Friday 25 May 2012
Menstrual Cycle
In the female reproductive system, the hormones control
the whole process of menstruation and pregnancy. The menstrual cycle is a sequence of events
which takes place approximately every 4 weeks (28 days) throughout the fertile
life of a woman, from the age of puberty to around 50 years of age.
After puberty, once a month, there is a surge of the
hormone, follicle stimulation hormone known as FSH, from the pituitary gland in
the brain that starts a few of the ova developing. FSH also affects the ovary
itself which starts making the female oestrogen. Oestrogen stimulates the
uterus to build up a thick, spongy lining with lots of blood vessels ready to
support the pregnancy.
As the oestrogen level rises, the production of FSH by the pituitary
gland falls and so the oetrogen level in turn will fall. The rise in oestrogen
level causes the production of the luteinising hormone (LH) to increase.
When the LH reaches its peak in the middle of the menstrual cycle, it
stimulates the release of the egg from the ovary.
About 14 days after the ova (egg) starts ripening, one of
them bursts out of its follicle. This is called ovulation. Once ovulation occurs the hormone levels from the
pituitary gland begin to drop dramatically.
After ovulation the remains of the follicle forms the corpus luteum (yellow body) which
secretes the hormone progesterone. Progesterone makes sure that for some days
the uterus lining stays thick and spongy and stimulates the growth of more
blood vessels, ready to receive a fertilised ovum. If pregnancy occurs the
embryo will immediately get a rich supply of food and oxygen.
About 10 days after ovulation (when no pregnancy has
occurred) the ovary reduces the levels of both oestrogen and progesterone. As
the chemical messages change again the blood vessels which are supplying the
thick spongy lining of the uterus close down. The lining detaches from the wall
of the uterus and is lost through the vagina as a monthly period or bleeding.
If the ovum has been fertilised, it will reach the uterus
and sink into the thick, spongy lining, attach itself (implant) and start to
develop.
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