The muscles of the vagina relax and contract rapidly, as do the muscles of the uterus. 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. Endorphins Orgasms cause a release of endorphins into one's spinal fluid and endorphins are also somewhat responsible for the emotion of happiness, pleasure, calming effect, etc.
Many researchers believe that strenuous exercise releases endorphins into the blood stream.
Others agree that endorphins are released during orgasm, as well as during laughter. Endorphins are a group of substances formed within the body that naturally relieve pain. They have a similar chemical structure to morphine. In addition to their analgesic affect, endorphins are thought to be involved in controlling the body's response to stress, regulating contractions of the intestinal wall, and determining mood. They may also regulate the release of hormones from the pituitary gland, notably growth hormone and the gonadotropin hormones.