5
Giving Birth Requires a Complex Series of
Hormonal Changes 498
• At some point near the end of pregnancy, labor is initi-
ated. A positive feedback mechanism involving the uterus,
hypothalamus, and posterior pituitary using the hormone
oxytocin produces wave-like uterine contractions. The
contractions first dilate the cervix to about 10 cm (stage 1),
expel the baby (stage 2 as shown), and deliver the placenta
following birth (stage 3).
^ 6 Aging Alters Reproductive Capacity 502
• The reproductive systems of both genders are intact but
inactive from birth to about 10 years of age. At puberty,
rhythmic secretions of GnRH, LH
, and FSH stimulate the
ovarian and menstrual cycles in the female and spermato-
genesis in the male, along with sex hormone production
and development of secondary sex characteristics in both
genders. Both genders become capable of sexual reproduc-
tion. Between ages 15 and 40 (as shown), when couples
often marry, both genders are fully capable of reproduction.
L a b o r • F ig u r e 1 6 .1 7 b
Stage
2.
Expulsion extends from complete dilation of cervix to
delivery of the baby
( 1 0
min to several hours).
• During the process of lactation, milk is produced and
ejected to feed the baby. The hormone prolactin maintains
milk production in the mammary glands. Milk ejection from
the glands occurs through a positive feedback mechanism
among the nipples, hypothalamus, anterior pituitary, and
mammary glands involving the hormone oxytocin.
R e p r o d u c tiv e c a p a b ilitie s th r o u g h o u t life • F ig u r e 1 6 .1 9
• Between ages 40 and 50, females undergo menopause,
the cessation of menses. The number of viable follicles is
exhausted, and the ovarian and menstrual cycles cease. The
female is no longer capable of reproduction.
• The reproductive capacity of healthy men extends beyond
middle age and into the 80s and 90s. However, after age
55, men may experience decreases in the number of viable
sperm, reduced muscle strength, and decreased sexual
desire.
506 CHAPTER 16
The Reproductive Systems
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