About the The Rectum and The Anus

From the colon, the stool moves into the rectum. Once or twice a day, stool comes in from the colon, and the rectum stretches to store it. As the amount of stool in the rectum increases, the person feels the urge to go to the bathroom.

In a normal resting state, to keep stool in, the internal anal sphincter is contracted and the puborectalis muscle creates an angle in the rectum. As stool fills the rectum, the internal sphincter relaxes, but a person can squeeze the external sphincter to hold stool in. Squeezing also causes the puborectalis muscle to contract, making the rectum more angled, which also helps to keep stool in.

While having a bowel movement, the rectum contracts, stool is pushed through an opening called the anus, and leaves the body. In order for this to happen, several muscles need to work in a coordinated way. Muscles in the abdominal wall contract, which increases pressure in the rectum and helps to push stool down. A person uses their pelvic oor muscles and the anal sphincter to control when stools are pushed out. The puborectalis muscle is a loop of muscle that wraps around the lower rectum. This muscle relaxes and allows the rectum to straighten. The anal sphincter and pelvic floor muscles also relax. This all happens at the same time to allow stool to pass.