The urethra is the opening that allows urine to leave the bladder. In men, the urethra is a thin tube-like structure that starts from the lower opening of the bladder and traverses the entire length of the penis. In women, it is a shorter opening coming off the lower opening of the bladder and is between 2.5 to 4 centimeters (cm) in length.
The urethra has a sphincter that is normally closed to keep urine inside the bladder. When the bladder fills with urine, there are both voluntary and involuntary controls to open the urethral sphincter to allow urine to come out.