The internal urethral sphincter is a muscle located at the base of the bladder, surrounding the urethra. Its primary function is to control the flow of urine from the bladder into the urethra, helping to maintain urinary continence. This sphincter is involuntary, meaning it operates automatically without conscious control. When the bladder fills with urine, the internal urethral sphincter remains contracted to prevent leakage. During urination, signals from the nervous system relax the sphincter, allowing urine to pass through the urethra. This process is essential for normal urinary function and is closely linked to the overall health of the urinary system, including the bladder and nervous system.