The sexual reactions of people have been studied in detail by W. Masters and W. Johnson (1966) through direct laboratory observation of more than 10,000 sexual acts of a group of volunteers from 700 men and women. The results of the study showed that they can be described as a cycle consisting of four phases that follow each other: excitement, plateau, orgasm, and resolution.

1. Excitement. Female reactions are locally manifested by moistening, an increase in the clitoris in diameter and in length; expansion and elongation of the vagina; the cervix and uterus are raised up, and labia are enlarged due to blood flow, smoothed and
slightly apart. In addition, in women, in 2/3 of cases, an erection of the nipples occurs, the breast swells, muscle tone increases, the pulse rate starts, blood pressure starts to rise; some women have a “sexy blush.”

In men, an erection of the penis occurs, the skin of the scrotum stiffens and thickens, the testicles partially pull up to the body. There may be an erection of the nipples, increasing muscle tone, increasing the pulse rate and blood pressure.

2. Plateau. In women, due to increased blood flow, there are changes in the color of small and large labia (their color varies from pink to bright red in birthless and from scarlet to dark cherry in those who gave birth), and the labia minora can increase by 2-3 , and the clitoris moves to the pubic bone and hides behind the folds of the lips. The vagina expands and extends even more, an orgasmic cuff is formed in the lower third of the vagina, thanks to which the inlet of the vagina narrows by 30% or more. The mammary glands increase in size to 1/3 (to a greater extent in those who have not breastfed), the nipples swell. In most women, sexual blush spreads through the body, muscle tension increases, respiration and heart rate increase, and blood pressure increases markedly.

In men, the penis reaches its maximum erection, the scrotum tightens, the testicles rise and increase in size up to 50%, from the bulbourethral glands two or three drops of mucoid secretion are secreted. There is an erection of the nipples, often sexual redness of the skin of the abdomen, the anterior surface of the chest, face and neck. Increases muscle tension, increase blood pressure, heart rate and respiration.

3. Orgasm. The highest stage of sexual satisfaction, which is manifested in women by a simultaneous wave-like contraction of the muscles of the uterus, lower third of the vagina and sphincter of the rectum at intervals of about 0.8 seconds, followed by a decrease in strength, duration and frequency of contractions. A “soft” orgasm can be accompanied by 3-5 contractions, whereas with an intense (strong) orgasm, their number reaches 10-15. There are no further changes in the condition of the clitoris, genital lips, breasts and nipples. Increases sexual blush, there is a loss of arbitrary control over muscle contractions. Blood pressure, respiratory rate and heart rate reach their maximum.

Male orgasm differs from the female in the presence of two clearly distinguishable stages. At the first stage, contractions of the vas deferens, prostate gland and seminal vesicles occur, which contributes to the output of sperm in the urethral bulb (subjectively, there is a feeling of inevitability of ejaculation). At the second stage of orgasm, the urethra and the penis itself are involved in the process of rhythmic contractions, which in a few seconds leads to ejaculation. Spasms of the muscles of the perineum, prostate, seminal ducts and urethra are repeated 3-4 times with an interval of 0.8 seconds, after which the intervals between contractions increase, and their intensity decreases. At the same time there are no noticeable changes in the state of the testicles and scrotum. Blood pressure, heart rate and respiration are maximized.

The orgasm lasts 3 to 15 seconds and is accompanied by a slight clouding of consciousness. With a bright orgasm, you can talk about a kind of “kinesthetic trance” with a focus on sensual ecstatic experiences.

4. Resolution. The phase is accompanied by a feeling of general relaxation and well-being, muscular relaxation. In women, the clitoris returns to its normal position, losing an erection. The walls of the vagina relax and return to an unexcited state. The uterus descends to its usual position, and the opening of the cervix expands by 20-30 minutes. Labia, breasts and nipples return to their original size, shape and color. The sexual blush disappears, respiration rate, pulse and arterial pressure return to normal.

Men lose penile erections, the skin of the scrotum relaxes and returns to normal thickness. The testicles take their original size and return to their original position in the scrotum. Pulse rate, respiration and blood pressure return to normal. Men for a certain time, which depends on their sexual constitution and increases with age, lose the ability to a new orgasm. Such a refractory period can last from several minutes to many hours. In women, the refractory period as such is absent, which is why they are potentially capable of multiple orgasms. The resolution phase lasts for 10–15 minutes, and if, as a result of sexual stimulation, an orgasm does not arise, up to 6–12 hours.