Circulatory adjustments at birthThese are brought about because of a shift from placental dependance for gas exchange in the fetus to pulmonary gas exchange in the neonate.1) Pulmonary circulationImmediately after birth lungs expand due to the first few breaths of the neonate. This causes a fall in pulmonary vascular resistance (O2 causes pulmonary vasodilatation). This results in increased flow into pulmonary trunk and arteries. The pulmonary artery pressure falls due to lowering of pulmonary vascular resistance. The pressure relations between the aorta and pulmonary trunk are reversed so that the blood flow through the ductus arteriosus is reversed → Instead of blood flowing from the pulmonary artery to aorta, the direction of flow through ductus, is from the aorta to pulmonary trunk. Increasing oxygen saturation causes the muscle of ductus to constrict →In full term neonates, the ductus  arteriosus closes within 10 to 25 days. 2) System circulation and circulation through the heartLoss of placental circulation and clamping of the cord after birth results in an increase in systemic vascular resistance. This tends to increase the aortic blood pressure and the left ventricular systolic pressure. The loss of placental circulation results in a sudden reduction of flow through ductus venosus which closes off  → Flow through ductus venosus disappears by the 7th day of postnatal life. The loss of placental flow results in a decrease in the volume of blood returning to right atrium → Right atrial pressure decreases. The left atrial pressure becomes higher than right atrial pressure and the septum primum which ats as a valve of fossa ovalis, approximates with the septum secundum to close off formen ovale. Functional closure of foramen ovale occurs very quickly. Over a period of months, the septum primtun and septum secundum become firmly adherent resulting in anatomical closure of the foramen ovale. After closure of ductus arteriosus, there is establishment of postnatal circulation : - The blood reaching the right atrium through IVC and SVC is emptied into the right ventricle from where it is pumped into pulmonary trunk. After coursing through lungs for gas exchange, it reaches the left atrium and ventricle. The left ventricle pumps it out for distribution in the body for oxygenation of the tissues. The venous return again comes back to right atrium through IVC and SVC. All of the blood leaving the right ventricle, after coursing through lungs, reaches the left ventricle → The two ventricles are connected in series and therefore, the output of right and left ventricles are same (in contrast to fetal circulation, where right ventricular output is more).