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MENSTRUAL CYCLE
- The regularly
recurring physiologic changes in the endometrium that culminate in
its shedding ( "menstruation" MENSTRUATION). Menstrual cycles
vary in length; the average is about 28 days.
- The menstrual
flow generally lasts about 5 days, although this too varies from person
to person. Women menstruate from puberty to menopause, except during
pregnancy. During the first 14 days of the 28-day menstrual cycle,
a follicle containing an ovum develops in the ovaries.
- As the menstrual
flow ceases, the lining of the uterus is stimulated by estrogen and
begins to increase in thickness to prepare for reproduction. On about
the fourteenth day of the cycle, "ovulation" OVULATION takes
place and the ovary discharges the ovum.
- The ruptured
follicle is transformed into a yellowish material called the corpus
luteum, which in turn secretes progesterone. Progesterone acts on
the endometrium, building up tissues with an enriched supply of blood
to nourish the future embryo.
- If conception
does not take place, the estrogen level in the blood falls, the endometrium
is no longer stimulated, and the uterus again becomes thinner. Blood
circulation slows, blood vessels contract, and the unused tissue breaks
down into the bloody discharge known as menstruation.
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