The human sexual response cycle has been outlined as a four phase process: excitement, plateau, orgasm, and resolution. The
excitement phase may last from a few minutes to several hours. During the excitement phase and the plateau phase that follows, both women and men experience increased muscle tension and heart rate, a sex flush (blotches on the skin of the chest and neck), nipple erections, and vasocongestion of the genitals that leads to the swelling of the clitoris and labia minora (inner lips) for women, and erection of the penis for men. In addition, the woman's vaginal walls "sweat" a lubricating fluid, her breasts enlarge, and her uterus elevates and increases in size. The outer third of the woman's vagina swells, and the pubococcygeal muscle that rings the vaginal opening tenses. Together these reactions form the "orgasmic platform." Several other responses occur in men. The testes elevate and increase in size, the scrotal skin thickens, and the Cowper's glands (pea-size organs located below the prostate gland) secrete a clear lubricating fluid.
The phases of sexual response are not necessarily distinct, and the clitoris and labia or penis may not become erect until the second, or plateau, phase. In this phase the clitoris may become painful to the touch as it withdraws under its hood. During the plateau phase the physiological responses begun in the excitement phase may extend and deepen in intensity.
During the orgasmic phase both women and men experience involuntary muscle spasms, peak blood pressure and heart rate, rapid breathing, contractions of the rectal sphincter, carpopedal spasms in the feet, and a sudden discharge of sexual tension. Female orgasms are characterized by multiple rhythmic contractions (approximately 0.8 seconds apart) of the uterus and the orgasmic platform that grips the penis during intercourse and facilitates orgasm for both men and women. Some women also experience the ejaculation of fluid during orgasms that are produced by the stimulation of the GrŠfenberg (G) spot. The G spot is located approximately an inch inside the vagina on the anterior wall, just behind the pubic bone. Stimulating the G spot may trigger the production and expulsion of fluid from the Skene's gland. The male counterpart to the Skene's gland is the prostate gland.
There is some debate as to whether orgasms attained by vaginal stimulation are different from those attained by clitoral stimulation alone. Masters and Johnson maintain that only one kind of physical female orgasm exists, whether the clitoris, the vagina, or both are stimulated. Other researchers suggest that there are at least three different kinds of female orgasms: the vulval orgasm, produced primarily by clitoral stimulation; the uterine orgasm, produced primarily by vaginal penetration and stimulation of the G spot; and the blended orgasm, produced by a combination of stimuli. While researchers disagree about types of orgasms, they do agree that orgasm itself, or the type of orgasm one experiences, is less important than the ability to express and enjoy one's sexuality.
For men orgasm is usually accompanied by ejaculation. A single ejaculate contains about 300 million sperm in a fluid medium called semen. Ejaculation is a two-phased process. In the emission phase seminal fluid accumulates in the urethral bulb of the prostate resulting in a sensation of ejaculatory inevitability. In the expulsion phase the neck of the urinary bladder closes to ensure that no urine will mix with the semen and the muscles at the base of the penis and of the penile urethra contract at intervals of approximately 0.8 seconds to force the semen out the urethral opening. Some men experience a "retrograde" or dry ejaculation as a result of genetics, illness, medication, surgery, or damage to the valves of the urethra that control the flow of semen and allow the ejaculate to empty into the bladder instead of being expelled via the penis.
During the resolution e both females and males return to normal levels of muscle tension, heart rate, and blood pressure. The sex flush vanishes, nipple erection subsides, and the vasocongestion that caused the swelling of the labia and clitoris, and the erection of the penis, disappears. In women the clitoris descends from under its hood and becomes less sensitive, vaginal lubrication ceases, breasts return to their normal size, the orgasmic platform dissipates, and the uterus returns to its pre-aroused state. In men the testes return to their normal size, the skin of the scrotal sac relaxes, and the Cowper's glands stop their secretions. Although many women and some men can move quickly through excitement, plateau, and orgasm again, most men experience a refractory period after orgasm and are not able to reexperience erection and orgasm again for a time ranging from a few minutes to hours or days, depending on age, genetics, and health.
The physiological differences of the sexual response cycle between the genders can be enormous. But the subjective experience of orgasm for both women and men appears to be so similar that researchers are not able to determine the gender of the author when presented with written descriptions of orgasmic experiences.