How the bowels and anus work
To understand constipation, it helps to know how the lower part of the gut works.
The muscles of the colon gently contract and relax all the time, rolling the waste matter about like clothes in a washing machine and pushing the faeces towards the rectum, which is the last section of the bowel before the anus (back passage). Several times a day, usually after meals, the colon makes some big muscular contractions to dump the faecal material in the rectum beyond it. By the time the faeces reach the rectum they are almost solid because most of the water has been absorbed. The hard and dry faeces of constipation occur when the colon absorbs too much water. This happens because its muscle contractions are slow or sluggish, causing the faeces to move through it more slowly.
The rectum and anal canal
- The innermost ring of muscle of the anal canal is called the ‘internal sphincter’. This muscle, which is not under our conscious control, maintains a resting pressure in our anal canal.
- The outermost ring of muscle of the anus is called the ‘external sphincter’. It also forms the pelvic floor eventually. This muscle is more like the sort of muscle that we have in our arms and legs, and we are able to control it (until the urge to pass faeces becomes overwhelming).
It is obvious that in babies the system of nerve messages that keep the anus shut is not in place – babies pass faeces as soon as the rectum fills. After about 18 months of age, the system develops, but in some children this can take a long time.
Written by: Mr Andrew Ramwell
Edited by: Mr Andrew Ramwell
Last updated: Thursday, November 7th 2013
Useful contacts for How the bowels and anus work
Click to see all the contacts that you may find useful in relation to constipation | How the bowels and anus work
535 people have
tackled this problem!
Tell us your thoughts
Did you find what you were looking for?
Add a comment
A problem shared is a problem halved: help others by sharing your frustrations or successes at tackling your health problem.
We have noticed that many of your queries are actually answered on the website, so please read carefully before submitting a comment. As all comments are moderated, there will be a delay before your comment appears. Please note that we cannot respond to individual requests for feedback.
How the bowels and anus work
- Causes of constipation
- Improving constipation – what you can do
- Constipation in children
- Constipation - when to see your doctor
- Are you constipated?
- Anal bleeding
- Anal itching
- Anal pain
- Irritable bowel syndrome
- Seeing your doctor about an anal problem
- Doc Spot – Irritable bowel syndrome
- Doc Spot – Laxatives
- Doc Spot – Strange faeces
- Dr Phil – Anal itching (video)
- Dr Phil – Ask me, I'm a doctor
- Dr Phil – Bowel cancer (video)
- Dr Phil – Cheeky bowel syndrome (poem)
- Dr Phil – Pile driver (poem)
An average person on a typical Western diet passes about 150 g (5 oz) of faeces each day. Faeces consist of about one-third solids and two-thirds water
The solid matter in faeces is cellulose from vegetables, dead cells cast off from the lining of the gut, bacteria, some salts and pigment from bile (which gives the brown colour)
Most of the waste matter from food is passed out in the faeces within 72 hours, but in healthy people up to 30% may remain in the colon for a week or more
Defecation is a very efficient process, normally taking only 10 seconds. Presumably it has to be quick - animals (and primitive man) cannot run from a predator easily if they are in the middle of defecating. This is probably why it is even quicker when we are frightened
In the USA, more than $800 million is spent on laxatives each year
Constipation results in more than 2–5 million visits to doctors in the USA each year
About 4,500,000 people in the USA say they are constipated most or all of the time (National Health Interview Survey)
In a UK survey, 6% of people said they had suffered from constipation in the past year, 13% had some difficulty in passing their faeces at least once a month, and 19% took laxatives at some time