internal urinary sphincter

The internal urinary sphincter is a ring of muscle that controls the exit of urine from the bladder. It is located at the base of the bladder, where it meets the urethra. This sphincter is part of the body's involuntary control system for urination.1

The bladder itself is a hollow, muscular organ that stores urine. When the bladder fills, its muscular walls stretch. The internal urinary sphincter remains closed to prevent urine leakage. When it's time to urinate, the bladder muscle contracts, and the internal sphincter relaxes, allowing urine to pass through the urethra. The external urinary sphincter, made of striated muscle, is under voluntary control and can help maintain continence even if the internal sphincter is not functioning properly.12