the colon, rectum, and appendix, mostly thought to arise from adenomatous polyps in the colon. It requires surgery.

Perspective and Prospects

The intuition that the colon is associated with waste accumulation and release dates to antiquity. Ancient Egyptian physicians also viewed “intestinal putrefaction” as the basic cause of disease, a concept later incorporated into the humoral doctrine of disease by ancient Greeks. This concept of autointoxication lasted throughout the centuries with few adaptations. In the nineteenth century, early studies showed the presence and activity of bacteria in the colon. It was then thought that colonic bacteria generate toxic amines that shorten life span. This theory was finally abandoned by the 1920s. Later research has focused on the molecular mechanisms of water and electrolyte movements and their regulation, as well as on the pathways that modulate the secretory and absorptive functions of the colon. Recent developments in genetics and immunology have allowed a deeper understanding of inflammatory diseases of the colon. The advent of endoscopic techniques has vastly improved microsurgery and cancer prevention.

Bibliography

Bäckhed, Fredrik, et al. “Host-Bacterial Mutualism in the Human Intestine.” Science 307 (March 25, 2005): 1915–1920.

Barrett, Kim E. “Functional Anatomy of the GI Tract and Organs Draining into It.” In Gastrointestinal Physiology. New York: McGraw-Hill, 2006.

"Digestive System." MedlinePlus, January 14, 2013.

Mahnke, Daus. "Colonoscopy." Health Library, February 6, 2013.

Sherwood, Lauralee. “The Digestive System.” In Human Physiology: From Cells to Systems. 8th ed. Belmont, Calif.: Brooks/Cole/Cengage Learning, 2013.

"Your Digestive System and How It Works." National Digestive Diseases Information Clearinghouse (NDDIC), April 23, 2013.

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