The distensibility of pulmonary veins of cats, in the diametric range of 100–1200 μm, was studied as a function of the venous pressure pv and pleural pressure pPL, while the alveolar air pressure was maintained at zero (atmospheric). The resulting percentage changes in diameter normalized with respect to the diameter at ΔP of 10 cm H2O (D10) are expressed as function of ΔP = pv − pPL. It was found that in most cases the diameter varies linearly with ΔP:  
D/D10=1+α(pvpPL)
where α is the compliance coefficient. The results show that smaller veins of the cat are more compliant than larger veins. For example, when pleural pressure is −10 cm H2O, the values of α for vessels in the ranges of diameters of 100–200 μm, 200–400 μm, 400–800 μm and 800–1200 μm are, respectively, 2.05, 1.44, 1.08 and 0.71 percent per cm H2O or Pa−1. The effects of lung inflation on the distensibility of pulmonary veins are also studied. Our results show that for vessel sizes in the range of 400–800 μm and 800–1200 μm the compliance constant α is not affected by inflation of the lungs (changes in pleural pressure to more negative values). For smaller veins in the size ranges 100–200 μm and 200–400 μm our results show an increase in compliance from 2.05 to 2.79 and from 1.44 to 2.01 percent per cm H2O or Pa−1, respectively, when pleural pressure is changed from −10 to −15 cm H2O. When the pleural pressure is more negative than −15 cm H2O, however, the compliance of the vessels in the foregoing two size ranges is observed to decrease.
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