Human sexual responses consist of four phases

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The responses of both women and men to sexual stimulation consist of four phases: excitement, plateau, orgasm, and resolution. As sexual excitement begins in a woman, her heart rate and blood pressure rise, muscular tension increases, her breasts swell, and her nipples become erect. Her external genitals, including the sensitive clitoris, swell as they become filled with blood, and the walls of the vagina secrete lubricating fluid that facilitates copulation.

As a woman's sexual excitement increases, she enters the plateau phase. Her blood pressure and heart rate rise further, her breathing becomes rapid, and the clitoris begins to re-tract—the greater the excitement, the greater the retraction. The sensitivity once focused in the clitoris spreads over the external genitals, and the clitoris itself becomes even more sensitive. Orgasm may last as long as a few minutes, and, unlike

Increased estrogen/ progesterone ratio

Increased contractility of uterine muscle

Growth of fetus

Increased stretch of uterus men, some women can experience several orgasms in rapid succession. During the resolution phase, blood drains from the genitals, and body physiology returns to close to normal.

In the male, as in the female, the excitement phase is marked by an increase in blood pressure, heart rate, and muscle tension. The penis fills with blood and becomes hard and erect. In the plateau phase, breathing becomes rapid, the diameter of the glans increases, and a clear lubricating fluid from the bulbourethral gland oozes from the penis. Pressure and friction against the nerve endings in the glans and in the skin along the shaft of the penis eventually trigger orgasm. Massive spasms of the muscles in the genital area and contractions in the accessory reproductive organs result in ejaculation.

Within a few minutes after ejaculation, the penis shrinks to its former size, and body physiology returns to resting conditions. The male sexual response includes a refractory period immediately after orgasm. During this period, which may last from minutes to hours, a man cannot achieve a full erection or another orgasm, regardless of the intensity of sexual stimulation.

rhythm method. The couple avoids sex from day 10 to day 20 of the ovarian cycle, when the woman is most likely to be fertile. The cycle can be tracked by use of a calendar, supplemented by the basal body temperature method, which is based on the observation that a woman's body temperature drops on the day of ovulation and rises sharply on the day after. Changes in the stickiness of the cervical mucus also help identify the day of ovulation.

However, sperm deposited in the female reproductive tract may remain viable for up to 6 days. Similarly, the egg remains viable for 12 to 36 hours after ovulation. These facts, added to individual variation in the timing of ovulation, result in an annual failure rate of between 15 and 35 percent for the rhythm method. In other words, 15 to 35 percent of women using only the rhythm method for 1 year will become pregnant during that time.

Another approach is to try to separate sperm and egg in space through coitus interruptus—withdrawal of the penis from the vagina before ejaculation. The annual failure rate of this method (mostly due to lack of willpower) may be as high as 40 percent.

Humans use a variety of technologies to control fertility

People use many methods to control the number of their children and the time between births. The only sure methods of preventing fertilization and pregnancy are complete abstinence from sexual activity or surgical removal of the gonads. Since those approaches are not acceptable to most people, they turn to a variety of other methods to prevent pregnancy. Many of these methods prevent fertilization or implantation (conception) and are therefore referred to as methods of contraception.

Some methods of contraception are used by the woman, others by the man. They vary from means of blocking gametogenesis to means of blocking implantation of a blastocyst. Contraceptive methods vary enormously in their effectiveness and in their acceptability to those who use them. Here we will review some of the most common methods and their relative failure rates (Table 43.1). Even with all the contraceptive methods available, a recent study revealed that almost half of the 6.3 million pregnancies that occur in the United States each year are unintended.

nontechnological approaches. An approach to contraception that does not involve physical or pharmacological technologies is to separate sperm and egg in time through the barrier methods. Techniques for placing a physical barrier between egg and sperm have been used for centuries. The condom is a sheath made of an impermeable material such as latex that can be fitted over the erect penis. A condom traps semen so that sperm do not enter the vagina. Latex condoms also help prevent the spread of many sexually transmitted diseases. In theory, use of a condom can be highly effective, with a failure rate near zero; in practice, the annual failure rate is about 15 percent due to leakage because of tearing or poor fit (e.g., with the loss of the erec-

43*1 Methods of Contraception

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