The colon and rectum

The colon and rectum make up the large bowel, which is part of our digestive system. The colon is where water is absorbed as part of the digestive process and the rectum is where waste matter (stools or faeces) is held until it's ready to be passed.

About two-thirds of cancers that develop in the large bowel occur in the colon and one-third are in the rectum.

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.

The large bowel – 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. After the colon, 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).

Close to the bowel are lymph nodes, also known as lymph glands, which are about the size of a baked bean. The lymph nodes are part of the lymphatic system. 

 

The colon

The colon is divided into four sections:

1. 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.
2. Transverse colon  The second section goes across the abdomen from right to left. 
3. Descending colon  The third section goes down the left hand side of the abdomen. 
4. Sigmoid colon  The final part of the colon is an ‘s’ shape bend which 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 long.

To help describe the position of a cancer within the rectum it is 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.