Some interesting facts about the human reproductive system:

bulletMen and women are each born with two gonads (two internal sex organs)—the testes (plural) in men, and the ovaries in women.
bulletThe human female is born with all the ova (eggs) she will ever have—about 2 milion. These are immature. By puberty, she will have only about 400,000 ova, the rest having dwindled away. During her reproductive life, a woman will only ovulate (release from the ovary) about 400 eggs. Which eggs are selected to mature or ripen to ovulation is still a biological mystery.
bulletThe number of times a woman's ovaries are stimulated to produce a mature ovum may be related to her lifetime risk for developing breast and/or ovarian cancer. Women who experience "ovarian rest" through taking the birth control pill, through pregnancies, through breast feeding, are at a lower lifetime risk for development of these diseases.
bulletThe ripening of an egg is stimulated by hormones released by the pituitary gland in the brain. The pituitary receives a chemical signal from the hypothalamus (the true "master" gland of the brain), and responds by releasing two hormones—FSH and LH—into the blood stream. FSH stimulates the maturing of an egg, while LH assists in this. As LH levels climb, it eventually triggers the release of the ovum (egg) in the process called ovulation (ovulation occurs approximately 12-24 hours after LH has reached its peak concentration). LH also stimulates the corpus luteum (the follicle that once contained the now-released egg) to produce estrogen and progesterone, two key hormones in a woman's cycle. Levels of progesterone and estrogen reach their peaks at around the 20th to 21st day of a woman's cycle. This peak causes changes in the endometrium (the lining of the uterus) which will make it better able to support the implantation of the embryo.
bulletIf a woman becomes pregnant, estrogen levels and progesterone levels remain high, and the progesterone produced by the corpus luteum helps maintain the lining of the uterus, so that the developing embryo has a good environment to attach to for further development. If the woman doesn't become pregnant, all levels of hormones decline, the corpus luteum degenerates, and a woman has her monthly menstrual flow. A normal menstrual flow is only about 2-3 ounces (4-6 tablespoons) of blood.
bulletIn males, FSH stimulates sperm production, while LH stimulates the production of testosterone, the key male sexual hormone.
bulletUnlike the ovaries which never produce new eggs after a female child is born, the male testes (once sperm production begins in puberty), can produce about 1,000 sperm per second, or about 30 billion per year. From the standpoint of the mathematics alone, 10-20 ejaculations of semen hold enough sperm to populate the earth!
bulletSperm cells contain receptors on the surface of the sperm head that are similar to odor receptors in the nose. This has lead researchers to believe that sperm can detect the "odor" of the released egg, and literally are following its scent to locate it and attempt to fertilize it.
bulletIt was once thought that the best time to either have sex to get pregnant, or to abstain from sex to keep from getting pregnant was at the time of ovulation. In reality, the best time to have sex if one to two days before ovulation, so that sperm cells have enough time to swim up through the uterus and the fallopian tubes to be waiting for the egg when it is ejected from the ovary at ovulation. Following ovulation, changes in a woman's hormones produce changes in the secretions at the cervix (the opening to the uterus, at the blind end of the vagina), and these thicken to form a barrier intended to protect the potentially developing embryo (if the egg has been fertilized). It is extremely difficult for sperm to penetrate this mucus barrier, and therefore more difficult for the egg to be fertilized after ovulation has occurred.
bulletSome highlights of male anatomy:
bulletThe seminiferous tubules fill the lobes (sections of tissue) of each testicle. Here, sperm cells develop. Though compacted into the testicles, the surface area provided by the tubules is enormous, and if stretched out end to end, would be much longer than a football field!
bulletThe epididymis is a tube that lies against the back wall of each testicle, that serves as a storage facility for sperm cells to finish maturing, and who have yet to be ejaculated. It is about 2 inches in length, but if unrolled (it is make up of many twisted passages) it would be about 10-20 feet in length if all sections were stretched end-to-end!
bulletThe vas deferens (also called the ductus deferens)—each epididymis empties into one of these tubes. Each vas deferens is about 16 inches long, and serves as the delivery tube for mature sperm.
bulletThe seminal vesicles are small glands that lie behind the bladder, and which secrete fluids (especially frustose, a sugar) which nourish the sperm and help them to become active (motile).
bulletThe prostate glad lies beneath the bladder, and is about 3/4 of an inch in diameter. Postatic fluid is milky and alkaline, and provides some of the texture and odor we associate with seminal fluid. The alkaline fluid helps neutralize some of the acidity in the vagina of the woman which could harm the sperm cells.
bulletCowper's glands (also called the bulbourethral glands—there are two of them, positioned below the protate) provide a drop or two of clear, slippery fluid, that appears at the opening of the urethra. It may help with the acidity of the vagina, and may provide some lubrication. The droplets from these glands may contain active sperm, and are released before ejaculation.