When synthesizing the available data of vessel elasticity of mammalian lung from the literature, the lack of data in the intermediate range of vessel sizes becomes evident. In an effort to fill this gap, the distensibility of pulmonary arteries of cats, in the range of 100–1600 μm diameter was studied as a function of the perfusion pressure. The resulting percentage changes in vessel diameter (D) were expressed as polynomials of “transmural” pressure, which is taken to be the difference between the perfusion pressure and the pleural pressure, pa − pPL, in the form  
where Do is the value of D when pa = pPL, and α and β are constants. Our results show that for vessels whose diameters Do are in the range of 100–200 μ;m, the mean values of D/Do are represented by α = 2.02 percent per cm H2O or 0.202 (KPa)−1, β = 0.046 percent per (cm H2O)2 or 0.046 (KPa)−2. For vessels with diameter Do greater than 200 μm, the pressure-diameter relationship is linear in the ranges tested, so that β = 0. The values of the compliance constant α (slopes of the curves) for vessels in the diameter (Do) ranges 200–300 μm, 300–400 μm, 400–600 μm, 600–1000 μm, and 1000–1600 μm are, respectively, 0.93, 0.78, 0.70, 1.10, and 2.61 percent per cm H2O, (i.e., these numbers × 10−1 (KPa)−1). Thus the compliance of the pulmonary arteries appears to be the smallest in the diameter range 400–600 μm; and that the compliance of vessels in the 1000–1600 μm range is more than twice that of the smaller vessels.
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