During this period, the woman's clitoris may become so sensitive that it is painful to the touch. The plateau phase extends to the brink of orgasm, which initiates the reversal of all of the changes begun during the excitement phase. The peak of sexual excitement is reached during the third phase. Involuntary muscle contractions, heightened blood pressure and heart rate, rapid intake of oxygen, sphincter muscle contraction, spasms of the carpopedel muscles in the feet, and sudden forceful release of sexual tension characterize the orgasmic phase. For men, orgasm generally climaxes in the ejaculation of semen, which contains millions of sperm.
Ejaculation consists of two steps.
During the first phase, called the emission phase, seminal fluid builds up in the urethral bulb of the prostate gland . As the fluid accumulates, the male senses he is about to ejaculate. This is often experienced as inevitable and uncontrollable. During the second phase, called the expulsion phase, the urinary bladder closes to block the possibility of urine mixing with the semen. At this point, muscles at the base of the penis begin a steady rhythmic contraction that finally expels the semen from the urethral opening at the head of the penis.