600
P A R T 7
Drugs acting on the reproductive system
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.
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
Anterior pituitary
Hypothalamus
Maturing
follicle
Graafian
follicle
Mens-
trual
Mens-
trual
Proliferative
1
5
10
15
20 25 28
Secretory
Ovary
Endometrium
Corpus
luteum
Corpus
albicans
Proges-
terone
Luteinising
hormone (LH)
Follicle-
stimulating
hormone (FSH)
Gonadotropin-releasing hormone (GnRH)
FIGURE 39.3
Relation of pituitary
and ovarian hormone levels to the
menstrual cycle and to ovarian and
endometrial function.
