Ejaculation is the release of semen from the penis. It is a normal part of the male sexual response cycle. During sexual intercourse or masturbation, semen collects in the ejaculatory ducts, which are located where the ends of the vas deferentia join the seminal vesicles within the prostate gland.

When excitation reaches its peak, a spinal reflex causes the rhythmic contractions of the smooth muscles within the urethra, penis and the prostate gland, and propels the semen through the urethra out the tip of the penis in spurts.

Once a man reaches a certain point of sexual arousal, he can no longer prevent ejaculation. This feeling of having reached the brink of control once these contractions start is known as ejaculatory inevitability.

The rhythmic contractions of the prostate, perineal muscles and shaft of the penis occur initially at 0.8-second intervals, just as in women, and account for the spurting action of the semen during ejaculation. The intervals between contractions become longer and the intensity of the contractions tapers off after the first three or four contractions.

The semen does not actually appear until a few seconds after the point of ejaculatory inevitability because of the distance the seminal fluid has to travel through the urethra. During ejaculation, the internal sphincter of the urinary bladder is tightly sealed to make sure that the seminal fluid travels forward and to prevent any urine from mixing with the semen.