Five changes happen essentially at once, and they all follow from two events: the lungs inflate and the placenta is removed. Pulmonary vascular resistance FALLS, because the alveoli open and the vessels dilate in response to rising oxygen. Systemic vascular resistance RISES, because the placenta was a large, very low-resistance circuit and it has just been disconnected, so left ventricular afterload rises. Left ventricular preload RISES, because pulmonary blood flow has increased and far more blood now returns through the pulmonary veins to the left atrium, and with it the left ventricular stroke volume rises. The pressure consequence is that LA pressure now exceeds RA pressure, which presses septum primum against septum secundum and closes the foramen ovale. The murmur question follows directly from the first two changes. In utero the duct shunted right to left, from the high-resistance pulmonary side to the aorta, silently. After birth the gradient reverses, since pulmonary resistance is now low and systemic resistance high, so flow runs left to right, aorta into pulmonary artery, throughout systole and diastole, producing the continuous machine-like murmur. If the duct fails to close it is treated with indomethacin; if the infant depends on it, prostaglandin E1 keeps it open instead.