Colon Cancer (Colorectal Cancer)

  • Medical Author:
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

  • Medical Editor: Jay W. Marks, MD
    Jay W. Marks, MD

    Jay W. Marks, MD

    Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

Quick GuideColon Cancer: Symptoms, Signs, Screening, Stages

Colon Cancer: Symptoms, Signs, Screening, Stages

What is the colon, and what does it do?

The colon and the rectum are the final portions of the tube that extends from the mouth to the anus. Food enters the mouth where it is chewed and then swallowed. It then travels through the esophagus and into the stomach. In the stomach, the food is ground into smaller particles and then enters the small intestine in a carefully controlled manner. In the small intestine, final digestion of food and absorption of the nutrients contained in the food occurs. The food that is not digested and absorbed enters the large intestine (colon) and finally the rectum. The large intestine acts primarily as a storage facility for waste; however, additional water, salts, and some vitamins are further removed. In addition, some of the undigested food, for example, fiber, is digested by colonic bacteria and some of the products of digestion are absorbed from the colon and into the body. (It is estimated that 10% of the energy derived from food comes from these products of bacterial digestion in the colon.) The remaining undigested food, dying cells from the lining of the intestines, and large numbers of bacteria are stored in the colon and then periodically passed into the rectum. Their arrival into the rectum initiates a bowel movement that empties the colonic contents from the body as stool.

Although the large intestine is a tube, it is structurally a complicated tube, more like a steel belted radial tire than a garden hose. The tube is comprised of four layers. The first is an inner layer of cells that line the cavity through which the undigested and digesting food travels, called the mucosa. The mucosa is attached to a thin second layer, the submucosa, that is attached itself to a layer of muscle, the muscularis. The entire tube is surrounded by fibrous (scar-like) tissue called the serosa. The most common cancers of the large intestine (the type called adenocarcinoma) arise from the mucosa, the inner layer of cells. These cells are exposed to toxins from food and bacteria as well as mechanical wear and tear, and they are relatively turning over rapidly (dying off and being replaced). Mistakes (usually a series of mistakes involving genes within the replacement cells) lead to abnormal cells and uncontrolled proliferation of the abnormal cells that give rise to cancer. The rapid turnover allows for more mistakes to occur as compared with tissues that do not turn over so rapidly (for example, liver tissue).

Medically Reviewed by a Doctor on 4/3/2017

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