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sphincter

sphinc·ter

/ˈsfiNGktər/

Noun

A sphincter is a ring of muscle that surrounds and, through constriction, controls an opening in the body. One example is the urethral sphincter that retains urine in the bladder. As the bladder approaches its maximum size, receptors deliver signals to the brain, indicating that it needs to be relieved. This is interpreted as the need to urinate. The sphincter then relaxes, causing micturition (urination) to occur.