The Biology of Reproduction

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To understand the different methods of birth control that are available today and to choose the best one for you, you will need to know something about the male and female reproductive systems and the process of conception. This chapter outlines how the systems work and how conception takes place.


The sexual organs of the male consist of the penis, scrotum, and testicles (on the outside), and the epididymis, the vas deferens, the prostate, and the urethra (on the inside) (see Figures 1.1 and 1.2).

The penis is composed mostly of soft, spongy tissue packed with a network of tiny blood vessels. Two sections of this tissue lie side by side along the upper part of the penis and help anchor it to the pubic bones. A third section lies underneath the entire length of the penis. At the tip this particular section broadens and forms the glans. The penis is covered with loose skin; in addition, on an uncir-cumcised penis, a fold of this skin, the foreskin, hangs over the glans. When a man is aroused sexually, the valve system in the blood vessels of the penis closes the usual exits in the blood network. As a

Sexual Arousal And Body Temperature
FIGURE 1.1 The Male Reproductive System (Frontal View)

result, the spongy tissues fill with blood and the penis becomes hard and erect, allowing it to penetrate the vagina.

The scrotum is the pouch of skin that hangs behind the penis and holds the testicles. It is sensitive to sexual stimulation and to changes in the temperature outside the body. The scrotum's function is to keep the testicles at the right temperature for producing sperm. It sometimes tightens up to hold the testicles snug against the body to keep them warm. At other times it becomes very loose, so the testicles can cool off.

The two testicles, or testes, are composed of delicate, tiny, tightly



Side View Erect Nipples
FIGURE 1.2 The Male Reproductive System (Side View)

coiled tubes that are lined with cells designed to manufacture either sperm or hormones. In the fetus, the testicles are formed in the abdomen and by the time of birth have descended into the scrotum. If the testes fail to descend or do not develop completely, this sometimes reduces sperm production and can be a cause of infertility.

During the teen years, the testicles grow and certain cells within the coil of tubes begin to secrete androgens. Androgens are the male hormones (such as testosterone) that cause the development of male sex characteristics such as facial hair. These hormones also stimulate other cells in the testicles to manufacture sperm. From puberty until sometime in old age, the continuous secretion of androgens results in a constant production of sperm.

After sperm have been formed, they move into the epididymis, the tube that coils along the back of each testicle, where they continue to develop. They are so tiny they cannot be seen without a microscope. The mature sperm is composed of an oval head that contains the chromosomes (the genetic material) and a long, whiplike tail that propels it with great vigor and speed. "Every one of them has an outboard motor, they know where they want to go, and there are millions of them," is the way a family planning expert recently described them. It takes a sperm about 72 days to grow to maturity, at which time it passes into another tube, the vas deferens, or sperm duct. Each vas runs from the epididymis up to the outside of the bladder. At this end the duct is wider and has enough space to store sperm in preparation for an ejaculation.

Just beneath the bladder lies the prostate gland, which produces secretions that help sperm survive after ejaculation. These secretions mix with the sperm after they leave the vas deferens. The mixture is called semen.

The urethra, the tube in which this mixing takes place, passes through the prostate, which enables the prostate secretions to enter the urethra through tiny ducts. The male urethra has two functions. When a man urinates, it transports urine from the bladder. During intercourse, it carries semen. When a man is sexually stimulated, the opening between the bladder and the urethra is closed to prevent urine from joining the semen. (After a man has been sexually aroused, it takes a short time for this system to reverse itself so he is able to urinate.)

At the peak of sexual excitement, the man's pelvic muscles tighten, forcing semen down the urethra and out the penis. This is called an orgasm, or ejaculating, or having a climax. Each ejaculate contains tens of millions of sperm—but only one actually gets to fertilize an egg.


If a woman is not familiar with her sexual anatomy, looking at the following illustration (Figure 1.3) may be helpful. However, looking at and touching your own anatomy may be an even better

Mons pubis

Mons pubis

Black Girls Monspubis

FIGURE 1.3 The Female Reproductive System (Exam View)

FIGURE 1.3 The Female Reproductive System (Exam View)

approach. You can use a mirror to see your external genitals if you wish, or you can rely on touch to determine where most of these structures are and how they feel. Being familiar with your body makes it easier to use birth control and may increase your pleasure during sex. If you want to insert a diaphragm, for instance, it is helpful to know just how long your vagina is and exactly where near its end your cervix lies.

The exterior sexual structures of the female are the mons pubis, the labia or "lips" of the vagina, the clitoris, the hymen, and the vaginal opening. The visible, exterior sexual structures together are called the vulva or external genitals.

The vagina leads to the internal reproductive organs: the cervix, the uterus, the fallopian tubes, and the ovaries. These lie inside the pelvis, supported and protected by the pelvic muscles and bones.

The mons pubis is the most noticeable part of the female genital area. It's the soft mound of fatty tissue below the belly that covers and protects the joining of the pubic bones, the pubic symphysis. During puberty, the mons becomes covered with hair that also covers the external genitals.

The opening to the vagina is protected by the labia. In some women, the labia majora, the outermost lips around the vaginal opening, are darker than the surrounding skin. The labia minora lie within the labia majora and are more delicate. They are sensitive to touch, and with sexual stimulation they become filled with blood and turn darker. The area of skin between the labia and the anus (the opening of the rectum) is the perineum, which often is also quite sensitive to stimulation.

The labia minora are joined at the front to form a soft fold of skin that looks like a little hood. This protects the clitoris, the most sensitive part of a woman's genitals. The clitoris contains many nerve endings which transmit sensory messages when the clitoris is touched, especially sexually. The clitoris has often been compared to the penis in its anatomy and reaction to stimulation. If you press the clitoris with your fingers, under its skin you can feel the firm but movable shaft that connects it to the pubic bones.

Between the clitoris and the vagina is a small opening for the urethra. As in males, the urethra is the tube that carries urine from the bladder.

In addition to being shielded by the labia, the vaginal opening in young girls may be partially covered by the hymen. This is a thin, usually stretchable tissue that is partially open to let menstrual blood flow through. In most women, the hymen opening is large enough to permit the use of tampons. When intercourse takes place, especially for the first time, the opening in the hymen often is stretched further by the penis. Sometimes it is pushed aside so hard it tears and may bleed a little.

The hymen varies a great deal from woman to woman and girl to girl in its flexibility and in the size of its opening. A woman who is sexually active may have a hymen that is still in one piece simply because it is very flexible. Or a woman with no sexual experience may have almost no hymen—just because that is the way her body developed.

The vagina is the passage that connects the external genitals to the cervix (Figure 1.4). Its length may vary from 2 1/2 to 4 inches. The vagina slants back slightly toward the spine. It is connected to the uterus at the cervix, which is the mouth of the uterus. Its muscu-


ions pubis

Female Reproductive System Side View

FIGURE 1.4 The Female Reproductive System (Side View)


ions pubis

FIGURE 1.4 The Female Reproductive System (Side View)

lar walls are lined with a folded mucous membrane and contain many blood vessels. When you examine it with your fingers, the vagina feels both soft and muscular. In most women, the lower third is sensitive to sexual stimulation and the upper portion is less responsive. The vagina's walls are folded in on themselves so closely they touch, yet they have a great capacity for expanding around an object such as a penis, a baby, or a diaphragm. The mucous membrane lining the vagina can change from almost dry to very wet because it secretes fluids, particularly around the time of ovulation, during pregnancy, and when the woman is sexually aroused.

The uterus resembles an upside-down pear about the size of a fist. The lower, narrow end of the uterus, or cervix, extends down into the vagina and has an opening called the os. The tip of the cervix is small and firm and feels like the tip of a nose. Before a woman has a child, the os is very small—about the diameter of a straw. The cervix and uterus have thick, muscular, very flexible walls designed to accommodate a fetus and the effort of childbirth.

Glands in the cervix secrete mucus that varies both in amount and consistency during the course of the month, depending on the levels of estrogen and progesterone, the hormones produced by the woman's body. When progesterone levels are high, for example, the cervix secretes a thick, nonstretchable mucus that effectively plugs the os against the entrance of sperm. During ovulation, however, estrogen levels are high and the mucus becomes thin and stretchy and is hospitable to sperm.

The cervix shifts position during the menstrual cycle, making it easier to reach on some days. A few days before ovulation it is high in the vagina and then, just before menstruation, it is lower. The os also changes throughout the cycle, opening wider before ovulation and before menstruation.

Sperm deposited inside the vagina or just outside it quickly propel themselves up into the os, through the uterus, and into the fallopian tubes, where fertilization takes place if an egg is waiting (Figure 1.5). How easily sperm move from the vagina and through the cervix depends on the type of cervical mucus present. The mucus produced near the time of ovulation protects sperm from the acid environment of the vagina and assists their progress toward the fallopian tubes.

After fertilization, the egg cell begins the process of dividing as it moves through the fallopian tubes and into the uterus. There it implants itself in the endometrium, or uterine lining, in order to continue developing into an embryo and then a fetus. When the fetus is fully developed, the muscular walls of the uterus will produce strong contractions to push it down through the cervix and vagina, which stretch to permit the passage of the baby.

In most women the uterus is slanted, with its top pointed toward the upper abdomen and the cervix aimed at the lower back. In some women, this angle is reversed and the cervix points forward, a differ-

Fallopian tube


Fallopian tube


Reproductive Gif
FIGURE 1.5 The Female Reproductive System (Frontal View)

ence that generally does not affect pregnancy or childbirth but can affect your choice of contraceptive. A forward tilt to the cervix may make the cervical cap or diaphragm less reliable because the forward slant can allow these devices to be easily dislodged during intercourse.

On each side of the uterus is a slender, delicate fallopian tube, which is three to four inches long. The end of each tube is shaped somewhat like a funnel with fringed ends, called the fimbria, that almost touch the ovary. The ovaries are small, oval shaped, and about the size of walnuts. There is one near each fallopian tube. They contain thousands of tiny egg follicles that hold a lifetime supply of undeveloped egg cells. The ovaries also produce the hormones needed for reproduction. When an egg matures, it is expelled from the ovary and the fimbria gather it into the fallopian tube. The walls of the fallopian tubes are lined with fine, hair-like projections called cilia. The cilia and muscular contractions of the tube move the egg along to the uterus.


At the time a baby girl is born, her two ovaries already contain all the egg cells she might ever need. Of the million or so eggs in her ovaries, only about 300 to 500 will actually be released (ovulated) during the years between puberty and menopause.

As a girl approaches puberty, her body steps up the production of the hormones that permit ovulation and reproduction. The most important hormones are estrogen, progesterone, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and gonadotropin-re-leasing hormone (GnRH). The production of each of these hormones increases and decreases in a regular pattern every month. During this cycle, changes in hormone levels cause an egg in one ovary to mature. Each egg develops within a follicle, a hollow, fluid-filled sphere of cells. When an egg is ready for fertilization, the follicle breaks open and the egg floats out and normally is captured by the fimbria. Usually only one egg cell develops each month; if two eggs mature and both are fertilized successfully, the result is fraternal twins. (Identical twins result when one fertilized egg cell divides into two.)

Ovulation usually takes place approximately halfway between menstrual periods, most commonly 14 days before the next menstrual period, although this can vary greatly from woman to woman and from month to month (see Chapter 14 on cycle-based fertility control). While the egg is maturing, the increased levels of hormones cause the lining of the uterus to prepare for nurturing a fertilized egg.

Physical and emotional events can have a considerable effect on hormone production and changes in hormone levels can alter the menstrual cycle; as a result, ovulation can occur at an unexpected time. In some instances, ovulation has been known to take place even during menstruation.

Some experts believe that sperm can survive in the reproductive tract for up to 7 days, and an egg cell can be fertilized anytime during the 24 hours after it has left the ovary. No one knows precisely how long eggs and sperm live in the human reproductive system and, therefore, fertilization is considered possible for about an 8-day span in each menstrual cycle. If an egg is unfertilized, it simply is absorbed by the uterus or flushed out with the menstrual blood.


In order to be fertilized, the egg must be penetrated by a single sperm. Although each ejaculate contains millions of sperm, usually only a few hundred reach the fallopian tubes, a journey that takes healthy sperm only a few minutes. If ovulation has recently occurred, the sperm will encounter an egg cell ready for fertilization. Egg cells are covered by a thick, tough, transparent layer called the zona pellucida, which functions as a sophisticated biological security system. The zona pellucida chemically controls the entry of sperm into the egg. Although there may be hundreds of tail-lashing sperm clustered around the egg, only one actually succeeds in penetrating that outer layer. As soon as it does, a chemical reaction automatically shuts out the rest. This reaction prevents the genetic confusion that would occur if the chromosomes of more than one sperm combined with the chromosomes of the egg.

When fertilization takes place, the genetic materials from the sperm and egg combine, and the egg cell begins to divide. While the egg is in the fallopian tube, there is a considerable risk that it may fail to develop. If it survives, it continues to divide, and by the time it reaches the uterus it is a cluster of cells the size of a speck of dust. By the sixth or seventh day after ovulation, the cluster begins to embed itself in the lining of the uterus, which has become thick, soft, and engorged with blood in preparation for nurturing an embryo. The egg cells continue to divide and by the eighteenth day after fertilization the cells that are destined to form the spinal cord can be detected. At this point the cluster of cells can be called a true embryo.


If the fallopian tube is abnormal in some way or damaged from pelvic inflammatory disease, the fertilized egg may not be able to travel through the tube to the uterus. Instead, it may start to grow in the fallopian tube, and the resulting pregnancy is called tubal, or ectopic. ("Ectopic" means "out of place.") Because fallopian tubes are not designed to sustain a pregnancy, the growing embryo usually ruptures or otherwise damages the tube and the tissues around it, causing severe abdominal pain and vaginal bleeding. Tubal pregnancies can be a life-threatening problem, requiring surgery to remove the embryo and to repair, if possible, the torn or scarred tissue. In the United States, about 1 in 100 pregnancies is ectopic, probably because of the relatively high rates of STDs and the pelvic inflammation that follows STDs.


If fertilization does not take place and no cluster of egg cells is implanted in the uterus, the production of hormones declines and the blood-filled endometrium, no longer needed to nourish a possible embryo, breaks up. Over several days it is expelled from the uterus, sometimes with the help of contractions that may range from mild to painful. This shedding of tissue and blood is called menstruation, or a monthly period. The monthly sequence of events is called the menstrual cycle and its sole purpose is to prepare your body for a probable pregnancy.

This cycle of events occurs every month unless an egg is fertilized and successfully attaches to the uterus lining. When such an attachment takes place, the lining is not expelled, there is no menstrual bleeding, and a pregnancy is under way.


When a woman becomes sexually aroused, a cascade of changes takes place throughout her body. Blood flow to the genital area increases, and her clitoris swells, becomes erect, and sometimes is so sensitive it hurts when it is touched. The labia minora swell and deepen in color, the vagina becomes moist, and its opening widens. The entire genital area seems to increase in sensitivity. In addition, the breasts swell and the nipples become erect. The woman's heart rate speeds up, she breathes faster and harder, and her muscles tighten. If sexual stimulation continues, arousal intensifies and leads to an orgasm. During orgasm, the muscles around the vagina, uterus, and rectum contract once or several times and then release. Afterwards, the process slowly begins to reverse. Muscles relax, blood flows out of the swollen tissues, and the clitoris and vagina return to their usual size.

When a man is sexually stimulated, his body undergoes many of the same physical changes. As we mentioned earlier, his penis becomes erect as blood flows into its tissues and stays there because the valves of the penile blood system have closed. His pelvic muscles tighten and force semen from the prostate and epididymis into the urethra. If stimulation continues, it triggers ejaculation. Afterward, his muscles relax, the valves in the blood vessels open, the blood drains away, and the penis becomes soft again.

After orgasm, most men experience an interval in which they have no physical response to further sexual stimulation. Women may or may not feel this same relaxation. Some women, if stimulation continues, experience multiple orgasms.


The various methods of birth control function at different points in the reproductive process. They either suppress ovulation entirely, stop the sperm and egg from meeting in the fallopian tubes, or create an environment hostile to fertilization and implantation.

• Barrier contraceptives such as condoms, the diaphragm, and the cervical cap physically block sperm from entering the cervix.

• The chemical spermicide in creams, jellies, foams, vaginal suppositories, and contraceptive film kills sperm upon contact.

• Birth control pills, Depo-Provera, and Norplant implants alter the normal levels of the hormones estrogen and progesterone in women. Pills combining small amounts of the synthetic forms of both hormones suppress ovulation. Norplant and pills containing only a synthetic progesterone-like hormone may also suppress ovulation but not consistently. These single-hormone methods thicken the cervical mucus to make it impervious to sperm. They also hinder the normal monthly changes in the lining of the uterus.

• Intrauterine devices (IUDs) with copper work by reducing the number and viability of sperm reaching the egg and by impeding the movement of the egg into the uterus.

• Sterilization is surgery to block the woman's fallopian or the man's vas deferens tubes, preventing egg and sperm from uniting.

• Fertility awareness methods teach a woman how to know when she is fertile. During her fertile days she and her partner either abstain from sex or use a barrier contraceptive.

• If unprotected intercourse takes place during a time a woman might be fertile, an emergency contraception method can prevent implantation if it is used soon after that intercourse (see Chapter 16 on emergency contraception).

• Abortion ends a pregnancy either via a suction procedure during the first 12 weeks (first trimester) or, in the second trimester, by using dilation and evacuation (D&E). Medical abortions that depend on a combination of drugs are becoming available for early first trimester abortions.

part one

Barrier Methods

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