Explanation: Normal closure of the ductus arteriosus (DA) is brought about by a number of
mechanisms.
Upon delivery the new born babies first breath inflates the lungs and this causes a rise in pO2.
Oxygen is known to cause constriction of the DA in vivo and in vitro. In addition the decreased
vascular resistance means the pressure within the lumen of the DA drops aiding closure.
In addition on inflation of the lungs, the lungs produce bradykinin which stimulates smooth
muscle constriction of the ductus.
The most important factor is thought to be the drop in Prostaglandin E2. Prostaglandin E2
maintains patency of the Ductus in the unborn child but after birth is metabolised in the lungs
and its levels fall rapidly within 3 hours of birth.