Female menstruation
Female menstruationPuberty - The arrival of the monthly �friend�.
The principle sign that a girl has become sexually mature and is capable of reproduction is the onset of menstruation, a period of cyclic bleeding from the vagina. (Smith 1997, pg 217) This is one of the key physiological functions of the female body. An ovarian, or estrus, cycle is characteristic of all female mammals, but menstruation, periodic uterine bleeding that accompanies the ovarian cycle, occurs only in women, female apes, and some monkeys.
The average length of the menstrual cycle is twenty-eight days, although somewhat shorter and longer cycles are quite common. The day that bleeding starts is counted as the first day of a given cycle. The menstrual cycle is controlled by mechanisms involving hormones released by the hypothalamus, the pituitary, and the ovaries. The cycle has four stages: the menstrual phase, the preovulatory phase, the ovulation phase, and the postovulatory phase. Refer to Graph One.
The preovulatory phase starts as soon as menstrual bleeding from the previous cycle has ended. The anterior pituitary produces large amounts of follicle stimulating hormone (FSH) and a small amount of luteinizing hormone (LH). Under the influence of FSH, an ovarian follicle begins to mature and produces, in turn, increasingly higher levels of estrogens. In response to estrogen stimulation, the uterine lining thickens with increased numbers of blood vessels and uterine glands.
In the ovulation phase at midcycle, the ovum is released as LH production surges and FSH output decreases. Gonadotropic hormones that control the output of ovarian hormones are in turn regulated by the level of ovarian hormones through a negative feedback mechanism. Thus, the increased level of estrogens depresses FSH production. Following ovulation, the follicle transforms into the corpus luteum ("yellow body")...
To view the complete essay, you be registered.