How the bowel works

The digestive system

Food is moved through the different sections of the digestive tract by a process called peristalsis, which is a series of wave-like muscle contractions. It usually takes between 24 and 72 hours for food to move through the digestive system. The sections are separated by bands of muscles, known as sphincters, which are valves that control the movement of food from one area of the digestive tract to another. It's important that the food stays in each of the sections long enough for the gut to absorb fluid and nutrients before expelling it as waste.

The small intestine (small bowel)

The small bowel is about 6-8m long and 2cm wide and is comprised of three parts:

  • duodenum
  • jejunum
  • ileum.

Food passes from the stomach through each of these three parts. The purpose of the small intestine is to absorb nutrients and much of the fluid from foods. As food moves from the small intestine to the colon it has a porridge-like consistency.

The colon (large intestine)

The colon, or large intestine (also known as the large bowel), starts at the final portion of the small intestine and ends at the rectum. The colon is about 2m long and 6-7cm wide. It is made up of the caecum, ascending, transverse, descending and sigmoid colon.

The colon is host to a countless number of micro-organisms that support the processing and elimination of waste. It can take between 12 and 48 hours for food to make its way through the colon.

The rectum and anus

Once food has passed through the bowel the waste moves to the rectum which stretches, triggering a message to the brain to say that the rectum is full and needs to be emptied. The pelvic floor muscles ensure that the anus remains closed until the person is ready to open their bowels.

The nerves can usually tell the brain whether it’s wind or stool that is filling the rectum.

In some neurological and spinal conditions the brain cannot tell whether the bowel is full of waste (faeces) or just wind. This can lead to accidental leakage.

For the bowel to function properly it needs:

  • the nerves of the rectum and anus to be sending the correct messages to the brain so that it can sense when stool or wind arrives in the rectum and can transmit messages to the muscles to hold it in
  • the internal and external anal sphincters to be working
  • stools which are not too soft or too hard

Normal bowel function

The frequency and consistency of bowel movements will vary from person to person. It averages between three times a day to three times a week, with the stool being soft and easy to pass.

Stool consistency is usually measured using the Bristol Stool Chart and ideally should be between 3 and 4.

A good position on the toilet is when knees are higher than hips (unless recent hip surgery), leaning slightly forward and with the elbows on knees, relax and breath easily and do not strain.

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