The nurse is explaining the physiologic mechanisms responsible for closure of the fetal structures or shunts at birth. What is an appropriate explanation?
a. When the umbilical cord is clamped, the left heart pressure is raised causing the foramen ovale to close between the right and left atria.
b. Increase in the blood oxygenation level at birth constricts the pulmonary arterioles, which dilates and closes the ductus arteriosus.
c. Clamping of the umbilical cord at birth causes a redistributing of blood, which increases blood flow through the ductus venosus and causes it to dilate.
d. When the neonate takes a breath, it causes the left heart pressure to rise and the foramen ovale to close.
ANS: A
Cord clamping causes a large stream of blood from the placenta to be cut off, raising neonatal left heart pressure, closing the foramen ovale between the right and left atria of the heart. Increase in blood oxygenation level at birth dilates pulmonary arterioles, which constricts (not dilates) and closes the ductus arteriosus. Cord clamping causes redistribution of blood, decreasing (not increasing) blood flow through the ductus venosus, which then constricts (not dilates) and closes the ductus venosus. Closure of the ductus venosus forces blood perfusion of the liver. The exact mechanism for the closure is unknown.
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