sphincter

n. ring-shaped muscle whose function is to constrict or close the body opening or passage which it surrounds

A sphincter is a structure, usually a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. There are over 40 different sphincters in the human body; some of these sphincters are microscopic in size.
See more at Wikipedia.org...

Sphincter /sphinc·ter/ [L.] a ringlike muscle which closes a natural orifice or passage.sphinc´teralsphincter´ic anal sphincter , sphincter a´ni [more]Sphincter - Community and Resources

Noun
1. a ring of muscle that contracts to close an opening
(synonym) anatomical sphincter, sphincter muscle
(hypernym) muscle, musculus
(hyponym) physiological sphincter

n. sphincter, ring-shaped muscle whose function is to constrict or close the body opening or passage which it surrounds