The Menstrual Cycle

The menstrual cycle is regulated by intricate relationships between the hypothalamus and various endocrine glands, including the pituitary gland, the adrenal glands, and the ovaries and uterus. The hypothalamus monitors hormone levels in the bloodstream throughout the cycle, releasing chemicals that stimulate the pituitary to produce two hormones that affect the ovaries: follicle-stimulating hormone (FSH) and lutein­izing (LOO-te-uh-ny-zing) hormone (LH). FSH stimulates the ovaries to produce estrogen and causes ova to mature in follicles (small sacs) within the ovaries. LH causes the ovary to release a mature ovum. LH also stimulates the development of the cor­pus luteum (the portion of the follicle that remains after the matured egg has been released), which produces the hormone progesterone.

The menstrual cycle is a self-regulating and dynamic process. Each hormone is secreted until the organ it acts on is stimulated; at that point the organ releases a sub­stance that circulates back through the system to reduce hormonal activity in the initiat­ing gland. This negative-feedback mechanism provides an internal control that regulates hormone fluctuation during the three phases of the menstrual cycle. These phases, described in the following paragraphs, are illustrated in I Figure 3.6.

Menstrual Phase

During the menstrual phase the uterus sheds the thickened inner layer of the endo­metrium, which is discharged through the cervix and vagina as menstrual flow. Men­strual flow typically consists of blood, mucus, and endometrial tissue.

O = Ovulation

M = 1st day of menstruation

1st day
of

menstruation

M

1 2 3 4 1 6 7 8 9 1 11 12 13 14 1 16 17 18 19 1 21 22 23 24 5 10 15 20

I Figure 3.6 Changes during the menstrual cycle.

The shedding of the endometrium is triggered by reduced progesterone and estro­gen levels in the bloodstream. As these hormone levels fall, the hypothalamus stimu­lates the pituitary gland to release FSH. This action initiates the second, or proliferative, phase of the menstrual cycle.

Female Sexual Anatomy and Physiology