Urethral sphincter: A muscular mechanism that controls the retention and release of urine from the bladder. There are two urethral sphincters: the internal and external urinary sphincters. Part of the muscular bladder wall acts as the internal urethral sphincter and prevents urine from leaving the bladder to enter the urethra. This sphincter cannot be willfully controlled but is under involuntary control by the brain. A layer of muscle called the urogenital diaphragm supplies support for the contents of the pelvis and acts as the external urethral sphincter. It provides a second means of stopping the escape of urine from the body. This sphincter is under voluntary control.