Human sexual response cycle

Excitement phase
The excitement phase (also known as the arousal phase) is the first stage of the human sexual response cycle. It occurs as the result of any erotic physical or mental stimulation, such as kissing, petting, or viewing erotic images, that lead to sexual arousal. During the excitement stage, the body begins becoming prepared for coitus, or sexual intercourse, in the plateau phase.

Among both sexes, the excitement phase results in an increase in heart rate (tachycardia), an increase in breathing rate, and a rise in blood pressure. An erection of the nipples, especially upon direct stimulation, will occur in nearly all females and approximately 60% of males. Vasocongestion of the skin, commonly referred to as the sex flush, will occur in approximately 50-75% of females and 25% of males. An increase in muscle tone (myotonia) of certain muscle groups, occurring voluntarily and involuntarily, begin during this phase among both sexes. Also, the external anal sphincter may contract randomly upon contact (or later during orgasm without contact).

In males, the beginning of the excitement phase is observed when the penis becomes partially erect, after only a few seconds of erotic stimulation. The erection may be partially lost and regained repeatedly during an extended excitement phase. Both testicles become drawn upward toward the perineum, notably in circumcised males where less skin is available to accommodate the erection. Also, the scrotum can tense and thicken during the erection process.

In females, the beginning of the excitement phase is observed when the nipples become erect. Additionally, venous patterns across the breasts become more visible and the breasts increase in size, which becomes more observable if lying on the back. The labia majora become flatter, thinner, and raise upwards and outwards in nulliparous women (those who have not given birth). In parous women (those that have given birth), they increase two or three fold in size. The labia minora increase in size and may protrude above the labia majora. The clitoris becomes tumescent, or swollen, like the glans of the penis. After only seconds of stimulation, vaginal lubrication is produced by the vasocongestion of the vaginal walls. They darken in color and become smoother than normal. Also, the uterus elevates, more vertically as time passes, and the inner two-thirds of the vagina stretch, usually a total of 7 to 10 cm.

Plateau phase
The plateau phase is the period of sexual excitement prior to orgasm.

The plateau phase is the second phase of the sexual cycle, after the excitement phase. Further increases in circulation and heart rate occur in both sexes, sexual pleasure increases with increased stimulation, muscle tension increases further.

During this phase, the male urinary bladder closes (so as to prevent urine from mixing with semen) and muscles at the base of the penis begin a steady rhythmic contraction. Males may start to secrete seminal fluid and the testicles rise closer to the body.

At this stage females show a number of effects. The areola and labia further increase in size, the clitoris withdraws slightly and the Bartholin glands produce further lubrication. The tissues of the outer third of the vagina swell considerably, and the pubococcygeus muscle tightens, reducing the diameter of the opening of the vagina and creating what Masters and Johnson refer to as the orgasmic platform. For the many women who never achieve orgasm, this is the peak of sexual excitement. Both men and women may also begin to vocalise involuntarily at this stage.

An effect of prolonged time in the Plateau stage without succession to the Orgasmic stage may result in frustration for either partner.

Orgasmic phase
For main article, see orgasm.

Orgasm is the conclusion of the plateau phase in a release of sexual tension. Both males and females experience quick cycles (typically 0.8 seconds apart) of muscle contraction of the anus and lower pelvic muscles, with women also experiencing uterine and vaginal contractions.

Males ejaculate approximately 2-5ml of semen, with the volume varying considerably depending on the period of abstinence and degree of sustained arousal prior to ejaculation.

Orgasms in both men and women are often associated with other involuntary actions, including vocalizations and muscular spasms in other areas of the body. Also, a generally euphoric sensation is associated with orgasm. Orgasm generally causes perceived tiredness, and both males and females often feel a need to rest afterwards. This is often attributed to the release of endorphins during orgasm causing relaxation and drowsiness, but can also be due to the body's need for a short rest after a bout of vigorous sexual activity.

Orgasms in females may also play a significant role in fertilization. The muscular spasms are theorized to aid in the locomotion of spermatozoa up the vaginal walls into the uterus. Some also hypothesize that if the woman remains in a horizontal position for some time, due to sexual exhaustion and post-orgasmic pleasure, the chances of impregnation will increase.

Resolution phase
The resolution phase occurs after orgasm and allows the muscles to relax, blood pressure to drop and the body to slow down from its excited state.

Generally males experience a refractory period, meaning orgasm cannot be achieved again until time has passed and, for some men, continued stimulation may cause discomfort or pain. The penis meanwhile returns to a flaccid state.

Females may not experience this refractory period and further stimulation may cause a return to the plateau stage. In some women, this allows the possibility of multiple orgasms. Otherwise, significant changes may also occur, such as the opening of the cervix and the reduction of blood flow to the genitals and nipples. Some women find continued stimulation painful in the resolution phase.