QUANTITATIVE CHANGES IN THE RAT PULMONARY VASCULATURE IN CHRONIC HYPOXIA - RELATION TO HAEMODYNAMIC CHANGES

The anatomical basis of resistance and compliance changes of the pulmonary arterial bed was studied in rats exposed to chronic hypoxia (10% O 2 , 3 weeks) and the findings were compared with those of normoxic rats. The lungs were perfused with a Ba-gelatine mixture at different pressures and studied by radiology and histology. The diameter of the pulmonary arteries (〉 0·5 mm), measured from X-rays, was less in chronically hypoxic than normoxic rats when filled at the same perfusion pressure. Diameters increased in both groups with increasing perfusion pressure but at a given pressure those of chronically hypoxic rats were always smaller than those of normoxic rats. We found evidence that arterial length was increased in chronically hypoxic rats. Arterioles of 50 µm or less in diameter adjacent to gas exchange units were of similar external diameter in normoxic and chronically hypoxic rats, but most of the latter had developed a muscular coat and a second elastic lamina internal to the single elastic lamina of control arterioles. These changes reduced the lumen by an estimated 10-14% and would increase pulmonary arteriolar resistance in chronically hypoxic rats, resulting in a changed pressure profile. We found no evidence of arteriolar loss in chronically hypoxic rats although at a given pressure, the Ba-gelatine mixture penetrated less far for reasons which are discussed.