How the bowels and anus work
To understand constipation, it helps to know how the lower part of the gut works.
The colon (large bowel) is the lower part of the gut. It is more than 1 metre long. Its job is to store faecal material and remove fluid from it, so that faeces are fairly solid and the body does not waste water. The colon may absorb 1 litre of fluid a day. The colon contains lots of helpful bacteria that break down food residues (turning some of them into wind) and manufacturing some vitamins. The muscles of the colon gently contract and relax all the time, rolling the waste matter about like clothes in a washing machine. Several times a day, usually after meals, the colon makes some big muscular contractions to dump the faecal material in the rectum beyond it.
- The ring of muscle at the top of the anal canal is called the ‘internal sphincter’. This muscle is not under our conscious control.
- The ring at the opening of the anus is called the ‘external sphincter’. 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.
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 too slowly.
Written by: Dr Margaret Stearn
Edited by: Dr Margaret Stearn
Last updated: Tuesday, April 5th 2011
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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