The colon and rectum

The large intestine is made up of the cecum, colon, rectum and anus. The colon and rectum are held in the abdomen by folds of tissue called mesenteries.

The cecum is a pouch-like passage that connects the colon to the ileum (the last part of the small intestine). If cancer develops in the cecum, it is treated like colon cancer.

The colon is the longest part of the large intestine. It receives almost completely digested food from the cecum, absorbs water and nutrients, and passes waste (stool or feces) to the rectum. The colon is divided into 4 parts:

• The ascending colon is the start of the colon. It is 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. It 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. It is on the left side of the abdomen.
• The sigmoid colon is the last part of the colon and connects to the rectum.

The proximal colon is the ascending colon and the transverse colon together. The distal colon is the descending colon and the sigmoid colon together.

The rectum is the lower part of the large intestine that connects to the sigmoid colon. It is about 15 cm (6 in) long. It receives waste from the colon and stores it until it passes out of the body through the anus.

Mesentery is made of fatty connective tissue that contains blood vessels, nerves, lymph nodes and lymph vessels. The mesocolon is a mesentery that attaches the colon to the wall of the abdomen. The rectum is surrounded by a mesentery called the mesorectum.

When part of the colon or rectum is removed to treat cancer, nearby mesentery is also removed. The lymph nodes within the mesentery are examined to see if they contain cancer cells.

The colon and rectum are made up of the following layers of tissues.

The mucosa is the inner lining of the colon and rectum. It is made up of:

• a thin layer of epithelial cells (called the epithelium)
• a layer of connective tissue (called the lamina propria)
• a thin layer of muscle (called the muscularis mucosa)

The submucosa is a layer of connective tissue that surrounds the mucosa. It contains mucous glands, blood vessels, lymph vessels and nerves.

The muscularis propria lies outside the submucosa. It is a thick layer of muscle. It has an inner ring of circular muscle fibres and an outer ring of long muscle fibres that surround the wall of the colon and rectum.

The colon and rectum:

• absorb water and some nutrients from what we eat and drink
• form and store stool
• move waste 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 the food through the colon and rectum. This movement is called peristalsis.

In the colon, bacteria break down food into smaller pieces. The inner layer of the mucosa (called the epithelium) absorbs, or takes up, water and some nutrients. The liquid waste remaining in the colon is formed into semi-solid stool.

The mucosa also makes mucus that helps stool move easily through the colon and rectum. As stool moves through the colon, more water is absorbed from it and 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, it pushes the stool out of the body through the anus.