How the bowel and anus work

How the bowel and anus work

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 them their 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
We think one stool a day is healthy, but this is because of the way we have trained ourselves – our bodies are designed to pass faeces more often. Other primates (such as chimps and monkeys) pass soft stools several times a day
Defaecation 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 passing a stool. This is probably why it is even quicker when we are frightened

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 manufacture 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 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 faeces arrive 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 faeces out. The wall of the anal canal wall is very muscular. The muscles keep the anus closed, except when faeces (‘stools’) are passed.

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.

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 lining of the gut is very slimy (so that faeces can pass along easily); the extra seal stops the slime (mucus) from leaking out. The spongy pads can become swollen. When this happens they are called piles (haemorrhoids).