Female Reproductive Physiology

The physiology of the female reproductive system is one of the most studied.  It is also easier to understand examples of a hormonal feedback system in human physiology. To clearly understand the process it is best to think of the whole system as a cycle of preparation for conception that once it fails to conceive resets and repeats approximately every 28 days.

This graphic of the ovarian and endometrial cycles gives a good depiction of the processes.  Refer to the illustration as you read the description below.  It will probably take reading this a few times to really understand the process.  Once you really understand this feedback cycle, it is much easier to remember and make sense of the female reproductive cycle.

By convention the process starts on day 1 of a woman’s menstrual flow.  At this time if the woman has failed to conceive, or more often purposely not conceived, the lining of the uterus, the endometrium, will begin to be shed as the menstrual flow.  The trigger for this response is the lack of production of the hormone called HCG that when produced by a developing embryo leads the ovary to continue to produce progesterone.  The progesterone is produced by the ovary from the time an egg is passed for almost exactly fourteen days.  This stops unless it receives the positive feedback from the embryo as HCG.  Without the progesterone to support the endometrial lining the lining is shed, resulting in a menses.

The ovary is stimulated by pituitary hormone called follicle stimulating hormone to produce a new egg.  The developing ovarian follicle will produce estrogen, which leads the endometrial lining to develop again.  By about day fourteen after the first day of the menses, the pituitary releases a surge of a hormone called luteinizing hormone, or LH. The LH stimulates the ovary to release the egg, a process called ovulation.  This usually happens about 14 days after the first day of the last menses, but it is much more variable than the relatively constant fourteen days from ovulation to the start of menstrual flow.

After ovulation the area that the egg was released from is called the corpus luteum.  This area produces progesterone which stimulates the endometrial lining to become ready to accept a fertilized egg.  It has more blood vessels, and is called the secretory phase.  This continues to develop for almost exactly 14 days.  If a fertilized egg implants in the endometrial lining, and a pregnancy begins, the embryo produces human chorionic gonadotropin, which supports the corpus luteum to continue to produce progesterone.  This supports the lining of the uterus so that no menses ensues.  If no pregnancy takes place, there is no HCG feedback to support the corpus luteum, the endometrium loses its support, and a menses starts.  This is again Day 1 of the next cycle.  In preparation for teaching sex education to a child having a solid understanding of the female reproductive physiology is important.