The human body

How do bowel movements work?

During a bowel movement, the indigestible parts of our food leave our body. Several parts of the end of the gastrointestinal tract control bowel movements: the rectum and anal canal, the sphincters (circular muscles), and hemorrhoids. These are embedded in the pelvic floor muscles.

Illustration: The lower end of the gastrointestinal tract - as described in the articleThe lower end of the gastrointestinal tract

Rectum and anal canal

The rectum, which is about 20 centimeters long, is the final section of the large intestine (colon). This is where the indigestible parts of our food collect until the bowel is emptied. The anal canal, situated between the rectum and anus, is closed off by sphincter muscles and hemorrhoids. It is between three and six centimeters long, and has a mucous membrane lining (anoderm). This lining contains a lot of nerves that send signals to help the brain do things like recognize the consistency of stool and how full the bowel is. These nerves also help you know when your bowel needs to be emptied.

Anal sphincters

There is an inner (internal) and outer (external) sphincter at the end of the anal canal. The inner sphincter makes sure that the rectum does not open when you are sleeping, for example. It is ring-like in shape and made up of smooth muscle.

The outer sphincter supports the inner sphincter, helping it perform its function. Unlike the inner sphincter, the outer sphincter can be controlled voluntarily. This makes it possible to empty your bowels or hold back your stool when you want to. But the outer sphincter can also act involuntarily, for instance to help or prevent the passage of stool.

Hemorrhoids

Hemorrhoids are “cushions” of tissue filled with blood vessels, found at the end of the rectum, just inside the anus. Together with the anal sphincters, they close off the bowel, preventing stool from leaving the body. Enlarged hemorrhoids can cause unpleasant symptoms. This is what most people mean when they talk about having hemorrhoids, also known as “piles.”

Pelvic floor

The pelvic floor is made up of muscles, ligaments and connective tissue. It physically supports the pelvic organs, and closes off the lower part of the pelvic cavity. Openings in this strong sheet of muscular tissue allow the rectum, urethra and vagina to pass through. The pelvic floor relaxes when you have a bowel movement, and contracts (squeezes) when you hold stool back.

What makes you feel the urge to “go”?

When the rectum fills up, stretch receptors in the wall of the anus are activated. Signals are sent along nerves to the part of the brain known as the cerebrum. There they are processed, and signals are sent back to the lining of the anus. The nerve impulse causes the inner sphincter to open, making you feel like you need to empty your bowels. The pelvic floor relaxes and drops down a little, allowing the muscles in the rectum to push the stool out.

If you follow the urge to empty your bowels, and go to the toilet, you voluntarily relax your outer sphincter muscle as well. But you can hold back stool for some time by tensing the muscles in your pelvic floor and abdomen, as well as your outer anal sphincter. After a bowel movement, the inner sphincter closes again too and you no longer feel the urge to go to the toilet for a while.