The Female Reproductive System

The reproductive system of the female is entirely internal, in contrast to that of the male. The organs and glands, in ascending order, are: the vagina, a passage lined with mucus membrane, about 3 to 5 inches in length and capable of great expansion. The vagina opens in the vulva and leads up to a blind vault, into which the cervix projects. The cervix is a fibrous ring of tissue, sometimes called the "neck of the uterus." It has a narrow opening (cervical os), and its mucus membrane lining produces abundant secretions, which range from thin and elastic to thick and impassable. These varying conditions are influenced by the woman's sex hormones.

The uterus is a thick-walled muscular organ about the size and shape of a pear in the nonpregnant state. The cavity of the uterus is very small and somewhat triangular in shape, widest at the top (fundus) and narrow at the bottom where the cervix is. The uterus is lined with mucous membrane called endometrium, which is also influenced by the hormones the woman secretes.

The Fallopian tubes are two trumpet-shaped, flexible muscular tubes, about 4 inches long and thinner in diameter than a lead pencil. They arise from the fundus of the uterus and extend one to each side, reaching upward and slightly behind the uterus. They are lined with membrane containing tiny hairlike projections (cilia), which move with the contraction of the musculature of the tubes to produce a progressive wave to sweep the ovum toward the uterus. In addition, they contain cells that produce secretions that affect sperm-egg interaction. The outermost ends of the Fallopian tubes are called the fimbriated ends. These are larger in diameter and contain fringed projections thought to draw the extruded ovum into the tube after ovulation, because the ovary is not in direct contact with the end of the tube. Below the fimbriated ends lie the ovaries, one on each side. About the size of walnuts, they have two important functions: to produce the female sex hormones estrogen and progesterone and to produce a ripened ovum once each menstrual cycle.

A woman's body operates in a cyclical manner during her reproductive years. The sequence of events in an idealized 28-day menstrual cycle follows this course: The first day of the cycle is the first day of menstruation, the shedding of the lining of the uterus from the previous cycle. Throughout the cycle, the pituitary, a small gland at the base of the brain dependent upon the hypothalamus, sends out follicular stimulating hormone (FSH), which stimulates the ovary to ripen a follicle and mature an ovum. Another hormone, luteinizing hormone (LH), is produced by the pituitary in somewhat higher amounts throughout the cycle. A large burst of LH that occurs right before ovulation is thought to help release the mature ovum at ovulation time.

Ovulation takes place on about the fourteenth day. The tiny ovum is extruded from the ovary, and the fringed projections of the fimbriated end of the nearest Fallopian tube help locate the ovum and wave it into the tube, where it moves along freely for about three days. Meanwhile the ruptured follicle that produced the ovum now becomes a functioning gland called the corpus luteum, named for its yellow luteal lining. This gland now produces the hormone progesterone, in addition to estrogen, which prepares the uterus with a lush secretory lining needed for implantation in case a pregnancy is begun.

If the ovum meets with sperm in optimal conditions in the tube, a conception occurs; and the tiny mass of cells travels through the tube into the body of the uterus, where it implants in the lining on about the twentieth day of the cycle (6 to 7 days after ovulation). Once implanted, the conceptus develops a feedback system with the corpus luteum, telling it to go on producing progesterone, thus preserving the lining and the pregnancy. This message is sent by the cells destined to become the placenta.

If fertilization does not occur, the ovum passes into the uterus and the corpus luteum receives no signal to continue to produce progesterone. In this case, on about the twenty-sixth day of the cycle, the corpus luteum deteriorates into a nonfunctioning gland and the progesterone level falls. The uterus begins to break down its rich lining and sloughs it off. The next menstrual period begins on the twenty-eighth day.