What is the bowel?

The digestive system

The ​digestive system​ system is also called the gut or gastrointestinal tract. Sometimes it’s written as GI tract or GIT for short.

The bowel goes from the stomach to the back passage. The back passage is also known as the anus. It is a hollow muscular tube.

The bowel processes all the food we eat and breaks it down into nutrients for the body to use. The bowel also gets rid of any solid waste matter from the body as poo. Other words for poo include faeces and stool.

The small bowel is also known as the small intestine. The large bowel is also known as the large intestine.  

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)

The small bowel

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

• duodenum pronounced dew-oh-dean-um. This is the top part of the small bowel, which connects to the stomach

• jejunum pronounced jej-you-num. This is the middle part of the small bowel

• ileum pronounced ill-ee-um. This is the lower part, which connects to the large 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 fluid each day. This means aiming to drink 6 to 8 glasses of fluid each day.

Read more about diet and cancer

Physical activity

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

Read tips on how to be more active

Getting older

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.

Medicine

Several medicines to treat a range of conditions can cause bowel problems. Examples of these include:

• types of painkillers called ​opioids​ such as morphine

• iron replacement medicine such as ferrous sulphate

• antibiotics such as co-amoxiclav or ciprofloxacin

• anti sickness such as ondansetron or metoclopramide 

Other conditions

Conditions other than cancer can change the way your bowels work can include:

• inflammatory bowel disease such as ​Crohn’s disease​ or ​ulcerative colitis​

• piles (haemorrhoids) 

• 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. 

Cancer medicines

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

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 healthcare team they might give you medicines to slow down your bowel and try to prevent diarrhoea. 

Surgery 

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.

Read about different types of bowel problems in people with cancer

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.