THE MICROVASCULAR ARCHITECTURE OF TERM FOETAL AND NEONATAL RABBIT LUNGS: A SCANNING ELECTRON MICROSCOPIC COMPARISON

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The microvascular architecture and the distribution of vascular resistance in the lungs of full term foetal and neonatal rabbits were studied by scanning electron microscopy of microvascular corrosion-casts and of specimens that had been perfusion-fixed by osmium. In the foetus, each potential air space was surrounded by its own capillary plexus; there were occasional, short anastamoses with adjacent plexuses. In the folded and compressed parenchymal tissue, two ‘lobule’ states were identified, one in which the vasculature was easy to perfuse, either with fixative or plastic, and another in which flow was inpeded. In the neonatal lung the parenchyma was inflated and the terminal air sacs rounded. The same capillary plexus arrangement was seen as in the foetus, but the perfusion of all the lobules was uniform. These findings suggest that the high pulmonary vascular resistance in the foetus is distributed unevenly. Such a system would allow maintenance of blood velocity sufficient to prevent sludging and stasis in the perfused, lower resistance areas, whilst total blood flow remained low. A cyclical pattern of vascular resistance and uniform lung development. The recruitment of the non-perfused areas at birth would rapidly drop pulmonary vascular resistance, and accommodate the vital increase in pulmonary blood flow at reduced perfusion pressures.