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The bowel

Men and women discussing bowel cancer

This page tells you about the bowel. You can find the following information

 

A quick guide to what's on this page

The bowel

The bowel is part of the digestive system. This is called the gastrointestinal tract or GI tract for short. The digestive system processes all the food we eat and turns it into energy for the body to use.

The bowel is divided into the small bowel (small intestine) and the large bowel (colon and rectum). Food passes down the food pipe (oesophagus) into the stomach. The food is digested and passes into the small bowel. Here the body absorbs nutrients from the food. The food then passes through the large bowel, which absorbs water and forms the waste matter into stool. The stool is stored in the back passage (rectum) until it is ready to be passed out of the body.

The small bowel

The small bowel is actually the longest part of the bowel. It is called small because it is narrower than the large bowel. Cancer of the small bowel is rare.

The large bowel

The large bowel is made up of the colon and rectum. It has walls made of several layers. Bowel cancers start in the innermost layer – the lining of the bowel. If left untreated, it can grow into the muscle layers underneath, and then through the bowel wall. Most bowel (colorectal) cancers take 5 to 10 years to develop. Most begin as a small growth on the bowel wall called a polyp or adenoma.

 

CR PDF Icon You can view and print the quick guides for all the pages in the about bowel cancer section.

 

 

How your bowel works

The bowel is part of the digestive system. The digestive system processes all the food we eat and turns it into energy for the body to use. It also gets rid of any solid waste matter from the body. The bowel is divided into the

 

The small bowel

The small bowel is actually the longest part of the bowel – about 20 feet long. The large bowel is about 5 feet long. The small bowel is called small because it is much narrower than the large bowel. When people talk about bowel cancer, they nearly always mean cancer of the large bowel. Cancer of the small bowel does happen, but it is much rarer.

Diagram showing the position of the small bowel

After you have swallowed your food, it passes down the food pipe (oesophagus) into your stomach. Digestion begins in the stomach and then the food passes into the small bowel, where digestion continues and the body absorbs nutrients from the food. The digested food then moves into the large bowel.

 

The large bowel

The large bowel absorbs water as the digested food passes through it, and the waste matter left behind forms into stool. The back passage (rectum) stores the stool until you are ready to pass it out of the body as a bowel motion.

The large bowel has 5 sections. Cancer can develop in any of these. They are the

  • Ascending colon – runs from the appendix at the end of the small bowel up the right side of the abdomen, with the first part of the ascending colon known as the caecum
  • Transverse colon – runs across the body from right to left, under the stomach
  • Descending colon – runs down the left side of the abdomen
  • Sigmoid colon – an 'S' shaped bend that joins the descending colon to the back passage
  • Rectum – the back passage which joins the bowel to its opening on the outside, the anus

Diagram showing the parts of the large bowel

The bowel walls are made up of several layers of body tissues. Bowel cancers start in the innermost layer – the lining of the bowel. Doctors think most bowel (colorectal) cancers take 5 to 10 years to develop. Most begin as a small growth on the bowel wall called a polyp or adenoma. If left untreated, they may become cancerous and grow into the muscle layers under the lining of the bowel and then through the bowel wall. The cancer can then spread into organs that are close to the bowel, such as the bladder, womb or prostate gland.

 

If bowel cancer spreads

Sometimes bowel cancer spreads to another part of the body through the lymphatic system. The lymphatic system is made up of lymph nodes, which are part of the body’s immune system. One of the first places bowel cancer spreads is to the lymph nodes in the abdomen.

Bowel cancer can also spread to other parts of the body through the bloodstream. The liver is a common place for colorectal cancer to spread because blood flows directly from the bowel to the liver (shown in the diagram below).

Diagram showing how the blood and lymph flow between the liver and bowel

 

More information

There are books and booklets about bowel (colorectal) cancer, some of which are free. You can find details on the bowel cancer reading list.

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Updated: 6 August 2013