cholesterol levels and falling HDL levels place postmenopausal
women at risk for cardiovascular disorders. At one time physi-
cians prescribed low-dose estrogen-progesterone preparations
to help women through this period and prevent skeletal and
cardiovascular complications. Tis seemed like a great idea and
by 2002, approximately 14 million American women were tak-
ing some form of estrogen-containing hormone replacement
therapy (HR±). Ten, the Women’s Health Initiative (WHI)
abruptly ended a clinical trial of 16,000 postmenopausal women,
reporting that those who took a popular progesterone-estrogen
medication had an increased risk of 51% in heart disease, 24%
in invasive breast cancer, 31% in stroke, and a doubled risk of
dementia compared to those taking placebos.
Te backlash of this information (aired in the popular press)
is still spreading through physicians’ offices and research labs. It
is restricting funding and the ability to find new volunteers for
future studies, and has dampened enthusiasm for HR± in both
the medical community and postmenopausal women. How-
ever, newer data suggest that the smallest dose of HR± for the
shortest time is OK to reduce symptoms in women who do not
have existing breast cancer or mutated
Tere is no equivalent of menopause in males, and healthy
men are able to father offspring well into their eighth decade
of life thanks to their small but enduring population of stem
cells (spermatogonia). However, aging men do exhibit a steady
decline in testosterone secretion and experience a longer latent
period aFer orgasm, a condition sometimes called
and testosterone replacement therapy is currently being pre-
scribed for more older men. Additionally, there is a noticeable
difference in sperm motility with aging. Sperm of a young man
can make it up the uterine tubes in 20–50 minutes, whereas
those of a 75-year-old take 2½ days for the same trip.
Check Your Understanding
What are the early signs of puberty’s onset in boys?
What is the definition of menopause?
For answers, see Appendix H.
Te reproductive system is unique among organ systems in at
least two ways: (1) It is nonfunctional during the first 10–15 years
of life, and (2) it is capable of interacting with the complementary
system of another person—indeed, it
do so to carry out its
biological function of pregnancy and birth. ±o be sure, having a
baby is not always what the interacting partners have in mind, and
we humans have devised a variety of techniques for preventing
this outcome (see
A Closer Look
on pp. 1090–1091).
Te major goal of the reproductive system is ensuring the
healthy function of its own organs so that conditions are optimal
for producing offspring. However, as illustrated in
System Connec-
, gonadal hormones do influence other body organs, and the
reproductive system depends on other body systems for oxygen
and nutrients and to carry away and dispose of its wastes.
Now that we know how the reproductive system prepares
itself for childbearing, we are ready to consider the events of
pregnancy and prenatal development of a new living being, the
topics of Chapter 28.
is the period of life when the reproductive organs
grow to adult size and become functional. As puberty nears,
these changes occur in response to rising levels of gonadal hor-
mones (testosterone in males and estrogen in females). Ear-
lier we described secondary sex characteristics and regulatory
events of puberty. But it is important to remember that puberty
represents the earliest time that reproduction is possible.
Te events of puberty occur in the same sequence in all indi-
viduals, but the age at which they occur varies widely.
In males, secretion of adrenal androgens, particularly dehy-
droepiandrosterone (DHEA), begins to rise several years before
the testosterone surge of puberty and initiates facial, pubic, and ax-
illary hair growth and other pubertal events. Te major event that
signals puberty’s onset in males is enlargement of the testes and
scrotum between the ages of 8 and 14. Te penis grows over the
next two years, and sexual maturation is evidenced by the presence
of mature sperm in the semen (see ²igure 27.11). In the meantime,
the young man has unexpected erections and occasional nocturnal
emissions (“wet dreams”) as his hormones surge and the hormonal
control axis struggles to achieve a normal balance.
In females, the first sign of puberty is budding breasts be-
tween the ages of 8 and 13, followed by the appearance of axil-
lary and pubic hair. Menarche usually occurs about two years
later. Dependable ovulation and fertility await the maturation
of the hormonal controls, which takes nearly two more years.
Epiphyseal plate closure and termination of skeletal growth
in height occurs in response to rising estrogen levels late in pu-
berty in both boys and girls.
Most women reach the peak of their reproductive abilities in
their late 20s. AFer that, ovarian function declines gradually,
presumably because the ovaries become less and less responsive
to gonadotropin signals. At age 30, there are still some 100,000
oocytes in the ovaries but the quality (hence fertility) has begun
to decline. By age 50, there are probably 1000 eggs leF (the pan-
try is nearly bare).
As estrogen production declines, many ovarian cycles become
anovulatory, while in others 2 to 4 oocytes per month are ovu-
lated, a sign of declining control. Increasingly frequent multiple
ovulations explain why twins and triplets are more common in
women who have deferred childbearing until their late 30s. In the
perimenopausal period, menstrual periods become erratic and in-
creasingly shorter. Eventually, ovulation and menstruation cease
entirely. Tis normally occurs between the ages of 46 and 54, an
event called
is considered to have oc-
curred when a whole year has passed without menstruation.
Although the ovaries continue to produce estrogen for a while
aFer menopause, they finally stop functioning as endocrine or-
gans. Without sufficient estrogen the reproductive organs and
breasts begin to atrophy, the vagina becomes dry, and vaginal
infections become increasingly common. Other sequels due
to lack of estrogen include irritability and depression (in some
women); intense vasodilation of the skin’s blood vessels, which
causes uncomfortable, sweat-drenching “hot flashes”; gradually
thinning skin; and loss of bone mass. Slowly rising total blood
previous page 1092 Human Anatomy and Physiology (9th ed ) 2012 read online next page 1094 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off