the umbilical vein. The reduced blood flow through the umbilical vein at birth will collapse and close the ductus venosus. Hence, the IVC will only carry deoxygenated blood from the infant’s organs and lower extremities. Second, as the infant breathes, the lungs will expand and fill the alveoli with oxygen. The increased oxygen content will dilate the pulmonary capillaries and also trigger the release of nitric oxide, which further dilates the blood vessels within the lungs. Together, these forces will decrease the pulmonary vascular resistance. With decreased resistance in the lungs, there will be increased blood flow to the lungs from the right ventricle of the heart through the pulmonary arteries, establishing the newborn’s pulmonary circulation. With each of the newborn’s breaths, blood perfuses the pulmonary capillary beds and undergoes oxygenation before exiting the lungs via the pulmonary veins and returning to the heart. Thus, as more blood flows through the pulmonary circulation, there will be a higher volume of blood returning to the left atrium from the lungs. The increased venous return will elevate the pressure of the left atrium until it exceeds the pressure of the right atrium. The difference in pressure between these two chambers of the heart will close the foramen ovale. Lastly, due to the decreased pulmonary vascular resistance, the pressure of the pulmonary artery will fall until it is lower than the pressure of the aorta. Since blood flows from high to low pressure systems, the direction of blood flow across the ductus arteriosus reverses. As the oxygen-rich blood from the aorta flows across the ductus arteriosus to the pulmonary artery, the ductus arteriosus will constrict in response to the high oxygen content of the blood. While oxygen serves as a vasoconstrictor of the ductus arteriosus, prostaglandins can keep the ductus arteriosus open to maintain blood flow to the lower extremities in cases of hypoplastic left heart syndrome where the mitral valve is shut. The removal of the placenta, a source of prostaglandin, is another mechanism by which the ductus arteriosus closes at birth. Within the next 2 to 3 weeks, the constriction results in decreased blood flow to the structure which induces the death of the tissue to keep the structure permanently closed. As a result of these changes, postnatal circulation will direct deoxygenated blood from the inferior and superior vena cava to the right heart, from which the blood will flow to