The bowel is part of the digestive system. It is divided into two parts:
- the small bowel
- the large bowel.
The large bowel is made up of the colon, rectum and anus.
Diagram of the digestive system
When you swallow food, it passes down the gullet (oesophagus) to the stomach. This is where digestion begins.
The food then enters the small bowel, where nutrients and minerals from food are absorbed. The digested food then moves into the colon. This is where water is absorbed.
The remaining waste matter (poo) is held in the rectum (back passage). Nerves and muscles in the rectum help to hold on to poo until you are ready to pass it through the anus.
The anus is the opening at the end of the large bowel. It contains a ring of muscle called the sphincter. This muscle helps to control when you empty your bowels.
Diagram of the digestive system
The small bowel is part of the digestive system. It is between the stomach and the large bowel (colon). The small bowel is between 4 and 6 metres long. It folds many times to fit inside the tummy (abdomen). It breaks down food, allowing vitamins, minerals and nutrients to be absorbed into the body.
The small bowel is made up of three main parts:
- the duodenum – the top section that is joined to the stomach
- the jejunum – the middle section
- the ileum – the lower section that is joined to the large bowel.
Around half of all small bowel cancers start in the duodenum.
Diagram of the position and sections of the small bowel
The colon is divided into 4 sections.
The final part of the colon is an ‘S’ shape bend that joins on to the rectum.
The lining of the colon is made up of layers of body tissue. Most colon cancers start in the inner lining of the bowel and develop from small growths called polyps.
To help describe where a cancer is, doctors divide the rectum into three: the upper, middle and lower third.
The upper third is the section directly after the sigmoid colon. The lower third is where the large bowel joins the anus. The middle third is in between.
The lining of the rectum is made up of layers of body tissue. Most rectal cancers start in the inner lining of the bowel and develop from small growths called polyps.
Below is a sample of the sources used in our bowel cancer information. If you would like more information about the sources we use, please contact us at email@example.com
R Glynne-Jones, PJ Nilson, C Aschele et al. ESMO-ESSO-ESTRO Clinical practice guidelines for diagnosis, treatment and follow up for anal cancer. July 2014. European Society of Medical Oncology. Available from www.esmo.org/Guidelines/Gastrointestinal-Cancers/Anal-Cancer (accessed October 2019).
National Institute for Health and Excellence (NICE). Colorectal cancer: diagnosis and management clinical guidelines. Updated December 2014. Available from www.nice.org.uk/guidance/cg131 (accessed October 2019).
Association of Coloproctology of Great Britain & Ireland (ACPGBI). Volume 19. Issue S1. Guidelines for the management of cancer of the colon, rectum and anus. 2017. Available from www.onlinelibrary.wiley.com/toc/14631318/19/S1 (accessed October 2019).
BMJ. Best practice colorectal cancer. Updated 2018. Available from www.bestpractice.bmj.com/topics/en-gb/258 (accessed October 2019).
This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Professor Tim Iveson, Consultant Medical Oncologist.
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