The First Phase of Sexual Response

Excitement can last for just a few minutes or extend for several hours. Characteristics of this phase include: an increasing level of muscle tension, a quickened heart rate, flushed skin (or some blotches of redness may occur on the chest and back), hardened or erect nipples, and the onset of vasocongestion, resulting in swelling of the woman's clitoris and labia minora and erection of the man's penis.
Other changes also occur. In the woman, the vaginal walls begin to produce a lubricating liquid, her uterus elevates and grows in size, and her breasts become larger. At the same time, the woman's vagina swells and the muscle that surrounds the vaginal opening, called the pubococygeal muscle, grows tighter.
These changes prepare the woman's body for orgasm and were called the "orgasmic platform" by Masters and Johnson. Additional changes in men include elevation and swelling of the testicles, tightening of the scrotal sac, and secretion of a lubricating liquid by the Cowper's glands.