Fetal and neonatal pulmonary circulation.

During fetal life, gas exchange is carried out in the placenta. The lung does not have a physiological role, apart from possible metabolic functions which include secretion of hormones, enzymatic conversion of inactive substances to functional hormones, and degradation of active materials to inactive metabolites. Blood flow through the lungs is quite low during fetal life; most of the systemic and umbilical venous blood returning to the heart is shunted through the foramen ovale to the left atrium and left ventricle, or through the ductus arteriosus directly from the pulmonary trunk to the descending aorta. About 85-90% of the blood ejected by the right ventricle is diverted from the lungs through the ductus arteriosus (48). Because the right ventri­ cle ejects about 66% of the combined ventricular output of the fetal heart, about 60% of the total output of the heart passes through the ductus arteriosus and thus does not enter the lungs. Should this volume of blood pass through the pulmonary circulation and return to the left ventricle, there would be an unnecessary increase in the volume of work placed on the heart. The low fetal pulmonary blood flow has been explained on the basis of a high pulmonary vascular resistance. Mter birth, however, a marked in­ crease in pulmonary blood flow is necessary to permit adequate gas ex­ change. This review considers: (0) pulmonary arterial pressures and flows in utero and changes during gestation; (b) patterns of fetal pulmonary blood flow; (c) factors that influence fetal pulmonary vascular responses; and (d) mechanisms responsible for the postnatal decrease in pulmonary vascular resistance.