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Male Reproductive System
Testes
The testes, which are contained in a pouch of skin called the scrotum, are located outside of the body because they require a lower temperature than the rest of the body in order to accomplish one of their main functions: the production of sperm cells (spermatogenesis). If the temperature of the scrotum rises by only a few degrees, the process of spermatogenesis may be seriously impaired. Each of the two testes are suspended from the body and held in place by a spermatic cord; the skin of the scrotum contains numerous sweat glands that assist in the cooling process.
The Penis
In the final phase of their journey, the sperm cells pass through the urethral duct, which runs through the center of the penis. The penis contains a large number of arteries, veins, and small blood vessels as well as erectile tissue, the last of which consists of three hollow, spongelike cylinders of tissue. When a male has an erection, these spongy tissues fill with blood and become firm. Unlike a number of other mammals--for instance, the whale--no bones are located within the human penis. Erection is caused solely by the relaxation of the blood vessels within the penis. Thus, when a man is under emotional or physical stress, he may experience some difficulty achieving a firm erection because the blood vessels may not relax sufficiently.
Female Reproductive System
The Uterus
The uterus, or womb, is about the size and shape of an inverted pear in women who have never borne children. During pregnancy, this muscular container increases enormously in size and weight. After childbirth the uterus tends to remain slightly larger in size but retains the same shape.
The bulk of the uterus is a network of dense muscular fibers interlaced in all directions; the contractions of childbirth begin when these muscles work to move the fetus toward the vagina.
At the bottom of the uterus is the cervix, a passageway between the uterus and vagina. It is normally very small and, at times, is blocked entirely by mucus that is secreted by the cervical glands. During birth, however, it opens wide enough to allow for passage of the fetus.
The Vagina
Like the uterus, the vagina is normally a small organ with little internal space: in the resting state, the walls of the vagina are touching each another. Also like the uterus, the muscles of the vagina are able to expand greatly during intercourse or childbirth, after which they return to their normal size. In shape, the vagina resembles an elongated "S" and is usually about 10 cm (4 in) long in the mature female. The vagina, in addition to muscular tissue, contains a rich network of blood vessels; when a woman is sexually aroused this network fills with blood in much the same way that the male's penis becomes erect due to increased blood flow. In turn, the pressure of this blood causes the mucous lining of the vagina to secrete drops of fluid; this lubrication response is a primary sign of female sexual arousal.
Female external sexual anatomy consists of the labia majora, labia minora, and clitoris. The labia majora, or outer lips, are two folds of skin that normally enclose the external genitals. The labia minora, or inner lips, are two smaller skin folds containing a rich network of blood vessels. They are normally pinkish in color but may show a variety of color changes when a woman is sexually stimulated.
The Clitoris
According to sex researchers William Masters and Virginia Johnson, the clitoris is a unique organ in human anatomy, having as its major purpose the sensation of sexual pleasure.
The clitoris, like the penis, to which it is homologous, is made up of erectile tissue--that is, when a woman is sexually stimulated, the clitoris fills with blood and becomes firm. Normally, the clitoral body is covered by a fold of skin called the clitoral hood.
In attempting to understand female sexuality, many people at one time thought that the size of the clitoris was related to the intensity of a woman's sexual response. Another theory suggested that the distance between the clitoris and the vaginal opening determined the ease of reaching orgasm during intercourse. Research, however, has shown that sexual arousal and orgasm may have little relationship with the size, shape, or position of the clitoris.
The ovaries produce and store eggs, the female gametes. An egg, or ovum, is barely visible to the unaided eye, and yet one egg is about 75,000 times larger than a single sperm cell. Each female is born with over 400,000 immature eggs in her ovaries and will not produce any new eggs during her lifetime. In most women only about 400 eggs actually mature. Other structure of the female reproductive system include the Fallopian tubes, the uterus, the cervix, the vagina, and the external genitalia. Females have two Fallopian tubes, one located next to each ovary. Each functions to carry eggs from the ovary to the uterus, and each can be the site of fertilization. The uterus is a muscular structure that functions to house the developing fetus if fertilization occurs. The lower entrance to the uterus is called the cervix. The vagina is a tube leading from the cervix to the outside of the body. It is the canal that accepts the penis durign the intercourse and through which the fetus passes during childbirth. The external genitalia include the inner and outer labia and the clitoris.
Each month the female reproductive system goes through a series of changes called the menstrual cycle. The word menstrual comes from the latin word mensis, which means "month". For most women, the menstrual cycle occurs over the course of about 28 days. During this time, an egg matures and is positioned to meet with a sperm cell in the Fallopian tube. If the egg is not fertilized, it is then discharged.
The menstrual cycle has four phases: 1. the follicular phase, 2. ovulation, 3. the luteal phase, and 4. menstruation
Follicular Phase
The follicular phase begins when the hypothalamus produces a releasing factor. The releasing factor stimulates the anterior lobe of the pituitary gland to release follicle-stimulating hormone, or FSH. FSH is transported via the bloodstream and causes a group of ovarian cells, called a follicle, to form around the egg. Simultaneously, FSH promotes the production of the hormone estrogen, which is released into the bloodstream. Estrogen in turn stimulates the lining of the uterus to thicken. In addition, estrogen causes the pituitary gland to produce luteinizing hormone, or LH. This hormone then causes the maturation of the egg.
Ovulation
When the egg has matured, the follicle moves to the wall of the ovary, ruptures, and releases the ripe egg. This event, called ovulation, is the second stage of the menstrual cycle. During ovulation tiny fingerlike projections draw the egg from the ovary into the Fallopian tube. The egg remains in the Fallopian tube for about four days and can be fertilized during this time. The egg, or zygote if the egg has been fertilized, then moves through the Fallopian tube to the uterus, which is now thick and lined with mucus.
Luteal Phase
During the luteal phase, the ruptured follicle left on the surface of the ovary devalops into a new structure called the corpus luteum. Another hormone produced by the pituitary, luteotropic hormone(LTH), stimulates the corpus luteum to send out steroid hormones, including estradiol and progesterone, which establish an even thicker lining on the inside wall of the uterus. In addition, the increased levels of these hormones in the blood cause the pituitary gland to stop producing FSH. The uterine lining will enable the uterus to nourish and protect the fertilized egg.
Menstruation
If the egg was fertilized in the Fallopian tube, the resulting embryo attaches to the lining of the uterus. If the egg was not fertized, it will not attach to the lining. In this case, the corpus luteum stops producing progesterone and the soft tissues of the uterine lining are sloughed off. These tissues along with blood and the unfertilized egg are discharged in the last stage of the cycle, called menstruation. They exit through the cervix and the vagina. Following menstruation the cycle begins again.
Most women menstruate until around age 45. At this time, called menopause, menstruation ceases because most of a women’s follicles have either matured and ruptured or they have degenerated. Without follicle cells the ovaries cannot secrete enough estrogen or progesterone to maintain the mentsrual cycle. The anterior lobe of the pituitary continues to secrete follicle-stimulating hormone for the remainder of a woman’s life.
Fertilization and development
Fertilization occurs when a sperm combines with an ovum in a Fallopian tube. Sperm enters the female during the act of sexual intercourse, in which the male places his penis in the females vagina. Muscular contractions during male orgasm eject semen from the body, through the urethra in the penis, and into the female. Hundreds of millions of sperm cells are released at once.
The egg in the Fallopian tube releases a chemical into the surrounding fluid that attracts the sperm. When sperm reach the egg, the first stick to the jellylike substance on the outer surface of the egg. Many sperm may attach themselves but only one sperm will actually penetrate the surface.
The egg membrane engulfs the head of a single sperm and the sperm nucleus breaks out of the head. Once this has occured, a membrane forms around the egg and prevents any other sperm from entering. The sperm nucleus then fuses with the egg nucleus.
Each gamete contains 23 chromosomes, the haploid (1N) number. Thus the union of sperm and egg causes the zygote to have 46 chromosomes, the diploid (2N) number. The presence of the diploid set of chromosomes initiates embryo devalopment. The development is the result of sequential gene expression.
Immediatly following fertilization, a phase known as cleavage occurs in which the zygote goes through many mitotic cell divisions while still in the fallopian tube. These divisions produce a ball of cells called morula. As the cells of the morula divide, they release fluid into the center of the sphere. The structure is now called a blastocysts, a sphere of cells with a large, fluid-filled cavity at its center. The outer layer of cells of the blastocysts, called the trophoblast, releases an enzyme that breaks down the epithelial tissue of the uterus and enables the blastocysts to embed itself in the thick lining. This process is called implantation.
The embryo will form a mass of cells on the inner surface of the blastocysts. This inner mass of cells has three primary germ layers: the ectoderm. the mesoderm, and the endodemr. These three layers will eventually form all the body organs. The ectoderm forms the skin, the skin glands, most cartilage, the nervous system, the pituitary gland, the lining from the mouth to the pharynx, a portion of the lining of the rectum, and the adrenal medulla. The mesoderm forms the connective tissue, the bone, most muscles, the kidneys and their ducts, the gonads and their ducts, the blood and blood vessels, the hearth, and the lymph system. The endoderm forms the lining of the alimentary canal from the pharynx to the rectum, the thyroid, the parathyroids, the trachea, the lungs, the bladder, and the liver.
The fertilized egg devalops inside the uterus during a nine-month period called gestation or pregnancy. In the first eight weeks of its devalopment, the organism is reffered to as an embryo. From eight weeks until birth, it is called a fetus. Physicians commonly divide a pregnancy into three equal periods called trimesters.
Through pregnancy the devaloping child is surounded and protected by four membranes that develop from the trophoblast. The first of these membranes is called the chorion. This membrane possesses a number of small, fingerlike projections called chorionic villi. The area where the chorionic villi meet the maternal blood supply is called the placenta.
A second membrane, called the amion, is a fluid-filled sac that surrounds the devaloping embryo. This sac is filled with amniotic fluid, which cushions the embryo and keeps it moist.
A third membrane is the yolk sac. The yolk sac provides some nourishment during early embryo devalopment.
The fourth membrane surrounding the embryo is the allantois. Along with the chorion, the allantois lenghtens to become the umbilical cord, a cord that contains arteries and veins that carry blood between the embryo and the placenta.
Cells within the placenta secrete chorionic gonadotropic hormone, which keeps the corpus luteum functioning. The corpus luteum produces estradiol and progesterone, two hormones that maintain the thick lining of the uterus. The high estrogen levels also stop folicle-stimulating hormone and luteinizing hormone production by the pituitary. As a result ovulation and menstruation do not occur during pregnancy.
The placenta is the life supporting link between mother and fetus. Most of what the mother ingests passes into the fluid that surrounds the capillaries in the placenta. These substances can then pass into the capillaries and into the fetal blood supply.
The most dramatic changes in the devalopment of a child take place in its first six to eight weeks. In the first two to three weeks, the human embryo resembles the embryos of other animals. But by the fifth week human features exist.
The brain, spinal cord, and nervous system begin forming in the third week. The hearth begins to beat at 21 days and develops a smooth rhythm at 28 days. By the fifth week eyes, ears, nasal organs, arms, legs, and the digestive system begin to devalop. At six weeks the fingers, toes, and the external ears form. Brain waves occur at this time. The embryo also begins to move, althought it is so small the mother cannot feel it turning.
When the first trimester ends the embryo is only about 5 cm long, but most of its organ systems are established. Throughout its devalopment the embryo, and later the fetus, is sensitive to viruses or toxins that may be passed to it from the mother.
In the second trimester the mother’s abdomen begins to swell as her uterus enlarges. The fetus skeleton begins to form and a layer of soft hair grows over its skin. The fetal hearhtbeat can be heard with the aid of a stethoscope. At this time the fetus also begins to wake and sleep. The mother may now fell the fetus start moving about. The fetus swallows, hiccups, sucks its thumb, and makes a fist. It also kicks its feet and curls its toes. By the end of the second trimester the fetus is about 32 cm long and its eyes are open.
In the third trimester the fetus is becoming modified to survive in the outside world. It grows quickly in size and weight, and devalops fat deposists that give it a rounded, less wrinkled appearence. The fetus can see light and darkness through its mothers abdominal wall and reacts to music and loud sounds. Ultrasound studies have led some researchers to beleive that learning may take place before birth occurs.
Birth begins about 270 days after the egg has been fertilized by the sperm cell. The pituitary gland of the fetus, prostaglandins in the fetal membranes, and glands within the mothers body all release hormones that initiate childbirth. Oxytoxin is secreted in large amounts, causing the smooth muscles of the uterus to contract. The amniotic sac breaks, and the fluid it contains flows out through the vagina. This is called breaking water. The tissues of the normally narrow cervix relax and enlarge, making it easier for the fetus to pass through. During the birth process, strong uterine contractions help push the baby through the cervix, out of the uterus, down the greatly expanded vagina, and out through the external genitalia.
Following birth the umbilical cord is tied and then cut. At this time, the baby’s lungs expand for the first time and the newborn begins to breathe on its own. The remains of the placenta and the amnion, the afterbirth, are then expelled from the mother’s body about ten minutes after the birth of the baby.
The male reproductive system basically is designed to produce and transport sperm cells. At the same time, the male genitals play an obvious role in sexual behavior, because reproduction cannot take place unless sperm cells are deposited in the female reproductive system. The major organs of the male reproductive system are the testes (testicles), the prostate, the seminal vesicles, the vas deferens, the epididymis, and the penis. Although the bladder empties through a duct that runs through the length of the penis, it is not considered part of the reproductive system.
Spermatogenesis
Within the scrotum, each testis is contained by a thick protective capsule, within which is a network of tightly coiled tubes called the seminiferous tubules; if uncoiled, these tubules would stretch to almost a mile in length. Spermatogenesis takes place within the seminiferous tubules. Sperm production fully occurs usually by the age of 16, even though it can begin before a boy reaches puberty. The male continues to produce sperm throughout his life but with a marked slowing of the process in the later years. A man often is able to father children when he is into his seventies or eighties, but the peak of his fertility is usually earlier in the life cycle. The testes also produce the male hormones, or androgens, in a number of large cells called the interstitial cells of Leydig, which are located between the seminiferous tubules. Like the production of sperm, the manufacture and secretion of these hormones begins about the time of puberty and continues throughout life. The hormones are manufactured in the testes and circulate throughout the body, affecting various organs.
Pathway of the Sperm
After sperm are produced in the seminiferous tubules, they move through the testes into another system of ducts called the epididymis. Although the epididymis is only about 3.8 cm (1.5 in) in length, it is so tightly coiled that it would measure about 12 m (20 ft) in length if extended. The sperm cells remain in this duct system and continue to mature for about 2 weeks and then pass into a longer transportation duct called the vas deferens. The male sterilization procedure, or vasectomy, is named for this duct because the operation involves cutting the vas deferens so that the sperm cells cannot travel from the testes to the penis.
Seminal Fluids
Before the sperm cells reach the penis, they travel through a number of internal organs: the prostate, the seminal vesicles, and Cowper's glands. The major function of these internal organs is to produce fluids that will provide the sperm cells with a nourishing and balanced environment. Only a very small proportion of the male ejaculate is actually made up of sperm cells; the remainder consists of the seminal fluids secreted by these internal organs. Because of this factor, a male who undergoes vasectomy will continue to ejaculate about the same volume of fluid as a fertile male. Although seminal fluids are not absolutely necessary in order for a man to be fertile, these fluids allow the sperm cells to live longer within the acidic environment of the vagina.
Formerly known as venereal diseases, sexually transmitted diseases (abbreviated as STDs) are highly specific infections almost always acquired during sexual contact. Bacteria, protozoans, or fungi that cause STDs generally attack only certain portions of the body, including the genital tract. They do not survive for long periods outside the human host, and transmission by such objects as clothing or toilet seats is extremely rare. The effects of STDs range from mild, annoying symptoms to life-threatening illnesses that spread from the reproductive tract to the entire body. The most serious diseases are AIDS, SYPHILIS, and GONORRHEA. CHLAMYDIA, the most common STD in the United States today, is not life-threatening but can have serious effects and is a leading cause of sterility. Lesser infections include HERPES, CHANCROID, genital warts, cold sores, and vaginitis. None of these diseases is acquired exclusively through sexual contact, but the sexual partners of a person with an STD are likely to be infected as well and should be examined and tested for the disease.
Common Syndromes
People with a sexually transmitted disease may have no symptoms or symptoms that are so mild that they do not prompt the person to seek medical attention. For example, almost half of the women with gonorrhea have no significant symptoms, and up to 3 percent of men will feel perfectly well. Some STDs have overlapping symptoms. Swelling of the lymph nodes in the groin may accompany many sexually transmitted infections and may be tender (herpes, chancroid) or painless (syphilis). Such swelling may be the only manifestation of a relatively rare STD called lymphogranuloma venereum. Unusual or increased discharge from the penis or vagina can be a symptom of a number of STDs, including gonorrhea, chlamydia, and particularly in women, vaginitis, which encompasses three different infections: bacterial, yeast, and TRICHOMONIASIS, which is caused by a protozoan.
Some STDs can cause skin lesions in the genital area. One of the first symptoms of syphilis is the chancre, a relatively painless ulcer found at the site of infection, which is usually, but not always, the genital area. Herpes causes sores in the genital area that are very similar to cold sores. Genital warts, caused by the human papilloma virus, resemble warts on other parts of the body and can be found both internally and externally in the genital region. Women who have had genital warts are at greater risk for cervical cancer.
Large, painful genital ulcers are symptomatic of the bacterial infection chancroid, common in Southeast Asia but rare in the United States. Small bumps around the genitals may be molluscum contagiosum, a viral disease spread by contact; this infection is also acquired nonvenereally by young children. Itchy, crusted scabies lesions result from infestation with the mite Sarcoptes scabiei, which burrows into the superficial layers of the skin. Itching in the pubic-hair region may result from infestation with Phthirus pubis, the crab louse, which is spread by close contact.
There are some sexually transmitted diseases that do not necessarily cause symptoms in the genital area. For example, some forms of infectious hepatitis can be acquired by sexual contact, and gastrointestinal infections such as shigellosis and amebiasis have also been sexually transmitted. AIDS is considered an STD because it is transmitted by the exchange of bodily fluids, which occurs during sexual intercourse.
Prevention
Any sexually active person runs the risk of acquiring an STD, and the risk increases with the number of sexual partners. A person having sex with a single individual but whose partner has multiple contacts is at high risk for acquiring a sexually transmitted infection. Because STDs may be present without any symptoms, asking partners about symptoms does not assure good health.
Mutual fidelity between sexual partners will keep the pair free of sexually transmitted infections; no other method provides complete protection against sexually transmitted diseases. If a person has multiple sexual partners, safe sex, in which no bodily fluids are exchanged, can prevent the transmitting of AIDS as well as other STDs. The condom is partially effective if it is worn during all genital contact, but some of the genital area is still left uncovered. Washing the genitals or urinating immediately after intercourse also offers incomplete protection. Individuals with multiple sexual partners should probably be examined for STDs at regular intervals.
