Human Physiology, 14th edition (2016)

730 Chapter 20

developing follicles secrete only estradiol, the corpus luteum

secretes both estradiol and progesterone. Progesterone levels

in the blood are negligible before ovulation but rise rapidly to

a peak level during the luteal phase, approximately one week

after ovulation (see figs. 20.32 and 20.33 ).

The high levels of progesterone combined with estra-

diol during the luteal phase exert an inhibitory, or negative

feedback, effect on FSH and LH secretion. There is also

evidence that the corpus luteum produces inhibin during the

luteal phase, which may help to suppress FSH secretion or

action. This serves to retard development of new follicles,

so that further ovulation does not normally occur during that

cycle. In this way, multiple ovulations (and possible preg-

nancies) on succeeding days of the cycle are prevented.

Ovulation occurs, therefore, as a result of the sequential
effects of FSH and LH on the ovarian follicles. By means of the
positive feedback effect of estradiol on LH secretion, the follicle
in a sense sets the time for its own ovulation. This is because
ovulation is triggered by an LH surge, and the LH surge is trig-
gered by the increased estradiol secretion that occurs while the
follicle grows. In this way, the graafian follicle is not normally
ovulated until it has reached the proper size and maturity.

Luteal Phase

After ovulation, the empty follicle is stimulated by LH to become
a new structure—the corpus luteum ( fig. 20.34 ). This change in
structure is accompanied by a change in function. Whereas the

Figure 20.33 The cycle of ovulation and menstruation. The downward arrows indicate the effects of the hormones.

FSH

FSH

LH

LH

Gonadotropin secretion

Ovarian events

Developing follicle Mature follicle

Early

corpus luteum

Ovarian cycle

Corpus luteum regresses

Follicular phase Ovulation

Days

Days

1

1

7142 1 2 8

Menstrual cycle

Progesterone

Progesterone

Estrogen

Estrogen

Ovarian hormone secretion

Thickness of endometrium

Menstruation Proliferative phase Secretory phase Menstruation

3 5 7 9 111315171921232527 1

Luteal phase