Physiology of Menstruation

• Menstruation is the cyclic shedding of the uterine lining (endometrium) in response to the hormonal fluctuations that occur during a woman's menstrual cycle.

• The cycle is typically divided into four phases:

1) Menstrual phase (Days 1-5)

• The menstrual phase marks the beginning of the cycle.

• Hormonal changes: Decreasing levels of estrogen and progesterone from the previous cycle cause the endometrial lining to shed.

• Menstrual bleeding: This shedding results in menstrual bleeding, which typically lasts 3-7 days.

2) Proliferative (Follicular) phase (Days 6-14)

• This phase is characterized by the growth of follicles in the ovaries and the repair and thickening of the endometrium.

A) Follicle-stimulating hormone (FSH) secretion

• The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to secrete FSH.

• FSH acts on the ovaries to stimulate the growth and development of ovarian follicles, each containing an immature egg (oocyte).

B) Estrogen production

• As the follicles develop, they secrete estrogen.

• Estrogen promotes the repair and thickening of the endometrial lining.

• Rising estrogen levels inhibit the further release of FSH, allowing a single dominant follicle to mature.

C) Luteinizing hormone (LH) surge

• High estrogen levels eventually trigger a surge in LH secretion from the anterior pituitary gland.

• This LH surge is responsible for initiating the process of ovulation.

3) Ovulation (Day 14)

• Ovulation is the release of a mature egg from the dominant ovarian follicle into the fallopian tube.

• The LH surge triggers a series of enzymatic reactions that cause the follicle to rupture and release the egg.

• The egg is now available for fertilization by sperm.

4) Luteal phase (Days 15-28)

• This phase begins after ovulation and is characterized by the following events:

A) Corpus luteum formation

• After the follicle ruptures and releases the egg, the remnants of the follicle transform into the corpus luteum.

• The corpus luteum is a temporary endocrine structure that secretes both progesterone and estrogen.

B) Progesterone production

• The corpus luteum primarily secretes progesterone, which stabilizes and thickens the endometrial lining, making it more receptive to a fertilized egg.

• Progesterone also inhibits further release of FSH and LH to prevent additional follicle development.

C) End of the luteal phase

• If fertilization does not occur, the corpus luteum degenerates around day 22-24.

• The drop in progesterone and estrogen levels results in the breakdown of the endometrial lining.

• This breakdown leads to menstruation, marking the start of a new cycle.