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About the bowel

The bowel is the part of the digestive system that goes from the stomach to the back passage (anus). It is a hollow, muscular tube.

Where the bowel is

The bowel is the lower part of the digestive system. The digestive system is also called the gut or gastrointestinal tract (or the GI tract or GIT for short). 

The digestive system processes all the food we eat and breaks it down into nutrients for the body. It also gets rid of any solid waste matter from the body as poo (also called faeces or stools).

The bowel is divided into the small bowel (or small intestine) and the large bowel (or colon and rectum). The small bowel is where you digest and absorb the nutrients from the food you eat. The large bowel absorbs water from the digested food and forms poo (stools).

Diagram showing the parts of the digestive system

Most of the bowel problems caused by cancer are related to the large bowel. It is less common to get problems related to the small bowel.

What affects the bowel

How your bowel works is affected by a number of different things:

Muscles and nerves

The nerves and muscles in your back passage help you have normal bowel movements. So if treatment or illness damages these nerves or muscles, you might have some difficulty passing your poo.

The muscles in your tummy (abdomen) and your intestinal muscles also play a part in moving the poo down into your rectum.

Eating and drinking

You need a certain amount of fluid in your body for your bowel to work properly. If you don’t have enough fluid you can get dehydrated. This can make it difficult to pass poo. So it is important to drink about 2 litres (about 8 tall glasses) of water each day.

What you eat also affects your bowel motions. Eating a diet high in fibre with lots of fresh fruit and vegetables helps to keep your stools soft and regular.

Remember to drink plenty of water if you are eating more fibre. Fibre draws water into the bowel so you could get dehydrated if you don’t drink enough.


A lack of daily exercise can reduce the muscle tone in your abdomen and bowel (intestines). This slows down the movement of stools through the gut. So taking regular exercise helps to keep your bowel working properly.

Ways of increasing daily exercise could include using the stairs rather than the lift, or getting off the bus one or two stops early.


As we get older, the way our bowel works tends to change. Bowel problems are more common in older people. Understandably, elderly people can become less active. They might also eat less fibre and not drink enough water. This can weaken their abdominal and bowel muscles.

Other conditions

Many conditions other than cancer can change the way your bowels work. With the right treatment, your doctor can usually control conditions such as inflammatory bowel disease, piles (haemorrhoids) and infections.

How cancer treatment affects the bowel

Constipation and diarrhoea are common side effects of many drugs used as cancer treatments. Your doctor or nurse will talk to you about the side effects of any new drugs they prescribe for you.

Radiotherapy to the pelvic area or to the back passage (rectum) can cause diarrhoea. This usually comes on shortly after the treatment starts and lasts for a few weeks after the course of treatment ends. Your doctor or nurse might give you medicines to slow down your bowel and try to prevent diarrhoea.

Let your doctor or nurse know if you notice any of these side effects. They will be able to give you advice about how to treat them.

Last reviewed: 
24 May 2017
  • Cancer and its management (7th edition)
    J Tobias and D Hochhauser
    Wiley-Blackwell, 2015

  • Cancer: Principles and practice of oncology (10th edition)
    VT De Vita, S Hellman and SA Rosenberg
    Lippincott, Williams and Wilkins, 2015

  • Symptom management in advanced cancer (4th edition)
    R Twycross, A Wilcock and S Toller
    Radcliffe Medical Press Ltd, 2009

  • The Royal Marsden Hospital Manual of Clinical Nursing Procedures (9th Edition)
    Editors: Lisa Dougherty and Sara Lister
    Wiley-Blackwell, 2015

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