Reproduction
625
muscle elsewhere in the body, which accounts for some of the
systemic symptoms that sometimes accompany the cramps,
such as nausea, vomiting, and headache.
After the initial period of vascular constriction, the
endometrial arterioles dilate, resulting in hemorrhage through
the weakened capillary walls. The menstrual fl ow consists of
this blood mixed with endometrial debris. Typical blood loss
per menstrual period is about 50 to 150 ml.
The major events of the menstrual cycle are summarized
in
Table 17–7
. This table, in essence, combines the informa-
tion in Figures 17–18 and 17–23.
Other Effects of Estrogen
and Progesterone
Estrogen has other effects in addition to its paracrine function
within the ovaries, its effects on the anterior pituitary and the
hypothalamus, and its uterine actions. They are summarized
in
Table 17–8
.
Progesterone also exerts a variety of effects (Table 17–8).
Because plasma progesterone is markedly elevated only after
ovulation has occurred, several of these effects can be used
to indicate whether ovulation has taken place. First, proges-
terone inhibits proliferation of the cells lining the vagina.
Second, there is a small increase (approximately 0.5°C) in
body temperature that usually occurs after ovulation and per-
sists throughout the luteal phase; this change is probably due
to an action of progesterone on temperature regulatory cen-
ters in the brain.
Note that in its myometrial and vaginal effects, as well
as several others listed in Table 17–8, progesterone exerts an
“antiestrogen effect,” probably by decreasing the number of
estrogen receptors. In contrast, the synthesis of progesterone
receptors is stimulated by estrogen in many tissues (for exam-
ple, the endometrium), and so responsiveness to progesterone
usually requires the presence of estrogen (
estrogen priming
).
Like all steroid hormones, both estrogen and progester-
one act in the cell nucleus, and their biochemical mechanism
of action is at the level of gene transcription.
In closing this section, brief mention should be made of
the transient physical and emotional symptoms that appear in
many women prior to the onset of menstrual fl ow and disap-
pear within a few days after the start of menstruation. The
symptoms—which may include painful or swollen breasts,
headache, backache, depression, anxiety, irritability, and
other physical, emotional, and behavioral changes—are often
attributed to estrogen or progesterone excess. The plasma
concentrations of these hormones, however, are usually nor-
mal in women having these symptoms, and their cause is not
actually known. In order of increasing severity of symptoms,
the overall problem is categorized as
premenstrual tension,
premenstrual syndrome (PMS)
,
or
premenstrual dys-
phoric disorder (PMDD)
,
the last-named being so severe as
to be temporarily disabling. These symptoms appear to result
Table 17–7
Summary of the Menstrual Cycle
Day(s)
Major Events
1–5
Estrogen and progesterone are low because the previous corpus luteum is regressing.
Therefore:
(a) Endometrial lining sloughs.
(b) Secretion of FSH and LH is released from inhibition, and their plasma concentrations increase.
Therefore:
Several growing follicles are stimulated to mature.
7
7–12
7–12
12–13
14
A single follicle (usually) becomes dominant.
Plasma estrogen increases because of secretion by the dominant follicle.
Therefore:
Endometrium is stimulated to proliferate.
LH and FSH decrease due to estrogen and inhibin negative feedback.
Therefore:
Degeneration (atresia) of nondominant follicles occurs.
LH surge is induced by increasing plasma estrogen.
Therefore:
(a) Oocyte is induced to complete its fi rst meiotic division and undergo cytoplasmic maturation.
(b) Follicle is stimulated to secrete digestive enzymes and prostaglandins.
Ovulation is mediated by follicular enzymes and prostaglandins.
15–25
Corpus luteum forms and, under the infl uence of low but adequate levels of LH, secretes estrogen and progesterone,
increasing plasma concentrations of these hormones.
Therefore:
(a) Secretory endometrium develops.
(b) Secretion of FSH and LH is inhibited, lowering their plasma concentrations.
Therefore:
No new follicles develop.
25–28
Corpus luteum degenerates (if implantation of the conceptus does not occur).
Therefore:
Plasma estrogen and progesterone concentrations decrease.
Therefore:
Endometrium begins to slough at conclusion of day 28, and a new cycle begins.
previous page 653 Vander's Human Physiology The Mechanisms of Body Function read online next page 655 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off