Sphincter (noun)

A ring-shaped muscle that contracts or relaxes to regulate the flow of substances such as food, air, or bile.

Origin:

Late 17th century: from greek sphinkter "band, bond", from sphingein "to bind".

Examples:

1. The sphincter muscles in the esophagus control the flow of food into the stomach.
2. The sphincter muscles in the anus control the release of feces.
3. The sphincter muscles in the iris regulate the amount of light entering the eye.
4. The sphincter muscles in the bronchi control the flow of air into the lungs.
5. The sphincter muscles in the bladder control the release of urine.