Large intestine

As this residue passes through the colon, nearly all of the water is absorbed, leaving a usually soft but formed substance called stool. Muscles in the wall of your colon separate the waste into small segments that are pushed into your lower colon and rectum. As the rectal walls are stretched, they signal the need for a bowel movement.

When the sphincter muscles in your anus relax, the rectal walls contract to increase pressure. Sometimes you have to use your abdominal muscles, which press on the outside of the colon and rectum. These coordinated muscle contractions expel the stool.