913
CHAPTER TWENTY-THREE
Reproductive System
e. Immature kidneys cannot concentrate urine very
well.
(1) The newborn may become dehydrated.
(2) Water and electrolyte imbalances may develop.
f. Homeostatic mechanisms may function imperfectly,
and body temperature may be unstable.
g. The cardiovascular system changes when placental
circulation ceases.
(1) Umbilical vessels constrict.
(2) The ductus venosus constricts.
(3) The foramen ovale is closed by a valve as
blood pressure in the right atrium falls and
blood pressure in the left atrium rises.
(4) The ductus arteriosus constricts.
2. Infancy
a. Infancy extends from the end of the fourth week to
one year of age.
b. Infancy is a period of rapid growth.
(1) The muscular and nervous systems mature,
and coordinated activities become possible.
(2) Communication begins.
c. Rapid growth depends on an adequate intake of
proteins, vitamins, and minerals in addition to
energy sources.
3. Childhood
a. Childhood extends from the end of the F
rst year to
puberty.
b. Primary teeth erupt and are replaced by secondary
teeth.
c. It is a period of rapid growth, development of
muscular control, and establishment of bladder
and bowel control.
4. Adolescence
a. Adolescence extends from puberty to adulthood.
b. Physiological and anatomical changes result in a
reproductively functional individual.
c. ±emales may be taller and stronger than males in
early adolescence, but the situation reverses in late
adolescence.
d. Adolescents develop high levels of motor skills,
their intellectual abilities increase, and they
continue to mature emotionally.
5. Adulthood
a. Adulthood extends from adolescence to old age.
b. The adult remains relatively unchanged
physiologically and anatomically for many years.
c. After age thirty, degenerative changes begin.
(1) Skeletal muscles lose strength.
(2) The cardiovascular system becomes less
efF
cient.
(3) The skin loses elasticity.
(4) The capacity to produce sex cells declines.
6. Senescence
a. Senescence is growing old.
b. Degenerative changes continue, and the body
becomes less able to cope with demands.
c. Changes occur because of prolonged use, effects of
disease, and cellular alterations.
d. An aging person usually loses some intellectual
functions, sensory functions, and physiological
coordinating capacities.
e. Death usually results from mechanical
disturbances in the cardiovascular system or from
disease processes that affect vital organs.
concentration of oxygen-carrying hemoglobin is
greater in fetal blood, and fetal hemoglobin has
greater afF
nity for oxygen.
c. Blood enters the fetus through the umbilical vein
and partially bypasses the liver by means of the
ductus venosus.
d. Blood enters the right atrium and partially
bypasses the lungs by means of the foramen ovale.
e. Blood entering the pulmonary trunk partially
bypasses the lungs by means of the ductus arteriosus.
f. Blood enters the umbilical arteries from the
internal iliac arteries.
7. Birth process
a. Pregnancy usually lasts thirty-eight weeks after
fertilization.
b. During pregnancy, placental progesterone inhibits
uterine contractions.
c. Several events occur at birth.
(1) A decreasing concentration of progesterone
and the release of prostaglandins may initiate
the birth process.
(2) The posterior pituitary gland releases oxytocin.
(3) Uterine muscles are stimulated to contract,
and labor begins.
(4) Positive feedback causes stronger contractions
and greater release of oxytocin.
d. ±ollowing the birth of the infant, placental tissues
are expelled.
8. Milk production and secretion
a. During pregnancy, the breasts change.
(1) Estrogens cause the ductile system to grow.
(2) Progesterone causes development of alveolar
glands.
(3) Prolactin is released during pregnancy, but
progesterone inhibits milk production.
b. ±ollowing childbirth, the concentrations of
placental hormones decline.
(1) The action of prolactin is no longer blocked.
(2) The mammary glands begin to secrete milk.
c. Refl
ex response to mechanical stimulation of the
nipple causes the posterior pituitary to release
oxytocin, which releases milk from the alveolar
ducts.
d. As long as milk is removed from glands, more milk
is produced; if milk is not removed, production
ceases.
e. During the period of milk production, the
reproductive cycle is partially inhibited.
23.4
POSTNATAL PERIOD (PAGE 904)
1. Neonatal period
a. This period extends from birth to the end of the
fourth week.
b. The newborn must begin to respire, obtain
nutrients, excrete wastes, and regulate its body
temperature.
c. The F rst breath must be powerful to expand the
lungs.
(1) Surfactant reduces surface tension.
(2) A variety of factors stimulate the F
rst breath.
d. The liver is immature and unable to supply
sufF
cient glucose, so the newborn depends
primarily on stored fat for energy.
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