The Masters & Johnson 'Model of Sexual Response', established in 1966, outlines the four stages of the human sexual response cycle: excitement, plateau, orgasm, and resolution. It revolutionised our understanding of human sexual physiology and dynamics.

Excitement-

The first stage is triggered by physical and mental erotic stimuli such as; kissing, touching or viewing erotic images that leads to sexual arousal.

During the excitement phase, the body prepares for sexual intercourse. People may experience an increased heart rate and breathing rate, a rising blood pressure, and often vasocongestion of the skin (AKA: "the sex flush"). 

In females, the vaginal walls swell with increased blood flow, and lubrication is produced to aid in comfort and arousal.

Plateau-

The period of sex prior to orgasm.

In both sexes, there is an increase in circulation and heart rate, increased sexual pleasure with increased stimulation, and further increased muscle tension. Respiration continues at an increased level. 

In females, vasocongestion causes further swelling of the clitoris, labia minora, and vaginal walls, though the clitoris may begin to retract under the clitoral hood. The muscles near the vaginal opening may tighten slightly, and the uterus continues to elevate, creating more space in the vaginal canal. Lubrication production slows compared to the excitement phase. The breasts may increase in size, the areolas can become more swollen, and while the nipples typically remain erect, in some cases, they may soften slightly despite ongoing arousal.

Orgasm-

The conclusion of the plateau phase.

It is accompanied by quick cycles of muscle contraction in the lower pelvic muscles, which surround the anus and the primary sexual organs. Females may experience uterine and vaginal contractions and external ejaculation. Orgasms are often associated with other involuntary actions, such as; vocalisation, muscle spasms in other parts of the body, and a general euphoric sensation. Heart rate continues to increase. 

Resolution-

Occurs after orgasm and allows the muscles to relax, blood pressure to drop, and the body to slow down from its excited state.

The refractory period (during the resolution phase) is the timeframe in which females have been recorded to have the capacity for multiple orgasms.