Normal Bowel Function
Most adults take bowel control for granted and need to give it little thought except for the few minutes a day that are spent emptying the bowel on the toilet. However, bowel control is actually a complex and incompletely understood process, involving delicate co-ordination of many different nerves and muscles.
The bowel is part of the digestive system and its' role is to digest the food that we eat, absorb the goodness and nutrients from the digested food into the blood stream and then to process and expel the waste products from the food that the body cannot use (See 'Normal bowel function leaflet' page 3) /patient-information-leaflets
The small bowel or small intestines is the part of the bowel where the useful parts of food are absorbed. the small bowel delivers 500-1000mls (1-2pints) of waste to the colon per day. The colon or large bowel is the waste processing part of the system. This wast eis the consistency of thick pea soup when it enters the beginning of the colon. It is the job of the colon to absorb fluid from this waste and, as it moves around the colon. to gradually form it into stools (also called faeces or bowel motions). Stool consistency can vary between hard lumps to very loose or mushy, often depending on how long the stools have been in the colon and how much water has been absorbed from them. Ideally stools should be formed into smooth sausage-shapes, which are comfortable to pass.
The left side of the colon and the rectum are the "storage tank" at the end of the large bowel. Normally the rectum is relatively empty. Stool does not enter the rectum from the colon on a continuous basis, but as a result of mass movements, which happen from time to time, especially before the need to go to the toilet is experienced. These mass movements are major waves of pressure, which can move stool through the whole length of the colon, like toothpaste being squeezed along the tube. Mass movements picture. Often a large part of the contents of the colon arrives in the rectum at once. (See 'Normal bowel function leaflet' page 6) /patient-information-leaflets
These mass movements are often triggered by the so called gastrio-colic response. Food arriving in the stomach when you eat a meal sets off a pressure wave in the colon some minutes later. this can lead to the need to empty the bowel, sometimes urgently, soon after eating. For many people the bowel is relatively quiet at night. The first meal of the day, together with physical activity involved in getting in and out of bed and washing and dressing, stimulates contractions in the colon and mass movements. This leads to a "call to stool", the feeling that the bowel needs emptying, shortly after breakfast.
Food usually takes an average of 1-3 days to be processed and up to 90% of the time is spent in the colon.
HOW OFTEN SHOULD I EMPTY THE BOWEL
There is no right or wrong answer to this. There is a very wide range of 'normal' bowel function between different people. It is by no means essential to have one bowel action per day, and indeed it is probably a minority of the total population who has this. Some people always go several times per day; others have several days between bowel actions. As long as stools are passes without excessive urgency (needing to rush to the toilet) with minimal effort and no straining, and without the use of laxatives, bowel function may be regarded as normal.
Perception of what is normal is based on personal experiences and growing up with other people. Most of us do not discuss bowel habit with our friends, or even our family. A few people become obsessed with the need for a daily bowel action and spend excessive amounts of time in the toilet or take laxatives to achieve this. Often this is unnecessary.
HOW IS BOWEL EMPTYING CONTROLLED
There are two rings of muscle around the anus or exit from the bowel. These two rings of muscle form the anal sphincter and are designed to hold in the bowel contents at all times except when you are sitting on the toilet and trying to empty the bowel.
The internal anal sphincter is an internal muscle responsible for keeping the anal canal closed at all times except when there is an urge to empty the bowel. You do not have to think about keeping this muscle closed, it happens automatically.
The external anal sphincter is the muscle that you use to hold on when the rectum is full and you feel that you need to empty the bowel.
When stool enters the rectum the internal anal sphincter muscle automatically relaxes and opens up the top of the anal canal. This is normal and allows stool to enter the upper anal canal to be 'sampled' by the very sensitive nerve cells in the upper anal canal. (See leaflet 'Normal bowel function' page 8) /patient-information-leaflets. People with normal sensation can easily tell the difference between wind (gas, also called flatus), which can safely be passed if it si socially convenient without fear of soiling, diarrhoea (very loose or runny stools needing urgent attention and access to a toilet) and a normal stool. Most people just know what it is in the rectum without really having to think about it.
FIGURE 3 ANAL SPHINCTER MUSCLES
Around the internal anal sphincter is the external anal sphincter, which is much thicker. This is the muscle around the anus that you can deliberately squeeze. Just like the muscles in the arm or leg, a person can decide when to use this muscle.
If a normal stool is sensed and it is not convenient to find a toilet at that moment, bowel emptying is delayed by squeezing the external anal sphincter. Squeezing the external anal sphincter ensures that the stool is not simply expelled as soon as it enters the rectum, and in fact the stool is pushed back up and out of the anal canal (See leaflet 'Normal bowel function' The large bowel page 4) /patient-information-leaflets. For most people this is not a deliberate action - you should not need to think - "I must squeeze my anal sphincter muscles so that I do not have a bowel accident" - but this is actually what you do, subconsciously without really thinking about it.
This external anal sphincter does not need to last all the time until the toilet is found. Stool is propelled back into the rectum, and the rectum relaxes and so the urge to empty the bowel is reisited and wears off.
For most people, an urge to empty the bowel is felt, but if the time and place are not right. it is possible to delay bowel emptying. and the feeling of needing to go wears off very soon. Most people can forget about the bowel for a while, and some can put off bowel emptying almost indefinitely, but may get reminders that the bowel is full at intervals until it is emptied. Continually resisting the urge to empty the bowel or ignoring the call to stool can lead to constipation, as the longer the stools stay in the colon and rectum, the more the fluid is absorbed and the harder the stools become.
For this mechanism to work properly you need several things:
- the nerves of the rectum and anus need to be sending the right messages to your brain so that you can feel when stool or gas arrives in the rectum and can send messages to the muscles that you want to hold on;
- the internal and external anal sphincters need to be undamaged and working properly;
- the stools should not be too soft or loose so that the sphincters can cope with holding on, but not so hard that they are difficult to pass;
- and you need the physcial ability to get to and onto the toilet and to hold on until the correct place is reached.
As you can imagine this is a delicate system and unfortuantely there are many things that can go wrong with it.
Related products
