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Menstruation
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Menstruation is vaginal bleeding that repeats in cycles and comes with a coordinated series of hormonal changes.
What bleeds is the endometrium, the layer that lines the inside of the uterus. During the first part of the cycle estradiol, a hormone the ovarian follicles produce as they grow, makes that endometrium develop. Progesterone comes next and matures it.
At the end of the luteal phase, which is the second half of the cycle, both hormones drop off sharply. The endometrium cannot sustain itself without them and sheds: that is menstruation. The endometrium is a lining that rebuilds itself every cycle and sheds when its hormonal support is withdrawn. That is why the endometrial cycle is divided into three phases: the proliferative, in which it grows; the secretory, in which it matures; and the menstrual, in which it sheds.
In brief
Menstruation is cyclic vaginal bleeding that accompanies a coordinated series of hormonal changes.
The sharp drop in progesterone and estradiol at the end of the luteal phase triggers shedding of the endometrium, which cannot sustain itself without those hormones.
The endometrium is a lining that rebuilds itself every cycle and sheds when its hormonal support is withdrawn.
The endometrial cycle has proliferative, secretory and menstrual phases; estradiol from the growing follicles builds the endometrium and progesterone matures it.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Menstrual Cycle, StatPearlsPublished
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