The colon and rectum

 

Once food has been swallowed, it passes down the gullet (oesophagus) to the stomach, where digestion begins. It then passes through the small bowel, where essential nutrients are taken into the body.

The digested food then moves into the colon, where water is absorbed. The remaining waste matter, known as stools or faeces, is held in the rectum (back passage) until it’s ready to be passed from the body through the anus as a bowel motion (stool).

The colon

The colon is divided into four sections:

Ascending colon

The first part of the colon starts at the bottom, right-hand side of the abdomen just after the small bowel and the appendix. This part goes up the right side of the abdomen.

Transverse colon

The second section goes across the abdomen from right to left.

Descending colon

The third section goes down the left-hand side of the abdomen.

Sigmoid colon

The final part of the colon is an ‘S’ shape bend that joins onto the rectum.

The rectum

The rectum is the last part of the large bowel before it opens to the outside at the anus. It is approximately 15cm (6 inches) long.

To help describe the position of a cancer within the rectum, it’s sometimes divided into three sections: upper, middle and lower.

The upper rectum is the section directly after the sigmoid colon, and the lower rectum is where the large bowel joins the anus. 

The walls of the colon and rectum are made up of layers of body tissue. Most colon and rectal cancers start in the innermost lining of the bowel and develop from small growths called polyps.