About the bowel

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 bowel goes from the stomach to the back passage (anus). It is a hollow muscular tube. It processes all the food we eat and breaks it down into nutrients for the body to use. 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 (colon and rectum). 

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.

About the large bowel and small bowel

The large bowel

This includes the:

  • colon
  • back passage (rectum)
  • bowel opening (anus)

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 the bowel called the caecum
  • the transverse colon - runs across the body from right to left, under the stomach
  • the descending colon - runs down the left side of the abdomen
  • the sigmoid colon - an 'S' shaped bend that joins the descending colon to the rectum (back passage)
Diagram showing the parts of the large bowel

The small bowel

The small bowel is made up of 3 parts, these are the:

  • duodenum (dew-oh-dean-um) - the top part of the small bowel, which connects to the stomach
  • jejunum (jej-you-num) - the middle part
  • ileum (ill-ee-um) - the lower part, which connects to the large bowel (colon)
Diagram showing the parts of the small bowel

What is a normal bowel habit?

How often you have a bowel movement can vary from person to person. Normal for some people can mean having a poo a few times a day, for others it might mean having a poo a few times a week. 

You can usually think of your bowel habit as normal if you:

  • have regular bowel movements (not necessarily once a day)
  • don’t have ongoing symptoms of constipation or diarrhoea
  • can have a bowel movement without straining or using laxatives

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

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

You need a certain amount of fluid in your body for your bowel to work properly. Fibre draws water into the bowel so you could get dehydrated if you don’t drink enough. This can make it difficult to pass poo. So it is important to drink plenty of water (about 6 to 8 glasses) each day.


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


As we get older, the way our bowels work tends to change. As we age we can become less active and our diet may change. This can have an impact on bowel function.

Tablets and medication 

Several tablets or medicines to treat a range of conditions can cause bowel problems. These include pain relief tablets, iron tablets, and antibiotics.

Other conditions

Conditions other than cancer can change the way your bowels work. This can include inflammatory bowel disease, piles (haemorrhoids) and infections. With the right treatment, your doctor can usually help control these conditions. 

How cancer treatment affects the bowel

Before you start your treatment, your doctor will explain the possible side effects and how to manage them. 

Drug treatment 

Constipation and diarrhoea are common side effects of many drugs used as cancer treatments including chemotherapy, immunotherapy and targeted cancer drugs. 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 can last for a few weeks or longer after your treatment ends. For some people, it might start after you finish your radiotherapy treatment.

Speak to your doctor, nurse, or radiographer they might give you medicines to slow down your bowel and try to prevent diarrhoea. 


You might have constipation or diarrhoea after surgery. It may be due to the surgery you have. Or it may be because you are less active afterward or your diet has changed.

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

  • Ross and Wilson Anatomy and Physiology in Health and Illness (14th edition)    
    A Waugh and A Grant
    Elsevier, 2022

  • Diarrhoea in adult cancer patients: ESMO Clinical Practice Guidelines

    P Bossi and others

    Annals of Oncology 2018, Volume 29, Supplement 4

  • The Royal Marsden Manual of Clinical Nursing Procedures (10th edition)
    S Lister, J Hofland and H Grafton 
    Wiley Blackwell, 2020

  • Gastrointestinal Tract Disorders in Older Age

    I Dumic and others

    Canadian Journal of Gastroenterology and Hepatology. 2019

  • Managing Constipation in Adults With Cancer

    R Wickham

    Journal of Advanced Practitioner in Oncology, 2017. Volume 8, Issue 2, Pages 149-161.

Last reviewed: 
09 Dec 2022
Next review due: 
09 Dec 2025

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