The Menstrual Cycle
The menstrual cycle is the term used to describe the monthly physiological changes in the female body for the purpose of sexual reproduction. The human menstrual cycle:
- may last 25-35 days
- involves the brain, ovaries, and uterus
- consists of two phases: the follicular phase and the luteal phase
The Follicular Phase
At the start of this phase, the brain sends out two hormones, the follicle stimulating hormone (FSH) and the luteinizing hormone (LH), to the ovaries. The ovaries usually respond by growing one egg in an ovarian follicle, a fluid-filled sack in the ovaries where eggs develop. The cells lining the follicle produce increasing amounts of estrogen as the follicle grows and the egg inside matures. The egg is fully matured when the brain reacts to high levels of estrogen and releases the luteinizing hormone.
Within 28-40 hours, the follicle ruptures and the egg is released. This is called ovulation. The ruptured follicle is then transformed into a corpus luteum. Corpus luteum is the tissue in the ovary that forms at the site of the ruptured follicle. At this point the luteal phase begins.
The Luteal Phase
During this phase, the ovary stimulates the uterus to prepare for pregnancy. The hormones estrogen and progesterone cause the uterine lining, or endometrium, to swell with glands and blood vessels. At the same time, the corpus luteum continually produces stable blood levels of estrogen and progesterone until approximately day 23 of the menstrual cycle. If pregnancy does not occur or the egg is not fertilized and implanted in the uterine wall by that time, the corpus luteum declines. This causes a sharp drop in hormone levels which results in menstruation, or the shedding of the uterine lining.