Morphometric estimation of pulmonary diffusion capacity. IV. The normal dog lung

The lungs of 22 dogs, ranging from 2.5 to 27 kg in body weight, were analysed by morphometry. The lung volume, the alveolar and capillary surface area and the capillary blood volume increased linearly with body weight. For a dog of 23 kg mean values are: Vl= 1500 ml. Sa = 90 m², S_c = 72 m², Vc = 120 ml. The arithmetic mean barrier thickness was estimated at 1.5 μ, its harmonic mean at 0.48 μ. The mean alveolar diameter was proportional to the cube root of lung volume; alveoli in the upper lung regions were consistently larger than those in the dependent parts. Calculation of diffusion capacity D_L yielded, for a dog of 23 kg, a maximal value of 174 ml· min^-1· mm Hg^-1 and a minimal value of 84, depending on the physical coefficients introduced. Dl was proportional to W^1.15. The relationships of the morphometrically estimated D_L for the dog to physiological estimates and to data on other species, including man, are discussed. It is concluded that morphometrieally estimated Dl must always be higher than physiological estimates because it supposes full availability of the lung for O₂ transfer.