output, is diverted away from the lungs through the widely patent ductus arteriosus to the descending thoracic aorta [27] . Midway through gestation, pulmonary blood fl ow is approximately 3%-4% of the total combined ventricular output. This value increases progressively, reaching about 6% at 80% gestation, when the release of surface active material into lung fl uid begins and up to a maximum of 8%-10% at or near term [21, 28, 29] . Fetal pulmonary arterial pressure also increases with

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... g gestation. At term, mean pulmonary arterial pressure is about 50 mm Hg, generally exceeding the mean descending aortic pressure by 1-2 mm Hg [28, 30] . Pulmonary vascular resistance, which is extremely high in early gestation, falls progressively as pulmonary arterial development advances, which increases the cross-sectional area of the pulmonary circulation; however, the pulmonary vascular resistance of the fetus is still much higher than that of the neonate after birth [27, 31] . A number of mechanisms have been implicated in the maintenance of the high pulmonary vascular resistance and pulmonary arterial pressure during fetal life. These include mechanical factors, the low oxygen tension of fetal pulmonary and systemic blood, leukotrienes, thromboxane, ET-1, NO, prostaglandin (PG) I 2 , plateletderived growth factor (PDGF), and K + channels (32). The Transitional Circulation The transition from the fetal to the neonatal pulmonary circulation is marked by a dramatic fall in pulmonary vascular resistance and rise in pulmonary blood fl ow, which increases 8-10-fold (up to 300-400 mL/min/kg body weight). These changes are associated with the initiation of ventilation of the lungs and the subsequent increase in pulmonary and systemic arterial blood oxygen tensions. The increase in pulmonary blood fl ow increases pulmonary venous return and left atrial pressure, allowing the foramen ovale to close. In addition, the ductus arteriosus constricts, functionally closing within several hours after birth, which effectively separates the pulmonary and systemic circulations. Mean pulmonary arterial pressure decreases, and by 24 hr of age is approximately 50% of mean systemic arterial pressure. Under normal conditions, adult values are reached 2-6 weeks after birth [29] . The decrease in pulmonary vascular resistance with ventilation and oxygenation at birth is regulated by a complex and incompletely understood interplay between metabolic and mechanical factors. In experiments, physical expansion of the fetal lamb lung without changing oxygen tension increases fetal pulmonary