About Colon Cancer

Colon cancer, also known as colorectal cancer, starts in the colon or the rectum. These cancers can also be called colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer are often grouped together because they have many features in common. 

 

To understand colorectal cancer, it helps to know about the normal structure and function of the colon and rectum. The colon and rectum make up the large intestine (or large bowel), which is part of the digestive system, also called the gastrointestinal (GI) system (see illustration below).

Most of the large intestine is made up of the colon, a muscular tube about 5 feet (1.5 meters) long. The parts of the colon are named by which way the food is traveling through them.

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  • The first section is called the ascending colon. It starts with a pouch called the cecum, where undigested food is comes in from the small intestine. It continues upward on the right side of the abdomen (belly).

  • The second section is called the transverse colon. It goes across the body from the right to the left side.

  • The third section is called the descending colon because it descends (travels down) on the left side.

  • The fourth section is called the sigmoid colon because of its “S” shape. The sigmoid colon joins the rectum, which then connects to the anus.

 

The ascending and transverse sections together are called the proximal colon. The descending and sigmoid colon are called the distal colon.

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