The colon and rectum are parts of the digestive system. They are sometimes called the large intestine or large bowel. The colon is a U-shaped tube made of muscle and found below the stomach. The rectum is a shorter tube connected to the colon. The colon and rectum are about 2 metres (6 feet) long, and form the end of the digestive system. They are surrounded by other organs including the spleen, liver, pancreas, reproductive organs and urinary bladder.
The large intestine or large bowel starts at the cecum. It is a pouch-like passage connecting the colon to the last part of the small intestine called the ileum. It is not technically a part of the colon but if cancer develops in the cecum, it is treated like colon cancer.
The colon is divided into 4 parts.
- The ascending colon is the start of the colon and found on the right side of the abdomen. It continues upward to a bend in the colon called the hepatic flexure.
- The transverse colon follows the ascending colon and hepatic flexure and lies across the upper part of the abdomen. It ends with a bend in the colon called the splenic flexure.
- The descending colon follows the transverse colon and splenic flexure, and is found on the left side of the abdomen.
- The sigmoid colon is the end of the colon and connects to the rectum.
The rectum is after the sigmoid colon, and about 15 cm (6 inches) long.
Colon and rectal cancers are grouped together as colorectal cancer because they are made of the same tissues and there isn’t a clear border between them.
The colon and rectum are held in the abdomen by folds of tissue called mesenteries. The mesocolon is a mesentery that attaches the colon to the wall of the abdomen. The rectum is also surrounded by a mesentery called the mesorectum. Mesentery is made of fatty connective tissue that contains blood vessels, nerves, lymph nodes and lymphatic vessels. When part of the colon or rectum is removed to treat cancer, nearby mesentery will also be removed. The lymph nodes within the mesentery are examined to see if they contain cancer cells.
The colon and rectum are made up of many layers of tissues including:
- mucosa which is the inner lining including the epithelium, lamina propria (connective tissue) and muscularis mucosa (thin layer of muscle)
- submucosa which is connective tissue containing glands, blood and lymphatic vessels, and nerves
- muscularis propria, a thick layer of muscle
- serosa which is the outer lining of the colon but not the rectum
The functions of the colon and rectum are to:
- absorb water and some nutrients from what we eat and drink
- form and store stool (waste), and move it out of the body
Partly broken down or digested food moves from the small intestine into the colon. Sections of the colon tighten and relax to move materials through the colon and rectum. This movement is called peristalsis.
Digestion continues in the colon as bacteria break down some materials into smaller pieces. Water and some nutrients are absorbed (taken in) by the inner layer which lines the colon (also called epithelium). The liquid waste remaining in the colon is formed into semi-solid stool (also called feces).
The inner lining of the colon or mucosa also makes mucus so stool can move easily through the colon and rectum. As stool moves to the end of the colon, it becomes more solid.
Stool leaves the colon and moves into the rectum. The rectum is a holding area for the stool. When the rectum is full, a signal is sent to the brain to push the stool out of the body through the anus.