The urethral sphincter is the circular muscular structure that surrounds the urethra and allows the bladder to store or empty urine. This muscle acts like a valve, relaxing or tightening to open or close the outlet of the bladder.
The urethra has an internal and external sphincter. The internal sphincter is the portion of the urethra found at the bladder neck that keeps the urethra closed when a person is at rest. The external sphincter is the portion of the urethra that is under a person’s voluntary control.