To understand constipation, it helps to know how the lower part of the gut works.

The Colon

The colon (large bowel) is the lower part of the gut. It is more than 1 metre long. As food moves through the colon it absorbs water and forms waste products (poo). This ensures that the poo is 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 and pushing the poo 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 poo reaches the rectum they are almost solid because most of the water has been absorbed. The hard and dry poo of constipation occur when the colon absorbs too much water. This happens because its muscle contractions are slow or sluggish, causing the poo to move through it more slowly.

The Rectum and Anal Canal

The large bowel (colon) leads into the last part of the gut, which is called the rectum. It is about 12–15 cm long. The final 3 cm of the gut is called the anal canal.

When poo arrives in the rectum, it sends a message to the nerve centres in the spinal cord, and these send a message to the sphincter muscles of the anal canal, making them relax to open the anus. If it is inconvenient for us to have our bowels open, the brain sends a message to the spinal cord to prevent the ‘open anus’ message being sent. We are not aware of this until the rectum becomes very full, when we have to make a conscious effort to keep the anus closed.

When we allow the anus to open, the muscles in the wall of the large bowel and rectum contract to push the poo out. The wall of the anal canal is very muscular. The muscles keep the anus closed, except when poo are passed.

  • 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 have a poo becomes overwhelming).

In babies the system of nerve messages that keep the anus shut is not in place – babies have a poo as soon as the rectum fills. After about 18 months of age, the system develops, and most children have control over their bowels by the age of 4.
A network of small veins lies under the lining of the anal canal. These veins form a soft, spongy pad that acts as an extra seal to keep the canal closed until you go to the lavatory. The spongy pads can become swollen. When this happens they are called piles (haemorrhoids).

Mucus Secretion

The lining of the gut is very slimy (so that poo can pass along easily); the extra seal stops the slime (mucus) from leaking out. Passage of mucus or slime from the bowel can be a normal phenomenon, but the amount varies between individuals and is increased in certain conditions. The commonest reason for increased mucus is irritable bowel syndrome but it can also be increased in inflammatory bowel disease (ulcerative colitis and Crohn’s disease) as well as some bowel cancers.


First published on:
Reviewed and edited by: Dr Kevin Barrett
Last updated: October 2020

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