Vertical gradient of alveolar size in lungs of dogs frozen intact.

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Abstract
Dog lungs were fixed in situ by freezing and alveolar size measured by histologic morphometric techniques. In the erect lung at functional residual capacity the apical alveoli were about 4 times larger by volume than the basal ones, most of the change in size being over the upper 10 cm of lung. The difference between apex and base increased to 11:1 when the animals were exposed to 3 G on a centrifuge. No difference in size was found when the lungs were expanded by 30 cm H2O pressure. In horizontal lungs, alveolar size was the same at the apex and base, but the most superior alveoli were larger than the dependent ones. In inverted dogs alveolar size was uniform from apex to base. The differences in alveolar volume can be explained if the transpulmonary pressure at any level is determined by the cross-sectional area of the lung and the weight of the lung below that level. This pressure apparently changes more rapidly in the upper part of the erect lung than in the lower part.

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