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What is bowel cancer

Men and women discussing bowel cancer

Bowel cancer means cancer that starts in the colon (large bowel) or rectum (back passage). It is also known as colorectal cancer.

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A quick guide to what's on this page

What is bowel cancer

Bowel cancer means cancer that starts in the colon (large bowel) or rectum (back passage). It is also known as colorectal cancer.

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.

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. Then the food waste (poo) passes into the large bowel (colon and rectum). 

The colon (large bowel)

The colon is the first part of the large bowel, which absorbs water and forms the waste matter into stool (poo). From the colon the poo passes into the rectum. 

The rectum (back passage)

The rectum stores poo until it is ready to be passed out of the body.

How bowel cancer grows

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

 

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.

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.

 

Colon cancer

The colon is the first part of the large bowel. It absorbs water as the digested food passes through it, and the waste matter left behind forms into stool (poo). The large bowel is about 5 feet long and has 4 sections. Cancer can develop in any of these. 

The parts of the colon are the

  • Ascending colon – runs up the right side of the abdomen. It is connected to the small intestine by a section of bowel called 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

Diagram showing the parts of the large bowel

 

Rectal cancer

Rectal cancer starts in the last part of the large bowel (the back passage). This part of the bowel stores poo (stool) until it is ready to be passed out of the body. 

Rectal cancer is also called cancer of the rectum.

 

How bowel cancer grows

The bowel walls are made up of several layers of body tissues. Bowel cancers start in the innermost layer – the lining. Most begin as a small growth 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.

Doctors think most bowel (colorectal) cancers take 5 to 10 years or more to develop. 

 

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 tummy (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

 

Related information

Read about

Treatment for bowel cancer

Diagnosing bowel cancer

Living with bowel cancer

Small bowel cancer

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: 11 August 2015