Menstruation of Physiology

• Menstruation is the cyclic shedding of the uterine lining (endometrium) in response to 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.

1. Follicle-Stimulating Hormone (FSH) Secretion:

   • The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to secrete FSH.
   • FSH stimulates the growth and development of ovarian follicles, each containing an immature egg (oocyte).

2. Estrogen Production in menstruation:

   • 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.

3. Luteinizing Hormone (LH) Surge:

   • High levels of estrogen eventually trigger a surge in LH secretion from the anterior pituitary gland.
   • The LH surge is responsible for initiating ovulation.

3. Ovulation Phase of Menstruation (Day 14):

• Ovulation is the release of a mature egg from the dominant ovarian follicle into the fallopian tube.
• The LH surge triggers enzymatic reactions that cause the dominant follicle to rupture and release the egg.
• The egg is now available for fertilization by sperm.

4. Luteal Phase of Menstruation (Days 15-28):

1. • This phase begins after ovulation and prepares the body for a potential pregnancy.

2. 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.

3. 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.

4. End of the Luteal Phase:

   • If fertilization does not occur, the corpus luteum degenerates around days 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.

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