Your body is like a pleasure powerhouse that cycles through a series of sexual responses each time you and a partner have sex. The phases of response differ somewhat for men and for women, but they both follow a fairly set physiological pattern, experts say.

Here's the science behind what happens when you and your partner get together, as explained by Ian Kerner, CSE, member of the American Association of Sexuality Educators, Counselors and Therapists (AASECT), Diplomate in Sex Therapy of the American Board of Sexology, and author of many books, including She Comes First, and Yvonne K. Fulbright, Ph.D., M.S. Ed., president and CEO of Sexuality Source, Inc. and the author of The Better Sex Guide to Extraordinary Lovemaking.

The First Stage

Also called the arousal or excitement stage, this starts the process of erotic arousal. In men, the penis becomes erect and the skin on the scrotum thickens. In women, within a short 10 to 30 seconds after arousal begins, the vagina starts getting lubricated and the clitoris also swells and becomes erect.

"A woman's nipples become erect as well," Fulbright says. "And the upper two thirds of her vagina expands dramatically."

Men and women both experience a so-called "sex flush," in which the heart rate increases, the muscles tense, and the body becomes more sensitive to stimulation.

A big difference between men and women is that men tend to experience desire more easily than women do. "Men cycle through the process of arousal more quickly than women do and they return to the pre-arousal state more quickly than women," Kerner says. "A woman is more likely to want to have that emotional connection."

The Second Stage

The second stage, or the plateau phase, both men and women become more aroused and the genitals become increasingly engorged with blood. For both men and women, breathing, pulse rate and blood pressure are increased.

"In a man, the penis is completely erect and the testes are so engorged with blood that they may be up to 50 percent larger than in an unaroused state," Fulbright explains. "In a woman, her body is preparing itself for orgasm with the formation of an orgasmic platform." The size of the entrance to the vagina gets smaller and there is a thickening of the walls of the outer third of the vagina.

The Third Stage

The third stage of sexual response is orgasm, and it's actually the shortest phase of the sexual response cycle. For men, in the "preliminary" or "emission" stage, the smooth muscles throughout the reproductive system actually contract, and the seminal fluids move into a widening of the urethra at the base of the penis. In the "expulsion" phase, the muscles at the base of the man's penis contract and force semen through the urethra and out the tip of the penis.

In a woman, both the pelvic and anal muscles contract, and so does the uterus, and contractions move in waves from the top of the uterus all the way down to the cervix.  Both men and women experience a sharp increase in pulse rate, breathing rate and blood pressure during orgasm.

The Fourth Stage

Most would believe that the sexual response ends after orgasm, but there is actually a fourth stage--returning to your unaroused state. For men, the penis loses the erection as the blood leaves the area. During the "refractory" period right after orgasm, men actually cannot have an erection or an orgasm.

For women, the uterus shrinks, the clitoris returns to its normal position and the swelling in the vagina does down. It's a somewhat slower process in a woman, taking anywhere from 15 to 30 minutes.

In women, Kerner explains, the blood does not flow out of the pelvic region as quickly as it does in men, which is why, as Kerner explains, women have the innate capacity to experience multiple orgasms."

"With the right kind of arousal, a woman can be hurled back into earlier stages of the cycle and experience the ride all over again, and quite rapidly at that," Fulbright says.