Very few physical activities are more pleasurable to humans than sexual intercourse. The sex drive is one of the strongest drives directing human behavior, and as such, it circumscribed by many rules, and customs.

Sexual intercourse acts as a channel for the strongest of human emotions such as love, tenderness, and personal commitment. Few subjects are at the same time more private and of more general interest. I will limit myself to a very narrow aspect of sexual behavior, and its immediate physiological effects. The emotional consequences are no less real, however, they are perhaps beyond my scope: (FEDT).

Until relatively recently, the physiology of human sexual activity was largely unknown. Perhaps because of the prevalence of strong social taboos against the open discussion of sexual matters, no research was carried out on the subject, and detailed information was lacking. Over the past thirty (30) years, however, investigations by William Masters and Virginia Johnson, as well as the thousands of researchers who followed them, have revealed much about the biological nature of human sexual activity.

Engaging in the sexual act is referred to by a variety of names, such as sexual intercourse, copulation, and coitus, as well as a host of informal terms. It is common to partition the physiological events that accompany intercourse into four phase - excitement, plateau, orgasm, and resolution, although there are no clear division between these phases.

Excitement

Sexual responce is initiated by the nervous sysstem. In both males and females, commands (signals) from the brain increase the respiratory rate, heaart rate, and blood pressure. The nipples of the woman becomes harden, and also become more sensitive.

Other changes increase the diameter of the blood vessels, leading to increased circulation. In some people, these changes may produce a reddening of the skin around the face, breasts, and genitals (the sex flush). Increased circulation also leads to vasocongestion, producing erection of the male's penis, and similar swelling of the female's clitoris, a small knob of tissue composed of a shaft and glans much like the penis, nevertheless, without the urethra running through it.

Women experience changes that prepare the vagina for sexual intercourse: the labia majora, and labia minor, lips of tissue that cover the opening to the vagina, swell and separate due to the increased circulation; the vaginal walls become moist; and the muscles encasing the vagina relax.

Plateau

The penetration of the vagina by the thrusting penis results in the repeated stimulation of nerve endings both in the tip of the penis, and in the clitoris. The clitoris, which is now swollen, becomes very sensitive and withdraws up into a sheath ("hood"). Once the clitoris has withdrawn, it is stimulated indirectly when the thrusting movements of the penis rub the clitoral hood against the clitoris.

The nervous stimulation produced by the repeated movements of the penis within the vagina elicits a continous response of the autonomic nervous system, thus, greatly intensifying the physiological changes initiated in the excitement phase. In the woman, pelvic thrusts may begin, while in the male the penis reaches its greatest length, and rigidity.

Orgasm

A climax during intercourse is reached the stimulation is sufficient to initiate a series of reflexive muscular contractions. The nerve impulses producing these contractions are associated with other nervous activity within the central nervous system, activity that people experience as intense pleasure.

In females, the contractions are initiated by impulses in the hypothalamus, which causes the posterior pituitary gland to release large amounts of oxytocin. This hormone (oxytocin) causes the muscles in the uterus and around the vaginal opening to contract and the cervix to be pulled upward. Contractions occur at intervals of about one per second. There may be one to several intense peaks of contractions or orgasm, or the peaks may be more numerous, nonetheless, less intense.

Analogous contractions take place in the male. The first contractions, which occur in the vas deferens and prostate gland, cause emission, the peristaltic movement of sperm and seminal fluid into a collecting zone of the urethra, located at the base of the penis. Shortly thereafter, violent contractions of the muscles at the base of the penis result in ejaculation of the collected semen through the penis.

As in the female, the contractions are spaced about one second apart, though in the male they continue for only a few seconds, and are almost invariable restricted to a single intense wave.

Resolution

After ejaculation, males rapidly lose their erection and enter a refractory period lasting 20 minutes , or more; sometimes more than an hour, or longer, in which sexual arousal is difficult to achieve and ejaculation is almost impossible. By contrast, most women can be aroused again almost immediately.

When sexual intercourse is completed, the bodies of both male, and female return over a period of several minuts to their normal physiological state.

Definition

Note: Sexual intercourse is a physiological series of events leading to the ultimate deposition of sperm within the female reproductive tract.