North American Registry of Midwives (NARM) Practice Exam

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What is the primary factor responsible for the closure of the ductus arteriosus?

Decrease in blood temperature
Increase in oxygen levels in the blood
Rapid decrease in heart rate
Increase in carbon dioxide levels

The correct answer is: Increase in oxygen levels in the blood

The closure of the ductus arteriosus is primarily influenced by an increase in oxygen levels in the blood. After birth, when a newborn takes its first breath, the lungs expand, allowing oxygen-rich blood to flow into the pulmonary circulation. This rise in oxygen levels prompts the smooth muscle in the walls of the ductus arteriosus to constrict. As a result, the ductus arteriosus, which previously allowed blood to bypass the non-functioning fetal lungs, begins to close off, effectively diverting blood to the now-functioning lungs for oxygenation. In addition to the change in oxygen levels, the transition from fetal to neonatal circulation is marked by several other physiological changes in the newborn, but the immediate and most significant trigger for the ductus arteriosus closure is indeed the increase in oxygenation that occurs with the onset of breathing.