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Endometriosis gets its name from the endometrium, which lines the inside of the uterine cavity. With endometriosis, there is ectopic endometrial tissue, which means endometrium can be present in other parts of the body, like the ovaries or fallopian tubes, or even as far as the lungs! Also, with endometriosis, the ectopic tissue behaves just like regular endometrium would, and it undergoes cyclic changes in response to the menstrual cycle.
Now, let’s look at the overall anatomy and physiology of the uterus. Remember that the uterine wall is comprised of three layers: the endometrium, which is the innermost, mucosal layer, and is itself made up of a basal layer, and a functional layer; the myometrium, which is the strong smooth muscle that gives the uterine wall its thickness; and the perimetrium, which is the outermost layer, and it’s basically a continuation of the peritoneal lining.
Now, during each menstrual cycle, an egg is released from the ovaries at ovulation, and estrogen and progesterone levels increase. Estrogen thickens the functional layer of the endometrium to prepare it for implantation. If there is no fertilization, hormone levels drop, and the functional layer sloughs off, and is eliminated along with a small amount of blood during menstruation. And after menstruation, the cycle repeats itself all over again!
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