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Pulmonary and ductus arteriosus circulation was studied in lambs before and after delivery. Flows were measured with electromagnetic flowmeters and pulmonary and systemic pressures with strain gauges. Before interrupting the umbilical circulation and expanding the lungs, pulmonary artery pressure was higher than system pressure and the direction of flow in the ductus was from right to left. Flow in the pulmonary artery varied from 154 to 258 ml/kg/min, of which about 60% passed through the ductus and the remaining through the lungs. After cutting the umbilical cord and expanding the lungs, systemic pressure rose and pulmonary pressure fell. This reversed the direction of flow in the ductus. Pulmonary artery flow fell temporarily after cord occlusion but increased when respiration was established. Ductus flow decreased progressively until the end of the experiment. Total flow to the lungs increased by four- to sixfold after respiration had been established and the increase was due to reduction in pulmonary vascular resistance.

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