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menstrual cycle
the regularly recurring physiologic changes in the endometrium that ends when it is ready to be shed (see menstruation). Menstrual cycles vary in length from 21 to 35 days, with the average being about 28 days. Women menstruate from puberty to menopause, except during pregnancy. The menstrual cycle is divided into the ovarian cycle, when changes are occurring primarily in an ovary, and the uterine cycle, when changes are occurring primarily in the uterus. If fertilization and conception do not take place, the estrogen level in the blood falls, the endometrium is no longer stimulated, and the uterus becomes thinner. Blood circulation slows, blood vessels contract, and the menstrual phase begins as unused tissue breaks down into the bloody discharge known as menstruation. The cycle then starts again.
Changes in the menstrual cycle in the human female. Solid lines indicate the course of events when the oocyte is not fertilized;
dotted lines indicate the course of events when fertilization occurs. Arrows indicate the actions of hormones of the pituitary
and the ovary in regulating the cycle.
Copyright 2007. An Elsevier publication. All rights reserved.
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