6-14. CHANGES DUE TO FERTILIZATION OF THE OVUM
If the ovum is fertilized by a sperm cell, the menstrual cycle ceases. After the
fertilized ovum passes through the fallopian tube, it implants into the already prepared
endometrium. The embryo (fertilized egg) grows rapidly and soon develops a placenta.
The placenta is a tissue that eventually covers about one-fourth of the uterus. The
placenta is located between the endometrium and the fetus. The placenta is supplied
with blood vessels from the mother and blood vessels from the embryo through the
umbilical cord. There is no direct exchange of blood between the mother and the
embryo; however, the embryo is able to receive nutrients, electrolytes, and oxygen from
the mother's blood by the processes of diffusion and active transport. Likewise, waste
products from the embryo's system are diffused from the embryo's blood to the mother's
blood. The fetus is surrounded by its own membranes and is supported by the amniotic
fluid in the amniotic sac filling the uterus. The endometrium and placenta are
maintained by high levels of progesterone, which acts to cause an increase in the
concentration of nutrients in the endometrium, reduce uterine contraction, and prepare
the breasts for lactation. For about the first trimester of pregnancy, the progesterones
are supplied by the corpus luteum. The corpus luteum, which normally degenerates
after two weeks, is itself maintained by another hormone, chorionic gonadotropin, which
is produced by the cells of the fetus (embryo) very soon after implantation. After the
first trimester of pregnancy, the corpus luteum degenerates and the progesterone
becomes produced by the placenta. If, at any time during this "change over" the
progesterone level falls too low, the endometrium will degenerate causing a
spontaneous abortion. The estrogens produced during pregnancy come from the same
sources as do the progesterones. The estrogens function to enlarge the uterus and the
breast.
6-15. MENOPAUSE
Women usually stop menstruating at about the age of 45. This is known as the
menopause. At this time, nearly all the primary follicles in the ovaries have been
released or have become involuted (returned to normal size). Since the primary follicles
supply most of the body's estrogen, the cyclic increase and decrease of estrogens
cannot occur. Thus, the menstrual cycle is ended. Some women experience various
effects (for example, hot flashes, fatigue, anxiety, and irritability) because of the
metabolic changes the body is undergoing because of the decreased production of
estrogen. The physician may prescribe estrogen therapy to the woman during this
time.
Section III. DISORDERS OF THE HUMAN REPRODUCTIVE SYSTEM
6-16. INTRODUCTION
There are numerous disorders of the human reproductive system that can occur.
This section of the lesson will consider some of these disorders.
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