Rectum and anus

Food passes along the gut, the nutrients and fluids are absorbed, and the waste is then expelled through rectum and anus.

The rectal canal and the sphincter muscles around the anus are richly supplied with nerves. When the rectum is full, the nerves sense the fullness and tell the brain whether it's gas or stool in there.

When we're ready to do the toilet, the brain tells the muscles, via the nerves, to relax. As the muscles relax, the anus opens and the rectum empties.

Anal sphincters

The external and internal anal sphincters surround the anal canal. The internal sphincter is a smooth muscle ring at the upper end of the anus. When the anal canal is empty, it is contracted. When it receives something from the gut, it stretches.

We don't consciously control this muscle ring. Its work is done automatically. But we can voluntarily squeeze the external anal sphincter, which is the ring of muscle lower down and around the anus.

The pelvic floor

The pelvic floor is a group of muscles often described as being shaped like a hammock or sling. The muscles are attached to the spine's tailbone or coccyx and continue to the pubic bone.

The outlets of the bladder and colon – urethra and rectum and anal canal – pass through the pelvic floor.

The pelvic floor supports the bladder and bowel (colon) in men, passing just under the prostate. It supports the uterus, bladder and bowel in women.

We can consciously control these muscles through a system of nerves. When we want to tense or relax them we can.

Ordinarily, the pelvic floor muscles are tense, ensuring continence by keeping the outlets closed. When we go to the toilet we relax these muscles.

The levator ani is an important muscle group in the pelvic floor group. It has a lever-type action. In other words, when we want to lift the pelvic floor area the levator ani helps accomplish that action.