Physiology of the Menstrual Cycle

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The menstrual cycle is a cyclic process in females that, on average, occurs every 28 days. The purpose of the menstrual cycle is to help prepare the body for potential fertilization, implantation, and, consequently, pregnancy. The happenings of the menstrual cycle are consequences of the simultaneously occurring ovarian cycle and uterine cycle. The ovarian cycle occurs in the ovaries and carries out the follicular phase, which spans the first 14 days of a new menstrual cycle. The result is ovulation. The uterine cycle works concurrently with the ovarian cycle and carries out the menstrual, proliferative, and secretory phases. Hormonally, gonadotropin-releasing hormone (GnRH) is released in the hypothalamus and travels to the anterior pituitary of the brain, thus increasing levels of follicule-stimulating hormone (FSH) and leutinizing hormone (LH). The brain acts directly on the follicle cells (in the follicular phase) and the corpus luteum (in the luteal phase) to stimulate a response. The result of hormonal secretions of the hypothalamus and anterior pituitary along with the ovarian and uterine cycle is menstruation. The overall cycle is regulated by the hormones that result from a release of GnRH from the brain. Negative feedback occurs in the early follicular phase and luteal phase of th ovarian cycle as GnRH is released, which causes an increase in FSH and LH levels in the anterior pituitary. In the early follicular phase, these hormones send signals through neurotransmitters that reach receptors on follicle cells and follicular development is enhanced. Simultaneously, plasma levels of estrogen increase and return to the brain to hinder continued LH and FSH secretion. At the end of the follicular phase, positive feedback occurs as high estrogen levels are passed back to the brain and estrogen continues to amount to high levels. In the luteal phase, the brain sends hormonal signals to the corpus luteum. As a result, levels of estrogen and...
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