Four-stage series of physiological reactions to sexual stimulation.
Persons do not necessarily progress through all four stages, nor need two persons pass through them simultaneously. If the two partners are of opposite sex, sexual response may lead to sexual intercourse . In excitement, muscles tense and heart rate increases; in males, the penis becomes erect; in females, the inner vagina widens, its walls become moist, and the clitoris enlarges. In the plateau stage, breathing accelerates and muscles continue tensing; the testes and glans of the penis enlarge; the outer vagina contracts and the clitoris retracts. At orgasm the muscle tension is quickly released; the penis contracts repeatedly, ejaculating semen ; the vagina contracts regularly. In resolution, the genitals of both return to their pre-arousal condition; men cannot become aroused for some minutes or hours; women can quickly become aroused again.