At birth, the most potent stimulus for constriction of the ductus arteriosus is an increase in what?

The most potent stimulus for the constriction of the ductus arteriosus at birth is an increase in arterial oxygen tension (PaO2). During fetal life, the ductus arteriosus allows blood to bypass the non-functioning lungs by connecting the pulmonary artery directly to the aorta, maintaining oxygen saturation through placental circulation. At birth, when the neonate takes its first breaths, there is a significant increase in oxygen availability because the lungs expand and begin to function.

As the lungs fill with air, the PaO2 increases due to better oxygen exchange, resulting in higher oxygen levels in the blood. This rise in PaO2 is essential because it triggers the vascular smooth muscle of the ductus arteriosus to constrict. The closure of this shunt is a normal part of the transition from fetal to neonatal circulation, leading to the eventual formation of the ligamentum arteriosum.

Higher levels of carbon dioxide (PCO2) and lower pH do not promote ductal constriction; rather, they may maintain ductal patency as they are associated with hypoxemia and metabolic acidosis, conditions that can occur without proper oxygenation. While CO2 levels can influence respiratory drive, it is the increase in oxygen (