905
CHAPTER TWENTY-THREE
Reproductive System
pressure in the right atrium falls. Also, as the lungs expand
with the F
rst breathing movements, resistance to blood fl
ow
through the pulmonary circuit decreases, more blood enters
the left atrium through the pulmonary veins, and blood pres-
sure in the left atrium increases.
As the blood pressure in the left atrium rises and that in
the right atrium falls, the valvelike septum primum on the
left side of the atrial septum closes the foramen ovale. In
most individuals, this valve gradually fuses with the tissues
along the margin of the foramen. In an adult, a depression
called the
fossa ovalis
marks the site of the past opening.
The ductus arteriosus, like other fetal vessels, constricts
after birth. After this, blood can no longer bypass the lungs
by moving from the pulmonary trunk directly into the aorta.
In an adult, a cord called the
ligamentum arteriosum
repre-
sents the ductus arteriosus.
In
patent ductus arteriosus
(PDA), the ductus arteriosus fails to close
completely. After birth, the metabolic rate and oxygen consump-
tion in neonatal tissues increase, in large part to maintain body tem-
perature. If the ductus arteriosus remains open, the neonate’s blood
oxygen concentration may be too low to adequately supply body
tissues, including the myocardium. If PDA is not corrected surgically,
the heart may fail, even though the myocardium is normal.
Changes in the newborn’s cardiovascular system are
gradual. Although constriction of the ductus arteriosus may
be functionally complete within F fteen minutes, the perma-
nent closure of the foramen ovale may take up to a year.
These cardiovascular changes are illustrated in
f
gure 23.32
and summarized in
table 23.8
.
±etal hemoglobin production falls after birth. By the time
an infant is four months old, most of the circulating hemo-
globin is the adult type.
PRACTICE
35
DeF
ne
neonatal period.
36
What factors stimulate the F
rst breath?
37
What does a newborn use for energy during its F
rst few days?
38
How do the kidneys of a newborn di±
er from those of an adult?
39
What is the fate of the foramen ovale? Of the ductus arteriosus?
Infancy
The period of continual development extending from the end
of the F rst four weeks to one year is called
infancy.
During
this time, the infant grows rapidly and may triple its birth
weight. Its teeth begin to erupt through the gums, and its
muscular and nervous systems mature so that coordinated
muscular activities become possible. The infant is soon able
to visually follow objects; reach for and grasp objects; and
sit, creep, and stand.
immediate need is to obtain oxygen and excrete carbon diox-
ide, so the F
rst breath is critical.
The F
rst breath must be particularly forceful because the
newborn’s lungs are collapsed and the airways are small,
offering considerable resistance to air movement. Also,
surface tension tends to hold the moist membranes of the
lungs together. However, the lungs of a full-term fetus con-
tinuously secrete
surfactant
(see chapter 19, p. 750), which
reduces surface tension. After the first powerful breath
begins to expand the lungs, breathing eases.
A newborn’s F
rst breath is stimulated by increasing con-
centration of carbon dioxide, decreasing pH, low oxygen
concentration, drop in body temperature, and mechanical
stimulation during and after birth. Also, in response to the
stress the fetus experiences during birth, blood concentra-
tions of epinephrine and norepinephrine rise signiF cantly
(see chapter 13, p. 514). These hormones promote normal
breathing by increasing the secretion of surfactant and dilat-
ing the airways.
±or energy, the fetus primarily uses glucose and fatty
acids in the pregnant woman’s blood. The newborn, on the
other hand, is suddenly without an external source of nutri-
ents. The mother will not produce mature milk for two to
three days, by which time the infant’s gastrointestinal tract
will be able to digest it. The early milk,
colostrum,
is an adap-
tation to the state of the newborn’s digestive physiology. The
newborn has a high metabolic rate, and its liver, not fully
mature, may be unable to supply enough glucose to support
metabolism. Instead, the newborn uses stored fat for energy.
A newborn’s kidneys are usually unable to produce con-
centrated urine, so they excrete a dilute fl
uid. ±or this rea-
son, the newborn may become dehydrated and develop a
water and electrolyte imbalance. Also, certain homeostatic
control mechanisms may not function adequately. ±or exam-
ple, during the F rst few days of life, body temperature may
respond to slight stimuli by fl
uctuating above or below the
normal level.
When the placenta ceases to function and breath-
ing begins, the newborn’s cardiovascular system changes.
±ollowing birth, the umbilical vessels constrict. The umbili-
cal arteries close first, and if the umbilical cord is not
clamped or severed for a minute or so, blood continues to
fl ow from the placenta to the newborn through the umbilical
vein, adding to the newborn’s blood volume.
The proximal portions of the umbilical arteries persist in
the adult as the
superior vesical arteries
that supply blood
to the urinary bladder. The more distal portions become
solid cords (lateral umbilical ligaments). The umbilical vein
becomes the cordlike
ligamentum teres
that extends from the
umbilicus to the liver in an adult. The ductus venosus con-
stricts shortly after birth and appears in the adult as a F
brous
cord (ligamentum venosum) superF
cially embedded in the
wall of the liver.
The foramen ovale closes as a result of blood pressure
changes in the right and left atria. As blood ceases to fl ow
from the umbilical vein into the inferior vena cava, the blood
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