The colon and rectum
The bowel is part of the digestive system. It’s divided into two parts: the small bowel and the large bowel. The large bowel is made up of the colon, rectum and anus.
When food has been swallowed, it passes down the gullet (oesophagus) to the stomach, where digestion begins.
From here, it enters 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 (stool or faeces) is held in the rectum (back passage). Nerves and muscles in the rectum help you to hold onto stools until you’re ready to pass them through the anus as a bowel motion.
The anus is the opening at the very end of the large bowel. It contains a ring of muscle called the sphincter. This opens and closes giving you control over when you pass bowel movements.
The colon is divided into four sections. These are labeled on the diagram above.
The first part of the colon starts at the bottom, right-hand side of your abdomen. It comes just after the small bowel. It goes up the right side of the abdomen.
The second section goes across the abdomen from your right to your left side.
The third section goes down the left-hand side of your abdomen.
The final part of the colon is an ‘S’ shape bend that joins onto the rectum.
The walls of the colon are made up of layers of body tissue. Most colon cancers start in the inner lining of the bowel and develop from small growths called polyps.
The rectum links the colon to the anus. It is about 15cm (6 inches) long.
To help describe where a cancer is, doctors divide the rectum into thirds: upper, middle and lower. The upper third is the section directly after the sigmoid colon (see diagram above). The lower third is where the large bowel joins the anus. The middle third is in between.
Most rectal cancers start in the inner lining of the bowel and develop from small growths called polyps.