CancerQuest >> Cancer by Type >> Colon and Rectal Cancer

Colon and Rectal Cancer: Anatomy

The colon and rectum are parts of the digestive, or gastrointestinal (GI), system. The purpose of this system is to break down food, absorb nutrients and water, and remove waste from the body.

Food matter is largely broken down in the stomach and then released into the small intestine. Most of the nutrients from food are absorbed in this region of the digestive system. The small intestine continues into the colon, or large intestine, which is divided into 4 regions (based on location): ascending colon, transverse colon, descending colon, and sigmoid colon

The main purpose of the colon is to absorb water and mineral nutrients from the food matter and store waste. Waste moves from the colon into the final 6 inches of the digestive system, called the rectum, and passes out of the body through the anus.

About 95% of colorectal cancers develop in glandular cells that make up the lining of the colon and rectum. A cancer that begins in a glandular cell is called an adenocarcinoma. Cancer usually starts in the innermost layer of the lining and slowly progresses through the other layers. The image below is a cross section view of the layers of the colon.