584 SECTION VICardiovascular Physiology
CHANGES IN FETAL CIRCULATION
& RESPIRATION AT BIRTH
Because of the patent ductus arteriosus and foramen ovale
(Figure 34–18), the left heart and right heart pump in parallel
in the fetus rather than in series as they do in the adult. At
birth, the placental circulation is cut off and the peripheral re-
sistance suddenly rises. The pressure in the aorta rises until it
exceeds that in the pulmonary artery. Meanwhile, because the
placental circulation has been cut off, the infant becomes in-
creasingly asphyxial. Finally, the infant gasps several times,
and the lungs expand. The markedly negative intrapleural
pressure (–30 to –50 mm Hg) during the gasps contributes to
the expansion of the lungs, but other factors are likely also in-
volved. The sucking action of the first breath plus constriction
of the umbilical veins squeezes as much as 100 mL of blood
from the placenta (the “placental transfusion”).
Once the lungs are expanded, the pulmonary vascular resis-
tance falls to less than 20% of the value in utero, and pulmonary
blood flow increases markedly. Blood returning from the lungs
raises the pressure in the left atrium, closing the foramen ovale
by pushing the valve that guards it against the interatrial sep-
tum. The ductus arteriosus constricts within a few hours after
birth, producing functional closure, and permanent anatomic
closure follows in the next 24–48 h due to extensive intimal
thickening. The mechanism producing the initial constriction
is not completely understood, but the increase in arterial O 2
tension plays an important role. Relatively high concentrations
of vasodilators are present in the ductus in utero—especially
prostaglandin F2a—and synthesis of these prostaglandins is
blocked by inhibition of cyclooxygenase at birth. In many pre-
mature infants the ductus fails to close spontaneously, but
closure can be produced by infusion of drugs that inhibit
cyclooxygenase. NO may also be involved in maintaining
ductal patency in this setting.
CHAPTER SUMMARY
■ Cerebrospinal fluid is produced predominantly in the choroid
plexus of the brain, in part via active transport mechanisms in
the choroid epithelial cells. Fluid is reabsorbed into the blood-
stream to maintain appropriate pressure in the setting of contin-
uous production.
■ The permeation of circulating substances into the brain is tight-
ly controlled. Water, CO 2 , and O 2 permeate freely. Other sub-
stances (such as glucose) require specific transport mechanisms,
whereas entry of macromolecules is negligible. The effectiveness
of the blood–brain barrier in preventing entry of xenobiotics is
bolstered by active efflux mediated by P-glycoprotein.
■ The coronary circulation supplies oxygen to the contracting
myocardium. Metabolic products and neural input induce vaso-
dilation as needed for oxygen demand. Blockage of coronary
arteries may lead to irreversible injury to heart tissue.
■ Control of cutaneous blood flow is a key facet of temperature
regulation, and is underpinned by varying levels of shunting
through arteriovenous anastomoses. Hypoxia, axon reflexes,and sympathetic input are all important determinants of flow
through the cutaneous vasculature.
■ The fetal circulation cooperates with that of the placenta and
uterus to deliver oxygen and nutrients to the growing fetus, as
well as carrying away waste products. Unique anatomic features
of the fetal circulation as well as biochemical properties of fetal
hemoglobin serve to ensure adequate O 2 supply, particularly to
the head. At birth, the foramen ovale and the ductus arteriosus
close such that the neonatal lungs now serve as the site for oxy-
gen exchange.MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.- Blood in which of the following vessels normally has the lowest
PO 2?
A) maternal artery
B) maternal uterine vein
C) maternal femoral vein
D) umbilical artery
E) umbilical vein - The pressure differential between the heart and the aorta is least
in the
A) left ventricle during systole.
B) left ventricle during diastole.
C) right ventricle during systole.
D) right ventricle during diastole.
E) left atrium during systole. - Injection of tissue plasminogen activator (t-PA) would probably
be most beneficial
A) after at least 1 y of uncomplicated recovery following occlu-
sion of a coronary artery.
B) after at least 2 mo of rest and recuperation following occlu-
sion of a coronary artery.
C) during the second week after occlusion of a coronary artery.
D) during the second day after occlusion of a coronary artery.
E) during the second hour after occlusion of a coronary artery. - Which of the following organs has the greatest blood flow per
100 g of tissue?
A) brain
B) heart muscle
C) skin
D) liver
E) kidneys - Which of the following does not dilate arterioles in the skin?
A) increased body temperature
B) epinephrine
C) bradykinin
D) substance P
E) vasopressin