Problem 2

Background and context

The hormones secreted by the hypothalamus, pituitary gland, and ovaries interact with each other to regulate the functioning of the female reproductive system as shown in Figure 43.15. 

• In the follicular phase, the gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus causes the anterior pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The rise in FSH and LH stimulates the growth of follicles. The growth of the follicles causes the ovaries to secrete a small amount of estradiol. This causes a negative feedback mechanism that inhibits the secretion of FSH and LH to stop more follicles from growing. 
• During the ovulatory phase, the estradiol levels keep rising. This initiates a positive feedback mechanism causing secretion of GnRH which in turn causes the anterior pituitary gland to secrete more FSH and LH. This causes the maturation of the dominant follicle and the LH surge causes the follicle to burst and the egg is released in the fallopian tube. The increasing levels of estrogen also cause the uterus lining to thicken.
• In the luteal phase, the ovarian follicle changes post-ovulation into the corpus luteum that secretes the female sex hormones estradiol and progesterone under LH stimulation. The increasing estradiol and progesterone levels continue to develop the uterine lining for implantation of the fertilized egg. If pregnancy does not take place, the corpus luteum disintegrates and the estradiol and progesterone levels decrease. This causes the shedding of the uterine lining leading to menstruation.