What is the reproductive system?

Structure and Functions

The reproductive system in each sex includes the organs that produce the gametes, called the gonads, and those that transport the gametes. In addition, the female mammary glands are also considered reproductive organs since they produce milk to nourish the newborn, a critical step in survival of the species.

The male gonads are the testes, which are located within the scrotum, a pouch of skin and muscle that is suspended from the body wall. In the adult, each egg-shaped testis measures about 2.5 centimeters by 4.0 centimeters. Internally, the testes contain seminiferous tubules, hollow tubes in which the
sperm develop. Besides the sperm cells, the seminiferous tubules also contain Sertoli cells, large cells in which the developing sperm are embedded. The Sertoli cells produce hormones, pass nutrients to the sperm, protect them from blood-borne toxins, and control their development. In spaces between adjacent seminiferous tubules are the interstitial cells of Leydig, which produce testosterone and other hormones.

Lying near each testis within the scrotum is the epididymis. Sperm undergo several stages of development within the testis, then move into the epididymis, where they proceed through further steps in maturation, including the development of the swimming ability that is necessary for fertilization of an ovum.

The narrow end of the epididymis is continuous with the long (45-centimeter) tubule called the vas deferens. The vas deferens leads upward from the epididymis and passes through a narrow ring of tissue, the inguinal canal, to enter the abdominal cavity. Within the abdominal cavity, the vas deferens loops over the top of the bladder, then turns downward to enter the prostate gland below the bladder. Near the end of the vas deferens, an enlarged area called the ampulla serves as a storage site for mature sperm.

Within the prostate gland, the vas deferens becomes a short segment of tubule known as the ejaculatory duct. The two ejaculatory ducts (one on each side) empty into the urethra, the tube that extends downward from the bladder. In the male, the
urethra carries either semen or urine, but not both at the same time: A valve-like structure below the bladder prohibits urine outflow when semen is moving through the system. The urethra passes through the penis to open to the outside at the tip of the penis.

Besides the urethra, the penis contains three columns of erectile tissue, spongy material with a large blood supply. During sexual excitement, the blood flow into the erectile tissue increases while the outflow decreases; the accumulation of blood within the erectile tissue causes the penis to increase in both length and diameter, a process known as erection. Erection allows the penis to become stiff enough to be inserted into the female’s vagina during intercourse. On the outside of the penis, the enlarged area at the tip, the glans penis, is well supplied with touch receptors that play a role in sexual excitement.

During intercourse, stimulation of the touch receptors on the penis results in ejaculation, the expulsion of semen from the male’s body. During ejaculation, sperm move out of the vasa deferentia, through the ejaculatory ducts, and then through the urethra. This sperm movement is caused not by the swimming of the sperm but by the contractions of involuntary muscles in the walls of the tubules. As sperm pass through the tubules, they are mixed with fluid secreted by three glands: the prostate, located immediately below the bladder; the seminal vesicles, which open into the vas deferens above the prostate; and the bulbourethral or Cowper’s gland, which lies below the prostate and opens into the urethra. The fluid secreted by these glands contains chemicals and nutrients that will ensure the survival of the sperm within the female tract.

The female’s vagina serves as the repository for sperm released during ejaculation and as the outlet for the fetus during childbirth. The outer opening of the vagina is located behind the urethra, which carries only urine and does not have a reproductive role in the female. Bartholin’s and Skene’s glands are located near the urethral and vaginal openings; these glands supply moisture and mucus to the female external genitals. The vaginal and urethral openings are located between folds of tissue, the labia majora and the labia minora. At the front junction of these folds is the clitoris, a small round structure containing many touch receptors; stimulation of the clitoris during intercourse is important in promoting sexual gratification in the female. The area that includes the labia, the vaginal and urethral openings, and the clitoris is known as
the vulva.

Internally, the vagina consists of a recess with elastic walls and a large blood supply, but little sense of feeling since there are only sparse touch receptors. The vagina slants upward and slightly backward from its outer opening. Near its upper end is the cervix, the lowest portion of the uterus. The cervix consists of strong connective tissue and contains glands that secrete mucus. The cervix has a narrow passageway, the cervical canal, that opens into the main part of the uterus.

The
uterus is about 7.5 centimeters long and 5.0 centimeters wide in the nonpregnant woman. The wall of the uterus is composed primarily of involuntary muscle controlled by nerves and hormones. The inner part of the uterus is hollow and is lined with a spongy layer of cells, the endometrium; the endometrium has a large blood supply and contains glands that secrete nutrients for the embryo during pregnancy. The endometrium undergoes growth during the menstrual cycle and is shed as the menstrual discharge if the woman does not become pregnant.

At either side of the upper end of the uterus are the oviducts, or Fallopian tubes, which are hollow tubes that open into the cavity of the uterus. The oviducts lead upward and sideways away from the uterus toward the ovaries, with their funnel-shaped ends adjacent, but not attached, to the ovaries.

The
ovaries are the female gonads; they produce ova, the female gametes. Each 3-centimeter-long ovary contains thousands of follicles, spherical structures that each contain one ovum. Hormonal signals cause growth of some of the follicles during the menstrual cycle, and, as they grow, the ova within them mature. The follicles are also sites of production of the hormones estrogen and progesterone. In the middle of the menstrual cycle, one follicle will ovulate, releasing its ovum, which will enter the oviduct to be transported toward the uterus.

During intercourse, after sperm are deposited in the vagina, the sperm will swim in the fluids of the female tract, passing upward through the cervical canal, the uterus, and the oviducts. If an ovum is present in one of the oviducts, it may be fertilized by a sperm. The fertilized ovum will then move downward to the uterus, where it will attach to the endometrium and develop into an embryo. At birth, uterine muscle contractions will cause stretching of the cervical canal and movement of the fetus through the cervix and vagina.

Milk production (lactation) in the woman’s breasts after childbirth will allow for the nourishment of the newborn. Milk is produced in glands within the breast; ducts carry the milk to openings in the nipple. In between the milk-producing glands are wedges of fat; it is the fat tissue that determines the size of the breast in the nonpregnant woman. Breast size is not related to milk-producing ability.

Disorders and Diseases

Abnormalities may exist in either the male or the female reproductive tract as a result of deviations during embryonic development, injury, or disease. Anatomical abnormalities in the reproductive system often can be corrected surgically.

In hypospadias, a problem during embryonic development of the male reproductive organs causes the urethral opening to be on the underside of the penis rather than at its tip. Hypospadias can occur independently or can be a sign of more serious problems. Urethral stricture or stenosis refers to a narrowing of the urethra; this can occur anywhere along its length, from the tip of the penis back to the prostate gland. Urethral stenosis causes difficulty in urination; it may be present from birth or result from later damage or infection. Cryptorchidism is the presence of one or both testes in the abdominal cavity instead of in the scrotum. In the male embryo, the testes begin development in the body cavity near the kidneys and then migrate into the scrotum during the last month or two before birth. In some male infants born with undescended testes, the testes will spontaneously move into the scrotum shortly after birth. If
not, then the cryptorchidism must be corrected surgically, usually within the first year of life, in order to prevent later infertility and other complications.

In females, problems during embryonic development can also lead to malformed reproductive organs. The vagina may be present but may not have an outer opening, or conversely, the outer opening may lead to an abnormally shallow vagina. The uterus may be divided into two separate halves (bicornuate uterus), and the vagina may also show such a division. It is also possible for a normal uterus to have an abnormal placement in the abdomen: It may be tilted backward or bent forward at an atypical angle. Surprisingly, variations in the anatomy of the uterus often have little effect on fertility.

Malfunctions in embryonic development of the reproductive organs can lead to hermaphroditism, a rare condition in which an individual has a mixture of male and female reproductive organs. A hermaphrodite may be either a genetic male or a genetic female. Hormone treatment and surgery can usually assure a fulfilling sex life in adulthood for such individuals.

In
inguinal hernia, the wall of the inguinal canal between the scrotum and abdominal cavity becomes weakened and stretched. A loop of the intestine may then become lodged in the canal, or the testis and epididymis may move upward to block the canal. The symptoms of inguinal hernia include pain during movement, especially during the lifting of heavy objects, and the presence of a soft lump in the herniated area.

A varicocele is a group of enlarged blood vessels within the scrotum. Varicoceles are thought to arise because of impaired blood flow from the testis. Normally, the blood flowing through the scrotum maintains the temperature of the testes a few degrees below that of the rest of the body. When the blood flow out of the scrotum is reduced in the presence of a varicocele, the temperature in the testes tends to increase. This increased temperature can cause infertility, since sperm production requires a local temperature that is lower than the normal body temperature. Increased scrotal temperature can be the cause of infertility in other situations as well: the wearing of tight clothing, prolonged soaking in hot water, or during episodes of fever. Infertility in these situations is usually temporary and self-correcting.

In women, stretching of the pelvic area during childbirth sometimes leads to
uterine prolapse, a condition in which the uterus sags into the vagina. Other causes of prolapse are developmental abnormalities, lifting heavy objects, and loss of muscle and tissue strength with aging. A prolapsed uterus is associated with pain during intercourse and may cause difficulty in urination. Temporary relief from a uterine prolapse can be achieved with the use of a pessary, a device worn in the vagina to support the uterus. Surgery to restore the supporting tissues in the pelvis may be necessary for long-term relief, but in some cases the uterus cannot be returned to its proper position and so must be removed, a process called hysterectomy.

In endometriosis, patches of endometrial tissue from the uterine lining attach to and grow on other organs in the pelvic cavity, affecting the shape and function of these organs. It is thought that the endometrium escapes into the pelvis through the oviducts during menstruation. The abnormally placed endometrial tissue can cause pain and infertility. Endometriosis can be treated with hormone therapy or with surgery to remove the endometrial patches. In more severe cases and when the woman does not wish to bear children in the future, the removal of the uterus may be required to control the invasion of other organs by the endometrial patches. The ovaries may also be removed in order to eliminate the source of hormones that produce the growth of the endometrium.

In polycystic ovary syndrome, one or both ovaries contain cysts that are formed from follicles that have failed to ovulate. Women with polycystic ovaries either do not menstruate or have irregular patterns of bleeding. Because ovulation does not occur, they are infertile. Another symptom of polycystic ovary syndrome is growth of hair in a male pattern on the face, neck, and chest; the hair growth is caused by certain hormones that are produced in abnormal amounts by the ovaries. Therapy usually involves hormone treatment in an attempt to establish ovulation and to prevent the deleterious effects of abnormal hormone levels on the body.

Both benign (nonspreading) and cancerous tumors may appear in the reproductive organs. Potential sites of tumor growth are the testes and prostate gland in the male and the ovaries, uterus, and breasts in the female. Only rarely do tumors cause pain, but they may be detected during routine self-examination of the testes or breasts or during a doctor’s examination. Treatment usually begins with surgery to remove the tumor, and x-ray therapy or chemotherapy prevents further tumor growth. Hormone treatment can also be useful in controlling the growth of some reproductive tumors. The exact factors that cause tumors to form are not well understood, but a family history of such problems, abnormal hormone levels, and exposure to radiation, pollutants, and toxins have all been implicated.

Prostate tumors should not be confused with nodular prostatic hyperplasia, an increase in the size of the prostate gland that occurs in about 75 percent of men over sixty years old. The enlargement appears to be caused by dihydrotestosterone (DHT), a hormone related to testosterone; the prostate gland itself is one site of conversion of testosterone into DHT. Prostate enlargement is associated with difficulty during urination and ejaculation. It has traditionally been managed with surgery to remove the gland or to reduce its size. More recently, several drugs have been approved to reduce the production of DHT and either prevent the progression of growth of the prostate or shrink the prostate in some individuals..

Perspective and Prospects

Rituals involving alteration of the reproductive organs have been performed since ancient times. Castration (removal of the testes) has been carried out for various reasons. In early times, men who were guards of noble women were castrated to control their sexual activity. During the Renaissance, castration of boys was performed to produce singers who would retain their clear, high-pitched voices, since it is testosterone that causes the deepening of the voice at puberty. More recently, castration has been espoused as a “treatment” for habitual rapists: Some judges sentence convicted sex offenders to castration, despite the fact that many authorities believe that rape is a manifestation of violent tendencies, rather than the result of excessive sexual desire.

In the United States and elsewhere, it is common for boys to undergo circumcision, or the removal of the foreskin, a flap of tissue that covers the glans of the penis. Parents have their sons circumcised in order to conform to religious and cultural practices. In the United States, the procedure is usually performed shortly after the boy’s birth, but in other cultures circumcision may occur during puberty rituals. One rationalization for performing circumcision is that removal of the foreskin helps to prevent the buildup of smegma, a thick secretion produced by glands located under the base of the foreskin. In fact, some studies have shown that circumcised boys are less likely to have urinary tract infections. There are no apparent effects of circumcision on sexual functioning. Researchers debate the advisability of circumcision, however, and the medical establishment has not established a definitive recommendation.

It is less well known that circumcision of women is practiced in some cultures. Indigenous groups who perform female circumcision
are found in the Pacific Islands, Asia, the Middle East, and Africa, and the practice has been carried to the United States by immigrants. The term female circumcision refers to three procedures that may be carried out singly or together. In simple circumcision, the flap of tissue covering the clitoris is removed. The entire visible part of the clitoris is removed in clitoridectomy. Infibulation is the sewing together of the labia to cover the vaginal opening, leaving only a small hole for the discharge of urine and menstrual fluid. A woman who has been infibulated cannot have intercourse or give birth; the tissue must be cut open to allow either of these events, after which the area may be sewn closed again.

Female circumcision may take place shortly after a girl’s birth or at puberty, and it is performed for a variety of reasons. Infibulation is a means of enforcing female abstinence from sexual activity. A wish to control women’s sexual desire is also given as a reason for performing simple circumcision and clitoridectomy. Also involved are the society’s views of what the ideal female organs should look like. From a medical standpoint, female circumcision is of concern because of pain and discomfort caused by the development of scar tissue in the vulval area. It is not known how many women die from infection or bleeding following these procedures, which are usually performed by other women under less-than-sanitary conditions.

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