An orgasm is an emotional and physical experience that occurs during a “sexual response cycle”. Before an orgasm, your body becomes increasingly excited. Breathing, heart rate and blood pressure increases. The pupils of the eyes dilate; the lips of the mouth darken, the nipples become erect, the clitoris swells, becomes hard and exposed, (much like the aroused penis). With increased excitement, the skin becomes flushed and it begins to sweat.
In women, the labia, clitoris, vagina and pelvic organs enlarge in very much the same way as the aroused penis enlarges. Sometimes there is a plateau of excitement which is held for several minutes before you are about to orgasm.
Orgasm is the point at which all the tension is suddenly released in a series of involuntary and pleasurable muscular contractions that may be felt in the vagina and/or uterus (some women do experience orgasms without contractions).
The orgasm happens when excitement seems to go over the edge -- a climax or crescendo is reached which may last several seconds or longer. During orgasm the body stiffens and the muscles contract. Involuntary muscle contractions and spasms may occur in various parts of the body, including your legs, stomach, arms, and back. The muscles of the vagina relax and contract rapidly, as do the muscles of the uterus. The glands of the vagina (Bartholin's glands) discharge a watery secretion, which acts to lubricate the vagina.
The main physical changes that occur during a sexual experience are a result of vasocongestion. This is the accumulation of blood in various parts of the body. Muscular tension increases and other changes occur throughout your body also.