McKenna's Pharmacology for Nursing, 2e

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P A R T 7  Drugs acting on the reproductive system

Hypothalamus

Gonadotropin-releasing hormone (GnRH)

Anterior pituitary

Luteinising hormone (LH)

Follicle- stimulating hormone (FSH)

Corpus luteum

Graafian follicle

Corpus albicans Proges- terone

Maturing follicle

Ovary

Endometrium

1

5

10

15

20 25 28

Mens- trual

Mens- trual

Proliferative

Secretory

FIGURE 39.3  Relation of pituitary and ovarian hormone levels to the menstrual cycle and to ovarian and endometrial function.

Pregnancy When the ovum is fertilised by a sperm, a new cell is produced that rapidly divides to produce the embryo. The embryo implants in the wall of the uterus, and the interface between the fetal cells and the uterus produces the placenta, a large, vascular organ that serves as a massive endocrine gland and a transfer point for nutrients from the mother to the fetus. The placenta maintains high levels of oestrogens and progesterone to support the uterus and the developing fetus. When the placenta ages, the levels of progesterone and oestrogens fall off. Eventually, the tendency to block uterine activity (an effect of progesterone) is overcome by the stimulation to increase uterine activity caused by oxytocin (a hypotha- lamic hormone stored in the posterior pituitary). At this

which would nurture a growing embryo; this is called a secretory endometrium. If pregnancy does not occur, after about 14 days the corpus luteum involutes, and the levels of oestrogen and progesterone drop off (Figure 39.3). The dropping levels of oestrogen and progesterone trigger the release of FSH and LH again, along with the start of another menstrual cycle. Lowered hormone levels also cause the inner lining of the uterus to slough off because it is no longer stimulated by the hormones. High levels of plasminogen in the uterus prevent clotting of the lining as the vessels shear off. Prostaglandins in the uterus stimulate uterine contraction to clamp off vessels as the lining sheds away. This causes menstrual cramps. This loss of the uterine lining, called menstruation , repeats approximately every 28 to 32 days. Figure 39.3 depicts the various phases of the menstrual cycle.