The Biology of the Menstrual Cycle.
Approximately once a month, reproductively mature females menstruate. For many this results in unpleasant emotional and physical surges, and increased expenditure on chocolate. The first instance of this decades long cycle is referred to as menarche, and generally it announces the tumultuous years of puberty, during which the ovaries (housed in the upper pelvic cavity either side of the uterus) begin cyclical changes under the influence of ovarian hormones released under the direction of the hypothalamus and anterior pituitary glands of the central nervous system. Menstruation can be divided into four stages, including: the follicular or preovulatory phase, the progesterone or luteal phase, the ischemic phase, and the menstrual phase.
The Follicular or Preovulatory Phase (Day 5+2 - Day 14 +2)
At the outer edge of each ovary sits a group of cells called primary follicles. Each primary follicle encloses an immature ovum (egg) known as an oogonium. When the ovaries are directed to release follicle stimulating hormone (FSH) up to twenty primary follicles begin maturing. Five to seven days after FSH is released, almost all the primary follicles and their housed oogonium will have degenerated. Generally, however, one primary follicle houses an oogonium which matures to become an ova or an egg, changing the primary follicle into a mature ovarian follicle.
The Progesterone or Luteal Phase (Day 14+2 - Day 28 +2)
Approximately five to seven days after the primary follicles begin maturing, the ovaries release a sudden surge of luteinizing hormone, paralleled by peaks in oestrogen and a second wave release of FSH. The combination of these hormonal peaks causes the mature ovarian follicle to rupture and release its egg into the ovarian, or fallopian, tube that connects the ovaries to the uterus. Blood fills the place of the now released egg; however the presence of luteinizing hormone causes this blood to be replaced by fat filled luteal cells. What was the mature ovarian follicle is now the corpus luteum. Many women are aware of this rupture and release, known as ovulation, as pain can be felt in the lower back and abdomen, and vaginal secretions become a thick, transparent, raw egg white-like mucous. Once ovulation has occurred, the levels of FSH and LH, and oestrogen all drop significantly; meantime a slow rise in the hormone progesterone occurs. After a few days of increasing progesterone levels, a slow, small rise in oestrogen occurs. At this stage, a woman is most fertile, and the lining of the uterus (the endometrium) becomes very thick under the influence of progesterone and oestrogen.
The Ischemic Phase (Day 27+2 - 28+2)
If a pregnancy does not occur, however, the levels of progesterone and oestrogen both drop steadily, and the corpus luteum degenerates. Subsequently, with nothing to support it hormonally, the circulatory system of the uterus constricts, resulting in ischemia - attenuate blood, and therefore oxygen, flow to the endometrium.
The Menstrual Phase (Day 1 - Day 5 + 2)
Without blood and its nutrients reaching it the endometrium begins to disintegrate and is released from the wall of the uterus in the form of blood. This bloody disintegrated uterine lining or endometrium travels along the hollow muscular tube that comprises the vagina, and is recognized as menstruation. The first day of bleeding is considered day 1 of the menstrual cycle. This shedding of the uterine lining or menstrual bleed, generally lasts between 4 and 7 days and occurs approximately every twenty-eight days.