**3. Normal closure of the ductus**

When a baby takes its first breath, the increased alveolar oxygen content leads to pulmonary vasodilation, resulting in a decreased ratio of pulmonary vascular resistance (RP) to systemic vascular resistance (RS). This drop in RP/RS can reverse the flow of blood across the ductus arteriosus from right-to-left to left-to-right (aorta to PA). At this point, the ductus is exposed to high systemic arterial pO2, which results in vasoconstriction of the ductus arteriosus through mechanisms not fully elucidated [3–5]. A sharp decline in circulating PGE and PGIE along with a rapid increase in pO2 following birth contributes to constriction, and ultimately functional closure of the ductus. Nearly all healthy, term infants achieve functional closure with 24–72 h [2–5]. Eventually, hypoxia and fibrosis cause the inner layers of the ductus to permanently close, leaving only a fibrous remnant called the ligamentum arteriosum.
