Two sphincters, two kinds of control
The anal canal is guarded by two ring-shaped muscles. The internal anal sphincter is made of smooth muscle—the kind that also works in places such as the stomach and blood vessels without conscious commands. It stays gently contracted most of the time, helping keep the canal closed. The external sphincter is skeletal muscle, which you can deliberately tighten.
How the body creates a pause
As stool enters and stretches the rectum, nerves trigger a reflex that briefly relaxes the internal sphincter. This response helps the body sense what has arrived and contributes to the urge to go. It does not mean the sphincter simply swings open and empties the rectum: the external sphincter and surrounding pelvic-floor muscles can remain contracted, holding stool in while you wait for a suitable moment.
When you choose to have a bowel movement, those voluntary muscles relax and the rectum contracts to help move stool out. The internal sphincter’s reflexive relaxation and the external sphincter’s voluntary control therefore work as a coordinated system. One responds automatically to what is happening inside; the other helps you decide when to act.
