Reproductive system

The reproductive system is a group of organized structures that make possible the creation, or reproduction, of new life for continuation of a species. Human reproduction is sexual, meaning that both a male and a female contribute genetic material in the creation of a new individual. During puberty, usually occurring between the ages of nine and fourteen, the reproductive systems of both sexes mature. The ovaries of a female release eggs (female sex cells) and a male's testes produce sperm (male sex cells). Reproduction occurs when a sperm unites with an egg, a process called fertilization.

The male reproductive system

The main tasks of the male reproductive system are to produce sperm cells and to introduce sperm into the female reproductive tract. Sperm are produced in the testes, the pair of male reproductive glands located in the scrotum, a skin-covered sac that hangs from the groin. Within each testis are hollow tubules called seminiferous tubules where sperm cells are produced. The testes also secrete the male hormone testosterone, which stimulates development of the reproductive structures and secondary sexual characteristics (such as deepened voice) at puberty.

After production, sperm cells move to a highly coiled tube called the epididymis, where they mature and are stored. During ejaculation (the ejection of sperm from the penis during orgasm), sperm travel from the epididymis through a long tube called the vas deferens to the urethra. The urethra is a single tube that extends from the bladder to the tip of the penis (and through which urine passes out of the body). Secretions from three different glands mix with sperm before it is ejaculated, forming the seminal fluid, or semen. Ejaculated semen may contain as many as 400 million sperm.

The penis is the male reproductive organ that delivers semen into the female reproductive tract. It consists of a shaft, the glans (or head region), and a foreskin. It is common practice in certain cultures and religions to have the foreskin removed, or circumcised. During sexual arousal, blood vessels in the tissue of the penis fill with blood, causing it to swell and reach a state of erection. The penis becomes longer, wider, and firm, allowing entry into the female vagina.

Words to Know

Fertilization: The union of an egg and a sperm that initiates the development of a new individual with genetic material from both parents.

Follicle: A sac in the ovary that contains a developing egg surrounded by a group of cells.

Gene: A section of a chromosome that carries instructions for the formation, functioning, and transmission of specific traits from one generation to another.

Hormone: A chemical produced in living cells that is carried by the blood to organs and tissues in distant parts of the body, where it regulates cellular activity.

Menstruation: The monthly shedding of the uterine lining and blood in a nonpregnant female.

Ovary: One of the paired female sex organs that produces eggs and sex hormones.

Ovulation: The release of a mature egg from an ovary.

Ovum: A female sex cell.

Puberty: The period of development of the sexual structures and secondary sexual characteristics in humans and higher primates.

Semen: Fluid containing sperm, nutrients, and mucus that is ejaculated from the penis during orgasm.

Sperm: A male sex cell.

Testis: One of a pair of male sex glands that produces sperm and sex hormones.

The female reproductive system

The main tasks of the female reproductive system are to produce ova, receive sperm from the penis, house and provide nutrients to the

developing embryo (fetus), give birth, and produce milk to feed offspring. Ova are produced in the ovaries, oval-shaped organs in the groin that also produce sex hormones. At birth, a female's ovaries contain hundreds of thousands of undeveloped eggs, each surrounded by a group of cells to form a follicle (sac). However, only about 360 to 480 follicles reach full maturity.

During puberty, the action of hormones cause several follicles to develop each month. Normally, just one follicle fully matures, rupturing and releasing an ovum through the ovary wall in a process called ovulation. The mature egg enters one of the paired fallopian tubes, where it may be fertilized by a sperm and move on to the uterus to develop into a fetus. The lining of the uterus, called the endometrium, prepares for pregnancy each month by becoming thicker. The lining is shed during menstruation if fertilization does not occur.

The uterus, or womb, is the organ in which a fetus develops and receives nutrients and oxygen. At its base lies the cervix, which widens during birth to allow passage of the fetus. The vagina is a muscular tube extending from the uterus to the outside of the body. It is the receptacle for sperm that is ejaculated during sexual intercourse and also forms part of the birth canal. The external genital organs, or vulva, include the labia, clitoris, and mons pubis. The labia are folds of skin on both sides of the openings to the vagina and urethra. The clitoris, a small, sensitive organ located in front of the labia, is comparable to the male penis. The mons pubis is a pad of fatty tissue above the clitoris.

During pregnancy, the female hormones estrogen and progesterone stimulate enlargement of the breasts and mammary glands. About two days after birth, blood levels of these hormones drop, and the pituitary gland releases the hormone prolactin, which stimulates the production of milk. Milk flows through small openings in the nipple of each breast to the nursing infant.

[See also Birth; Contraception; Hormone; Reproduction; Sexually transmitted diseases ]

reproductive system in animals, the anatomical organs concerned with production of offspring. In humans and other mammals the female reproductive system produces the female reproductive cells (the eggs, or ova) and contains an organ in which development of the fetus takes place; the male reproductive system produces the male reproductive cells, the sperm, and contains an organ that deposits the sperm within the female.

In the Human Female

In the human female reproductive system, ova are produced in the ovaries , two small organs set in the pelvic cavity below and to either side of the navel. The ovaries also secrete, in cyclic fashion, the hormones estrogen and progesterone (see menstruation ). After an ovum matures, it passes into the uterine tube, or fallopian tube . If sperm are present as a result of sexual intercourse or artificial insemination , fertilization occurs within the tube. The ovum, either fertilized or unfertilized, then passes down the fallopian tube, aided by cilia in the tube, and into the womb, or uterus , a pear-shaped organ specialized for development of a fertilized egg.

An inner uterine layer of tissue, the endometrium, undergoes cyclic changes as a result of the changing levels of the hormones secreted by the ovaries. The endometrium is thickest during the part of the menstrual cycle in which a fertilized ovum would be expected to enter the uterus and is thinnest just after menstruation. If no fertilized egg is present toward the end of the cycle, the thickened endometrium degenerates and sloughs off and menstruation occurs; if a fertilized egg is present it becomes embedded in the endometrium about a week after fertilization. The developing embryo produces trophoblastic cells and these, along with cells from the endometrium, form the placenta, the organ in which gas, food, and waste exchange between mother and embryo takes place. The embryo also forms the amniotic sac within which it develops.

The lower end of the uterus is called the cervix. The vagina, a passage connecting the uterus with the external genitals, receives the penis and the sperm ejaculated from it during sexual intercourse. It also serves as an exit passageway for menstrual blood and for the baby during birth . The external genitals, or vulva, include the clitoris, erectile tissue that responds to sexual stimulation, and the labia, which are composed of elongated folds of skin. After birth the infant is fed with milk from the breasts, or mammary glands , which are also sometimes considered part of the reproductive system.

In the Human Male

In the male reproductive system sperm are produced in the seminiferous tubules of the testes , two organs contained in the scrotum, an external sac in the groin. The testes also produce the male hormone testosterone and a portion of the seminal fluid, the liquid in which sperm are carried. The external location of the scrotum ensures the relatively low temperature that is necessary for the normal development of sperm. After formation, the sperm pass from the testes into the tubular epididymis, and from there into another passage, the vas deferens. The seminal vesicle, which produces nutrient seminal fluid, and the prostate gland , which produces alkaline prostatic fluid, are both connected to the ejaculatory duct leading into the urethra .

The first stage of the male sexual act, erection, results from nerve impulses from the autonomic nervous system that dilate the arteries of the penis, thus allowing arterial blood to flow into erectile tissues of the organ. During intercourse, contractions in the ducts of the testes, epididymis, and ductus deferens cause expulsion of sperm into the urethra and their mixture with the seminal and prostatic fluids. These substances, together with mucus secreted by accessory glands known as Cowper's glands, form the semen, which is discharged from the penile urethra during ejaculation.

Human Reproductive Disorders

Disorders that may affect the proper functioning of the reproductive system include abnormal hormone secretion, sexually transmitted diseases , and the presence of cancerous tissue in the region. Such problems frequently affect fertility and may complicate pregnancy.

See infertility . See also fertility drug ; in vitro fertilization .

reproductive system The gonads, which produce the GERM CELLS, and all associated organs and tissues in either sex that provide the mechanisms for fertilization and development of an embryo: in the male, the testes, vasa deferentia, prostate, seminal vesicles, and penis; in the female, the ovaries, Fallopian tubes, uterus, vagina, and mammary glands (breasts). Also, in both sexes, the relevant sex hormones and the tissues that produce them.

Stuart Judge

reproductive system The organs that are involved in the process of sexual reproduction in an organism. The reproductive system of a flowering plant is found in the flower and consists of the stamens (male organs) and carpels (female organs). In mammals the reproductive system consists of the testes, epididymis, sperm duct, and penis in the male and the ovaries, fallopian tubes, and uterus in the female (see illustration).

reproductive system (ree-prŏ-duk-tiv) n. the combination of organs and tissues associated with the process of reproduction. In males it includes the testes, vasa deferentia, prostate gland, seminal vesicles, urethra, and penis; in females it includes the ovaries, Fallopian tubes, uterus, vagina, and vulva.