Basics of Human Reproduction

The Female Reproductive System

• Ovaries: A woman is born with all the eggs she will ever produce already stored in her two ovaries. At the time of ovulation, which occurs each month about 14 days before the onset of menses, one ovary produces an egg, called an ovum. This egg is stored in a follicle until it is ejected into the fallopian tubes. The spent follicle becomes the corpus luteum, which releases the hormone progesterone to ready the uterine lining. The follicular phase refers to the time before ovulation. Once the egg is released, and before the woman’s period, she is in the luteal phase.
• Fallopian tubes: Each ovary has its own fallopian tube. The fluted ends of these flexible tubes are tipped with small feathery projections that are waiting to catch the ovum. Tiny hairlike cilia move the egg down the tube where it waits to be fertilized. Each month, a woman is fertile only during the time an egg is available in the fallopian tubes.
• Uterus: The uterus is a hollow organ that becomes the home to a developing fetus. During the first half of the menstrual cycle, the uterine lining thickens in preparation to nourish a fertilized egg. If the egg fertilizes, it moves down the fallopian tube to the waiting uterus and implants in (attaches to) the lining, where it will eventually become a fetus. If the egg is not fertilized, it is shed along with the uterine lining when the woman has her period.
• Cervix: The cervix is a ring of muscle at the base of the uterus. The opening to the cervix is called the os. Sperm enter the cervix through the os and swim up through the uterus to the fallopian tubes, looking for a waiting egg.
• Vagina: The vagina is the muscular tube that connects the woman’s uterus to the outside. It is where a man places his penis during intercourse and is the path through which the uterine lining is shed. Though normally narrow, the vagina can stretch to accommodate the head of a newborn baby.

Female Hormones

The Male Reproductive System

• Testes: The testes (or testicles) make sperm and produce the hormone testosterone. Because sperm production cannot occur at body temperature, the scrotum, which houses the testes, hangs away from the body. This keeps them a few degrees cooler.
• Epididymis: These are coiled tubes at the back of the testes which house immature sperm. The sperm mature after about 20 days.
• Penis: When the penis is erect, its normally soft, spongy tissue engorges with blood. Upon ejaculation, the sperm travel through the vas deferens where they are mixed with fluids from the prostate gland and seminal vesicles. The milky fluid that results is called semen. Semen nourishes the sperm, allows it to move around inside the woman’s body, and helps neutralize acids that are found in the female reproductive tract. The semen is ejaculated through the urethra, a tube within the penis.

Male Hormones