The reproductive system consists of internal and external sex organs and glands necessary for sexual reproduction. Which structures are present depends on a given individual's sex. Sex hormones are chemicals released into the blood to direct the development of male and female sex organs, secondary sex characteristics, and maintain reproductive cycles. The female reproductive system is also responsible for fetal protection, growth, and development during pregnancy; childbirth; and the nutrition and immunity of the child shortly after birth.
Reproductive organs are mostly present in the pelvic region. They are also known as gonads if they are inside the body and genitalia when they are outside the body. The female breasts are also considered part of the reproductive system because of their role in infant nutrition. The mammary glands produce milk, in a process called lactation, in response to hormones during and immediately following pregnancy. The milk contains nutrients as well as antibodies that help the newborn develop immunity.
The other female reproductive organs are mainly internal. The vagina is a tubular tract extending from below the external urethra opening to the cervix, which is a cone-shaped opening to the uterus, also known as the womb. The uterus consists of a muscular wall and accepts fertilized eggs for gestation and fetal growth (pregnancy). Eggs are released from the ovaries, two round organs located above the uterus on either side of the pelvic region. They alternately release an egg into the fallopian tubes during ovulation. The fallopian tubes contain cilia, which help carry the egg to the uterus and are also known as oviducts. The tubes are the usual site of egg fertilization by sperm.
The primary female sex hormone is estrogen, which is produced by the breasts, ovaries, placenta, adrenal gland, and corpus luteum (non-matured egg). Estrogen and its metabolites regulate the menstrual cycle and promote the maturation of female secondary sex characteristics. It is present mainly in women, but fat tissue produces the hormone in men as well. A hormone present only in women, and is increased during pregnancy, is progesterone. Progesterone is a steroid molecule that supports gestation, regulates estrogen effects during the menstrual cycle, and inhibits lactation and uterine contraction.
Estrogen is produced from androgens. Androgens are steroid hormones, meaning they are derived from cholesterol, that also control the development of masculine secondary sex characteristics and male reproductive organs. A subset of androgens is produced by the adrenal gland. The major androgen is testosterone, which is secreted by the testes, ovaries, and adrenal gland, though it is most prevalent in men. The hormone influences libido, promotes the maturation of male sex organs, and influences male pattern hair growth and muscle mass.
The male reproductive organs are mainly external, though there are several glands important to reproduction. The testes are round glands that produce sperm and are protected by a sac of skin called the scrotum, or scrotal sacs. Normally there are two testicles that descend from the pelvic/abdominal region around puberty. The seminal vesicles produce 70% of the total seminal fluid, which is important for the energy nutrition of sperm. The prostate gland stores and secretes an alkaline solution into the seminal fluid to aid in fertilization. The gland also contains smooth muscles that aid in ejaculation. Sperm cannot survive if the environments of the penis and vagina are not alkaline. The bulbourethral, or cowper's, glands also aid in neutralizing the female environment. However, the penis is the organ most associated with the male reproduction system. It is also part of the urinary tract since it contains the urethra in addition to the seminal tract. It is made of cavernous tissue that responds to increased blood flow and directs semen towards the uterus for fertilization.