In women, the urethra is comparatively short (about 2 cm) and has only the one purpose of conveying urine. It is set within the muscle of the front wall of the vagina and has its external opening just in front of the vaginal opening.
In men, the urethra is considerably longer (about 20 cm) and runs from the bladder through the prostate down through the penis so that its external opening is at the tip of the penis. It serves as a passageway not only for urine but also for the ejaculation of semen, and so is also part of the male reproductive system. It is not possible for both semen and urine to be expelled at the same time, because when a man urinates, the process automatically seals the opening through which seminal fluid enters the urethra.