Introduction

The reproductive system is the only body system that's completely unique to either males or females. Both reproductive systems depend on each other for the procreation process.

Reproductive Characteristics

The reproductive organs of both sexes are designed to allow fertilization of the germ or "seed" cells called gametes. The female gametes are called ova, plural for ovum or egg. The male gametes are called spermatozoa, or sperm. Gametes are produced in organs called gonads, specifically the ovaries and testes, which are the essential reproductive organs.

The reproductive system has primary and secondary sex characteristics. Primary sex characteristics are directly related to the growth and function of the reproductive organs themselves. Secondary sex characteristics refer to those "masculine" and "feminine" body features, such as beard growth and breast development. The hormones that develop all these characteristics also cause the sex drive, leading men and women to reproduce.

The complex human organism constantly grows and changes. Humans grow from embryo, fetus, infancy, and childhood to adolescence, young adulthood, middle age (40–65), and old age (after 65). Amazingly, it all begins with fertilization.

Fertilization is the fusing of ovum and sperm. Ova and sperm cells differ from other body cells in that they carry only half the usual number of chromosomes. When fertilization occurs, and the gametes from the male combine with the gametes from the female, the hereditary material for a new individual is complete.

At this point, the male has contributed his reproductive component. The female system has only just begun. It provides nourishment to the developing life for the approximately 40-week gestation period inside the female's uterus. Once the baby is born, the female reproductive system lactates. This lactation provides milk, the perfect nutrition for the child's early growth.

The development of both males and females is essentially identical except for the development of the reproductive organs. Early in the gestation period, the fetal reproductive organs are said to be undifferentiated, meaning the fetus is neither male nor female, and the basic structures have the potential to become either sex. Differentiation begins at two months of fetal age. The fetus will then take on either male or female reproductive characteristics.

The Male Reproductive System

Let's look at the male reproductive system and its organs. The penis and scrotum are called external genitalia. They're located on the outside of the body.

The remaining male reproductive organs are all inside the body. Internal genitalia include the testes, duct system, and accessory glands.

The male urinary system and the male reproductive system are intertwined. The specialty area of medicine that treats male urinary and reproductive disorders is called urology.

The penis is the male sex organ composed of three parts:

A shaft
A tip called the glans
A cuff of skin called the prepuce (or foreskin)

Sexual arousal causes two important reflex actions of the penis: erection and ejaculation. Erection of the penis is made possible by this organ's porous tissue. Arteries bring blood into the penis. This pressure squeezes against the veins, preventing the blood from going back out again. The penis becomes engorged with blood, causing the erectile tissue of the penis to become firm. Ejaculation of semen occurs when sexual arousal reaches a peak, known as orgasm.

A skin-covered sac called the scrotum contains two testes (or testicles), which produce millions of sperm cells each day. This organ is located in a vulnerable position, not encased in the safety of the pelvic cavity. There is a good reason for its location. The body's internal temperature is too hot for spermatogenesis, or the creation of spermatozoa. The scrotum provides a cooler environment. It does this by adjusting position relative to the body to maintain the ideal temperature.

The teste originally develop within the abdominal cavity of the male embryo. A couple of months before birth, they descend into the scrotum through a passageway called the inguinal canal. The inguinal canal shrinks shortly after birth. This leaves just enough room for the spermatic cord to pass through, but not enough for the testes to return to the abdomen.

Each testis has about 250 lobules. They hold the seminiferous tubules, threadlike coils where the sperm-manufacturing cells are located.

Leydig cells in the testes produce a class of hormones called androgens. These hormones are produced in response to the luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are both secreted by the pituitary gland. The most important androgen produced by these cells is testosterone, the male sex hormone.

At age 12 or 13 is the typical period of sexual maturation known as puberty. At this age, testosterone stimulates the development of secondary sexual characteristics, such as a deep voice, broad chest, and growth of facial hair.

A male begins producing sperm at puberty. He continues to produce sperm and testosterone throughout his lifetime.

Once the sperm is produced, it's secreted from the seminiferous tubules into the epididymis, a 16-foot-long tube coiled tightly into a two-inch space along the length of the testicle.

A single sperm looks like a microscopic tadpole. Its oblong head region contains genetic material. The tail, called a flagellum, enables the sperm to "swim" through the female reproductive tract. Each ejaculation of semen contains approximately one teaspoon's worth of fluid, which contains 300 million sperm.

The duct system includes four parts:

Epididymis
Vas deferens
Urethra
Ejaculatory ducts

The main function of these ducts is to transport sperm from the testes to the urethra for ejaculation as semen. The epididymis performs two functions:

Storing sperm produced in the seminiferous tubules
Continuing to develop the sperm

When sperm enter the epididymis, they're not fully developed. They're unable to swim or fertilize an egg. They mature while they move through the body of the epididymis. Once matured, they can perform their functions.

The matured sperm is stored in the tail end of the epididymis. The epididymis leads to the two-foot-long vas deferens, also called ductus deferens. The vas deferens is a firm, hollow tube that carries sperm straight through the spermatic cord.

The spermatic cord is a sheath of connective tissue that also encases the blood vessels and nerves on their way to and from the testes. The vas deferens enters the pelvis and crosses over and behind the urinary bladder, where it meets the accessory gland, the seminal vesicle.

As the vas deferens emerges from the seminal vesicle, it joins with the ejaculatory duct in the prostate. The converged ducts pass through the Cowper's glands, and then join the urethra, the tube leading from the bladder to the outside of the body.

From there, the semen (sperm mixed with the seminal fluids) passes out of the body through the urethral orifice or meatus. As you know, the urethra also expels urine from the body, but during ejaculation, the sphincter to the bladder stays closed to keep urine in.

The seminal fluids contained in semen are secreted by the accessory sex glands. Seminal fluid consists of proteins, minerals, fructose, enzymes, mucus, and citric acid.

The main accessory glands of the male reproductive system include seminal vesicles, prostate gland, and Cowper's glands.

A large percentage of seminal fluid comes from two seminal vesicles, small glands located at the base of the bladder. The fluid picked up within the seminal vesicles makes up about 65 percent of the fluid in semen. These thick, yellowish, sugar-rich secretions provide energy for the sperm's long trip through the female reproductive system to reach ovum.

Another part of seminal fluid comes from the prostate gland, a doughnut-shaped structure located just beneath the bladder and surrounding the beginning of the urethra. The prostate closes the urethra during ejaculation, preventing the passage of urine while semen is being ejaculated.

This gland's thin, milky, alkaline secretions help the sperm move along and stay healthy in the acidic environments of the urethra and the vagina. Furthermore, the prostate's musculature helps push the sperm forward during ejaculation.

Finally, a small but important amount of fluid comes from the pea-shaped bulbourethral glands, also called Cowper's glands, located just below the prostate. Their alkaline secretions empty into the urethra to lubricate it so the sperm will have an easy passage.

Male Reproductive System Disorders

Erectile dysfunction (ED), also referred to broadly as impotence, is an ability to achieve or maintain an erection for sexual intercourse. The cause for this condition may be physical (disease, injury, or side effects of certain drugs) or psychological (depression, stress, or fear of sexual failure).

Diseases that may cause erectile dysfunction include hypertension, diabetes, nerve disease, and abnormal hormone production. Certain medications, such as beta blockers and antidepressants, are also known to cause ED.

Impotence is actually any male condition that interferes with reproduction. Besides ED, other forms of it include ejaculation problems, low sperm count, or lack of sexual desire. Sperm production may be inhibited by chemotherapy.

Treatment is usually selected based on the cause of the erectile dysfunction. Many psychological measures are available to treat ED. Healthy lifestyle choices such as smoking cessation, decrease in alcohol consumption, and weight reduction help treat ED.

Some commonly used medications to treat the condition are

Sildenafil citrate (Viagra)
Vardenafil (Levitra)
Tadalafil (Cialis)
Avanafil (Stendra)

While sildenafil and vardenafil are taken 30–60 minutes prior to sexual activity and their effects last for up to 4 hours, tadalafil is longer-acting, lasting for up to 36 hours. Avanafil is faster-acting, usually taken 15 to 30 minutes before sexual activity. All of these medications can cause hypotension (sudden decrease in blood pressure). Patients taking blood pressure medications are recommended to avoid taking these medications. ED medications are contraindicated—meaning advised against due to a conflict with another medication—in patients taking nitrates such as nitroglycerin; taking both can result in hypotension.

Priapism is a disorder in which an erection persists for more than four hours without stimulation. This is a very painful condition and is considered a medical emergency. The condition can be caused by certain medications, such as the drugs given for erectile dysfunction. It can also be caused by the use of some recreational drugs, such as cocaine.

Untreated, this condition can cause damage to blood vessels in the penis, blood clotting, and ischemia (restriction of blood supply), which can lead to gangrene. Priapism generally requires emergency room admission, with treatments requiring aspiration of blood from the penis, medications such as phenylephrine, and surgical interventions when necessary.

The Female Reproductive System

The ovum is considered the largest cell in the female body. A female is born with all of the ova she will ever have—about one million immature sex cells. However, only about 400,000 of those will develop as the female matures, and of those, only 350 to 500 will ripen and be released during the fertility period between puberty and menopause.

Usually, only one ovum a month leaves the ovary and travels down the fallopian tube, where it may meet up with a sperm. This process is called ovulation.

*At times, more than one ovum may be released, especially when using certain fertility treatments; the fertilization of two ova will result in fraternal or dizygotic twins. If one ovum is fertilized and then splits to form two embryos, it will result in identical twins.

Since sperm live for about 48 hours after ejaculation and the ovum lives for only 24 hours after ovulation, intercourse has to occur the day before, the day of, or the day after ovulation for fertilization to occur.

An unfertilized ovum will simply disintegrate. A fertilized ovum will become an embryo and move down to the uterus to implant itself there for the nine-month job of becoming a fully developed baby.

The medical specialty that treats the female reproductive system actually has two components called obstetrics and gynecology. The obstetric component deals with pregnancy and childbirth, whereas the gynecologic component deals with diseases and illnesses affecting the female reproductive tract.

The female reproductive organs include the ovaries, fallopian tubes, uterus, vagina, and accessory glands.

Unlike the male's, the female's reproductive system functions cyclically. Males have a relatively constant hormone level and are always producing sperm. Female hormone levels and ovulation patterns follow a monthly menstrual cycle. Every month, the uterus gets itself ready for pregnancy. Menstruation occurs when pregnancy doesn't happen.

Menstruation is the process of discarding old tissues so that fresh ones can be prepared for the next potential embryo.

The ovaries are located on each side of the uterus. They're shaped like large almonds. Ovaries contain thousands of tiny sacs called Graafian follicles. Each Graafian follicle contains an ovum in varying stages of maturation. After the onset of puberty, once a month, a Graafian follicle ruptures and a mature ovum leaves the ovary for the journey down the fallopian tube. This process is called ovulation.

Then, a very important physiological event happens. The ruptured follicle transforms into a yellow, glandular structure called the corpus luteum, literally meaning "yellow body." The corpus luteum secretes the female hormone progesterone, which plays a major part in the menstrual cycle. Progesterone stimulates the uterus to prepare for pregnancy. During pregnancy, it causes many changes in the body to provide the proper environment for a fetus.

When pregnancy doesn't occur, the corpus luteum stops producing progesterone and decays, forming a mass of scar tissue. Without the supply of progesterone, the uterus can't maintain its lining, so it sloughs it off and menstruation results.

Beginning at puberty, the ovaries start secreting the female sex hormone estrogen, which causes the reproductive organs and the secondary sex characteristics that shape a woman's figure to develop. The secretion of estrogen causes the female's first menstrual cycle, or menarche.

Similar to the male system, the pituitary hormones FSH and LH stimulate the production of female hormones at puberty, and they continue to influence ova formation and ovulation. High levels of progesterone and estrogen in the bloodstream during pregnancy cause the pituitary to shut off its production of FSH and LH, which stops the ovulation process. Birth control pills contain enough estrogen and/or progesterone to trick the pituitary into stopping ovulation.

Even though they're called oviducts, the fallopian tubes aren't actually attached to the ovaries; they open into the peritoneal cavity (the abdomen). Each fallopian tube curves around to the edge of the ovary. Its fingerlike fringed edges, called fimbriae, catch the released ovum. This section of the fallopian tubes is called the infundibulum.

Peristalsis of the fallopian tube, combined with the sweeping movement of hairs called cilia, moves the ovum toward the next portion of the fallopian tube called the ampulla.

If intercourse has taken place in the past day or two and no contraceptive has been used, chances are good that a sperm will fertilize the ovum.

If the ovum is fertilized, it will continue on through the isthmus (the section of fallopian tubes that enters the uterus), exit into the uterus through the intramural oviduct, and attach itself to the uterine wall.

If the ovum wasn't fertilized during its journey through the fallopian tube, it will disintegrate, and the uterus will get the message that fertilization hasn't taken place, initiating the menstrual period.

About the size and shape of a pear, the uterus is mostly muscle—one of the strongest muscles in the human body. The uterus does the important and delicate job of holding, nurturing, and growing the fetus and laboring during childbirth to deliver the baby.

The thin, skin-like outer layer of uterine tissue is called the perimetrium. The middle layer is the muscular part, called the myometrium. The inner lining of the uterus, the endometrium, is a mucous membrane with a rich supply of blood vessels. This layer has two parts: (1) the basal endometrium stays in place all the time, and (2) the functional endometrium develops to prepare for pregnancy, then sloughs away at the end of the menstruation cycle. The rounded, upper portion of the uterus where the fallopian tubes enter is the fundus, and the large center part is called the corpus, or body. The cervix is the rounded bulb at the bottom that protrudes into the innermost portion of the vagina.

The vagina is a muscular yet elastic tube lined with mucous membrane in folds called rugae. It's a passage for the entry of sperm and the exit of menstrual fluid and babies. The vagina opens between the anus and the urethra.

Bartholin's glands, also called greater vestibular glands, secrete mucus-like fluid from duct orifices on either side of the vagina. They're analogous to the bulbourethral or Cowper's glands in the male. A number of smaller vestibular glands secrete mucus near the opening of the urethra.

The breasts are also glands of the reproductive system, attached by connective tissue to the muscles of the chest. Female breast sized is determined by the amount of adipose tissue (fat) that surrounds the actual mammary glands (milk-secreting glands). This fat amount has absolutely nothing to do with the quantity of milk produced by a nursing mother.

The milk-producing mammary gland is actually a modified sweat gland, but it doesn't work full-time like the sweat gland. The mammary gland starts secreting milk three days after parturition (childbirth). During this time, the suckling child will ingest colostrum, a thin, yellow fluid containing protein and lactose (milk sugar) but little fat. Release of the hormone prolactin from the pituitary stimulates the production of milk, and the hormone oxytocin releases the milk from the glands. The milk-secreting tissue is made up of 15 to 20 lobes that converge at the nipple. These lobes are composed of many different smaller lobes, which in turn are made of many milk-secreting cells in grapelike clusters. There's one lactiferous duct per lobe, and each duct empties through the nipple. Thus, there are 15 to 20 pores from which milk leaves the breast. The medical name for the nipple is the mammary papilla, and the pigmented area surrounding the nipple is the areola.

The menstrual cycle is designed to prepare an egg for fertilization with sperm. The process of an ovary ejecting a mature egg into the fallopian tube is known as ovulation, which generally occurs once every 28 days, although the length of this cycle varies among females. The menstrual cycle repeats itself about 13 times per year for 30 or 40 years in most women.

There are four phases of the menstrual cycle:

Even though the menstrual phase is technically the last phase of the cycle, we count it as the beginning because it's the only phase that's measurable without special equipment. The first day of a woman's period counts as the first day of her menstrual cycle. During this time, the uterus sheds the functional endometrium because there was no fertilized ovum to implant. The normal time range for this phase is 3–7 days.
During the follicular or proliferative stage, a Graafian follicle grows an ovum to maturity, and estrogen signals the basal endometrium to start growing a new functional endometrium. The cervix starts to produce less-acidic mucus that will actually assist the sperm in their journey to the fallopian tubes. This phase lasts 6–12 days.
The ovulation phase lasts one or two days and typically occurs midway through the cycle. When the ovum has nearly matured, the estrogen level is high enough to trigger the pituitary gland to release luteinizing hormone (LH). This causes the ovary to release the ovum. As you know, the ovum then enters the fallopian tube and awaits fertilization by sperm. If fertilized, the ovum will immediately begin to develop, still moving through the fallopian tube. It will take about three days to implant into the endometrium.
The luteal phase, or secretory phase, is dominated by the corpus luteum, the "yellow body" created when the ovum left the ovary. The corpus luteum produces hormones to make the endometrium receptive to implantation and early pregnancy. During this phase, a woman's body temperature increases. After ovulation, the hormones FSH and LH sustain the corpus luteum. If the egg is fertilized, the resultant embryo will produce the hormone human chorionic gonadotropin (HCG), which can support the corpus luteum instead of LH. If the LH is too low and there's no embryo to produce HCG, the corpus luteum stops producing hormones to prepare the body for pregnancy. This will be the signal for the endometrium to slough off, starting the cycle all over again. This final phase lasts about two weeks.

Pregnancy starts when a sperm cell reaches the egg and penetrates the epithelial, or outside, layer of the egg and enters it. When the sperm makes it to the center and combines its nucleus with the egg's nucleus, a fertilized egg cell called a zygote is created. As soon as the zygote divides itself into two cells, it's called an embryo. Cell division continues at a rapid pace; by the time the embryo implants itself on the uterine wall a few days later, it's a cluster of 16 cells.

Four weeks later, it has a brain and spinal cord. After the second month, the embryo is referred to as a fetus.

Many hormones are present during pregnancy. Particularly important is human chorionic gonadotropin (HCG). This is the hormone that's produced by the embryo and sustains the corpus luteum in place of LH. It's also the identifying hormone in urine tests for pregnancy.

This hormone signals the corpus luteum to grow larger and produce the massive quantities of estrogen and progesterone needed for the uterus to grow.

Most people consider menopause, which is the cessation of menstruation, to be climacteric or "the change of life." This condition is a normal consequence of aging. In healthy women, the menstrual cycle will continue repeating until menopause, with some interruptions if pregnancies occur.

Menopause usually occurs between the ages of 45 to 55, but it can occur earlier or later. Menopause that occurs before age 35 is considered "premature." If it occurs after age 58, it's considered "delayed."

The cause of menopause is the cessation of the production of eggs and hormones by the ovaries. Without these hormones, the endometrium never receives a signal to start preparing for pregnancy, so there's nothing to slough off at the end of the cycle. Therefore, the menstrual period ends. Once these events have occurred, the woman can no longer bear children.

Menopause, however, isn't an instantaneous event; it occurs gradually over a few years. The woman's menses become less and less frequent because the ovaries release ova erratically. When they do release them, the cycle continues as usual. If another egg isn't matured after the start of menses, the next period will be skipped. Eventually, the ovaries stop producing progesterone and estrogen, and the cycle ceases altogether.

Menopause results in dryness and atrophy of the reproductive organs. Many women experience hot flashes, fatigue, headaches, depression, anxiety, or irritability.

Female Reproductive System Disorders

Endometriosis is a condition where the endometrium (the innermost layer of the uterus) grows out of the uterus. The exact cause of endometriosis is unknown. Some women may have this condition without any symptoms, while others may experience symptoms such as pain during menstruation or during sexual intercourse.

Endometriosis may also interfere with fertility. In younger women who wish to have children, hormone therapy is given as a treatment. Depending on the severity of the condition, the offending tissues may be removed in surgery and a complete hysterectomy performed to prevent a reoccurrence. Some medications that are also useful to treat endometriosis include

Danazol (Danocrine)
Nafarelin (Synarel)
Lupron Depot

Bacteria, viruses, and parasites can cause infection that may ascend from the vagina and cervix, through the uterus and fallopian tubes, all the way to the ovaries. Pelvic inflammatory disease (PID) is characterized by inflammation of the cervix (cervicitis), uterus (endometritis), fallopian tubes (salpingitis), ovaries (oophoritis), and sometimes the connective tissue of the uterus (parametritis). Abdominal pain, fever, and vaginal discharge of pus are all symptoms of PID. The most common cause of PID is gonorrhea, a bacterial infection that's discussed later.

The Trichomonas parasite and a fungus named Candida albicans are common causes of vaginitis. Infection with Trichomonas results in a foul-smelling, whitish-yellowish discharge, or leukorrhea, which in turn causes itching, burning, and soreness in the vaginal area. Candidiasis, more commonly known as a "yeast" infection, causes a clumpy discharge and intense itching, termed pruritus vulvae. Any itching is referred to as pruritus, in medical terms.

Ironically, antibiotics used to combat an infection elsewhere in the body can also lead to vaginitis by killing the harmless microorganisms that live there and keep the harmful population low. Antibiotic-resistant fungi and viruses can then thrive.

Male and Female Reproductive System Disorders

Human immunodeficiency virus (HIV) is a viral disease that's sexually transmitted. You'll learn more about HIV and its treatments in a later discussion of immune system disorders.

Some commonly used HIV drugs include

Zidovudine (Retrovir)
Didanosine (Videx)
Stavudine (Zerit)
Abacavir (Ziagen)
Tenofovir (Viread)
Saquinavir (Invirase)
Indinavir (Crixivan)
Efavirenz (Sustiva)
Lopinavir + Ritonavir (Kaletra)

Gonorrhea is caused by the bacterium Neisseria gonorrhoeae . Men receive more symptoms, such as dysuria (painful urination) and discharge of pus from the penis. Although women may also have dysuria and vaginal discharge, the symptoms are typically milder than those in men. Gonorrhea can cause pelvic inflammatory disease (PID) and infertility in women if left untreated. Gonorrhea can be diagnosed with tests that detect the bacterial genes in a urethral or cervical swab sample.

Infertility is defined as difficulty in conceiving a child. This is a unique medical condition that involves a couple rather than a single individual.

There are many causes of female and male infertility. A woman can be infertile because of an ovulation disorder, endometriosis, tubal disease, uterus abnormalities, chronic infection, or hormone irregularities. A man can be infertile because of low sperm count, a defect in sperm transport, testicular dysfunction, or hormonal irregularities. Infertility treatment is tailored to the causing factor.

Common medications that are used to treat infertility include

Clomiphene (Clomid)
Human chorionic gonadotropin (Pregnyl)
Letrozole (Femara)
Hormone replacement therapy with estrogen, progestin, and testosterone

Key Points

Male gonads are called testes, and female gonads are called ovaries.
Testes produce testosterone and sperm.
Ovaries produce estrogen, progesterone, and ova, or eggs.
Secretion of sex hormones is controlled by hormones released from the pituitary gland.
Sex hormones determine the secondary sex characteristics specific to the male and female body.
The glands of the male and female reproductive system produce secretions and hormones to facilitate the fertilization of the egg and sperm.
Once fertilization takes place, an embryo travels to the uterus and implants there to grow into a baby.
Unlike the male reproductive system, the female reproductive system is more complex and plays an extensive role in growing, nourishing, and delivering a baby.
The main disorders of the male reproductive system include prostate disease, erectile dysfunction, priapism, infections, and STDs.
The main disorders of the female reproductive system include endometriosis, infections, PID, STDs, and menstrual cycle dysfunctions.

Next up

Previous

Respiratory and Endocrine Systems

Description