Structure and Function of the Reproductive System

The function of the reproductive system is to produce offspring. The male and female reproductive organs (gonads) are different in structure to accomplish this function. Puberty is the age at which the reproductive organs mature sufficiently to allow reproduction.

The reproductive organs of both the male and female produce sex cells called gametes. The combination of genes of the female gamete (ova) and male gamete (sperm) is called fertilization (Fig. 21-1). From the time of conception to 2 weeks, the fertilized ovum is called a zygote. From 2 to 8 weeks’ gestation, the growing cells are called an embryo. From 8 weeks to birth, the unborn baby is called a fetus. During the first 30 days of life, the infant is considered to be a neonate.

Male Organs of Reproduction

The organs of reproduction of the male are shown in Fig. 21-2. The testes, about 4 to 5 cm long, produce the sperm. The testes also secrete an androgenic hormone (testosterone), causing the appearance of secondary sexual characteristics such as facial and body hair, deepened voice, increased muscle mass, and thickening of the bones.

The epididymis is a tube on the surface of each testis that stores the sperm while they mature. Sperm are transported by the vas deferens into the ejaculatory duct below the bladder. The seminal vesicle adds fluid that increases the volume and nourishes the sperm. The prostate gland, located below the bladder, secretes a fluid that protects the sperm.

The penis becomes rigid and elevated when filled with blood (erectile). The penis encloses the urethra. The glans penis is covered with a loose-fitting, retractable casing called the foreskin (prepuce), which may be removed (circumcision). Both semen and urine are excreted through the urethra, but the systems operate separately. Semen is a thick, white secretion that contains the sperm and fluid. Expulsion of semen is called ejaculation. Approximately 200 million sperm are in each ejaculation.

The National Cancer Institute (NCI) recommends a monthly self-examination of the testes to detect testicular cancer (Fig. 21-3). For 2009, the NCI estimated that 8400 new cases and 380 deaths occurred as a result of testicular cancer in the United States. Testicular cancer represents about 1% of all cancers in men. However, it is the most common form of cancer in men between 15 and 40 years of age.

Female Organs of Reproduction

The organs of reproduction in the female are shown in Fig. 21-4. The ovaries are glands that produce eggs (ova) and the hormones estrogen and progesterone.

Brain Byte

The ovaries of a newborn girl contain about 600,000 immature eggs.

The hormones prepare an egg (ovum) for fertilization, regulate the menstrual cycle, and help maintain pregnancy by promoting the growth of the placenta.

Brain Byte

The largest cell in the female body is the egg.

The fallopian tubes (oviducts) transport the mature ovum from the ovary to the uterus. Fertilization usually occurs in the fallopian tubes during the 5 days required for the ovum to move to the uterus. If the ovum is not fertilized, it degenerates and is excreted as part of the menstrual cycle.

The uterus is a muscular structure about the size of a small finger. The zygote is implanted in the uterus after conception. The cervix, or neck of the uterus, thins and opens for delivery of a fetus. The inner layer of the uterus, called the endometrium, is shed during each menstrual cycle.

The vagina is a muscular tube that extends from the cervix to the exterior of the body. It is the site of sexual intercourse and the passageway (birth canal) for delivery of the fetus.

The external structures of the female reproductive system are collectively called the vulva (Fig. 21-5). The labia majora are folds of adipose tissue that protect the vaginal opening. The mons pubis is a pad of fat that joins the labia majora. The labia minora are pinkish folds of skin between the labia majora.

The clitoris is a small projection of erectile tissue located between the labia minora. The Bartholin glands secrete mucus and a lubricating fluid into the vaginal opening. The mammary glands (breasts) enlarge during puberty. The mammary glands have a system of ducts that secrete milk (lactation) after pregnancy.

Menstrual Cycle

The shedding of blood tissues of the uterus (menstruation) in the female signals the onset of puberty at about 10 to 16 years of age. The menstrual cycle, which lasts about 28 days, is a complex process of hormone secretion and tissue changes in the uterus (Table 21-1). A mature ovum is released from an ovary on about the 14th day of each cycle (Fig. 21-6). Some women experience pain a few hours after ovulation called mittelschmerz. This “middle pain” is believed to be caused by irritation of the peritoneum. If the released ovum is not fertilized, the lining of the uterus (endometrium) is released from the body along with the ovum. The sloughing of this bloody tissue, or menses, lasts from 3 to 7 days. The menstrual cycle continues until 45 to 50 years of age. The cessation of the cycle is called menopause (climacteric). Reduction in hormone production that occurs with menopause may cause a variety of symptoms, including “hot flashes,” or transient periods of feeling warm. Reduced levels of both estrogen and pituitary gonadotropins occur with menopause. Without these hormones, the woman’s ability to conceive or reproduce ceases.

Endometrial ablation is the removal or destruction of the lining of the uterus. Methods used to destroy the tissue include laser beam, heat, electricity, freezing, and microwave. It is an alternative to hysterectomy used to control heavy, prolonged menstrual bleeding. The procedure reduces or stops the menstrual cycle.

Pregnancy

Growth of an offspring in the uterus lasts about 280 days (9 months), or through the period of pregnancy. Pregnancy results from the union of the ovum and sperm, usually in the fallopian tube. The fertilized egg is known as a zygote for about 3 days. It is then considered to be the morula and enters the uterus. As a blastocyst, it implants in the uterine wall and is considered an embryo through the eighth week. From the eighth week until birth, it is considered a fetus.

Changes in the female reproductive system during pregnancy include an increase in muscle mass of the uterus and an elongation of the vagina. The uterus increases to 16 times its normal size during pregnancy. The secretions, vascularity, and elasticity of the cervix and vagina also increase in preparation for the delivery of the fetus.

The amniotic sac is a membrane that surrounds the fetus in the uterus. The sac is filled with fluid to cushion and protect the fetus against infection and temperature changes. A portion of the uterus forms the placenta, which filters the blood of the mother to provide oxygen and nutrients for the fetus.

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