In-situ pulmonary vascular morphology and lung volume in the fetal and neonatal rat.

In situ cross-sectional morphology of the pulmonary arteries, veins, and lung volumes were studied following rapid whole-body freezing of the fetal and neonatal rat. In the fetus, the right and left main pulmonary arteries were much smaller than the pulmonary trunk, and the ratio of the internal diameter of the right or the left pulmonary artery to the diameter of the pulmonary trunk was 0.44 and 0.38, respectively. One day after birth, the right and the left main pulmonary arteries enlarged, and the ratio of the right pulmonary artery to the pulmonary trunk increased to 0.82. The proximal right and left pulmonary arteries immediately adjacent to the bifurcation enlarged slowly, and showed diffuse narrowing at the proximal portions adjacent to the bifurcation. The narrowing disappeared 3 and 4 days after birth as the proximal pulmonary artery enlarged and the ratio of the right pulmonary artery to the pulmonary trunk reached 0.8. The pulmonary vein showed rapid enlargement in these neonates, concordant with the enlargement of the pulmonary arteries. Lung volume increased to 131% of the fetal volume 10 min after caesarean section. These studies quantitated neonatal pulmonary vascular adaptation. Delayed enlargement of the pulmonary arteries adjacent to the bifurcation in the neonatal period was shown, and presented a morphologic basis for the physiologic gradient and transient systolic murmur in the pulmonary arteries in human neonates.