While most of us are sure that we like to have sex, most of us also haven't spent much time thinking about what happens physiologically while we are engaged in the act. Masters and Johnson (two groundbreaking sex therapists) coined the term "sexual-response cycle" to mean the sequence of events that happens to the body when a person becomes sexually aroused and participates in sexually stimulating activities (intercourse, masturbation, foreplay, etc.).
The sexual-response cycle is divided into four phases: excitement, plateau, orgasm and resolution. There is no distinct beginning or end to each phase -- they're actually all part of a continuous process of sexual response.
Keep in mind that this is a very general outline of what happens to each of us as we become sexually aroused. There is much variation among individuals, as well as between different sexual events.
Both men and women go through all four phases, except the timing is different. Men typically reach orgasm first during intercourse, while women may take up to 15 minutes to get to the same place. This makes the likelihood of simultaneous orgasm during intercourse a rare event.
This phase usually begins within 10 to 30 seconds after erotic stimulation, and can last anywhere from a few minutes to many hours.
Men: The penis becomes slightly erect. A man's nipples may also become erect.
Phase Two: Plateau
The changes that started in the excitement phase continue to progress.
Men: The testes are drawn up into the scrotum. The penis becomes fully erect.
Women: The vaginal lips become puffier. The tissues of the walls of the outer third of the vagina swell with blood, and the opening to the vagina narrows. The clitoris disappears into its hood. The inner labia (lips) change color (although it's a bit hard to notice). For women who've never had children, the lips turn from pink to bright red. In women who've had children, the color turns from bright red to deep purple.
Both: Breathing and pulse rates quicken. A "sex flush" may appear on the stomach, chest, shoulders, neck or face. Muscles tense in the thighs, hips, hands and buttocks, and spasms may begin.
Phase Three: Orgasm
This is the climax of the cycle. It is also the shortest of the four phases, usually only lasting a few seconds.
Men: First, seminal fluid collects in the urethral bulb. This is when a man may have the sensation that orgasm is certain, or "ejaculatory inevitability." Next, semen is ejaculated from the penis. Contractions occur in the penis during the orgasmic phase.
Women: The first third of the vaginal walls contract rhythmically every eight-tenths of a second. (The number and intensity of the contractions vary depending on the individual orgasm.) The muscles of the uterus also contract barely noticeably.
Both: Breathing, pulse rate and blood pressure continue to rise. Muscle tension and blood-vessel engorgement reach a peak. Sometimes orgasm comes with a grasping-type muscular reflex of the hands and feet.
Phase Four: Resolution
This phase is a return to the normal resting state. It can last from a few minutes to a half-hour or longer. This stage is generally longer for women than men.
Men: The penis returns to its normal flaccid state. There is usually a refractory period, where it's impossible to orgasm again until a certain amount of time has passed. The amount of time varies among men by age, physical fitness and other factors.
Women: The uterus and clitoris return to their normal positions. Some women may be able to respond to additional stimulation with additional orgasms.
Both: Swelling recedes, any sex flush disappears, and there is a general relaxation of muscle tension.
Understanding what's happening to you and your partner's bodies during sex can only aid in the full enjoyment of the experience. Combine this with some good communication skills, and you've found the key to unlock sexual pleasure and your heart's desires.