Pulmonary transit time and diffusing capacity in mammals

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Abstract
Allometry is used as a tool to explain the apparent mismatch of O2 consumption and diffusing capacity in the mammalian lung. By combining equations for pulmonary capillary volume and cardiac output, it is apparent that erythrocyte transit time through the lung must scale disproportionately to body mass. This inequality is a consequence of physical and mechanical properties setting optimal cardiac and respiratory frequencies. Because of much shorter transit times, the mean alveolar-capillary O2 pressure difference increases as body size decreases. The time course of O2 binding to Hb may limit maximum O2 consumption in the smallest mammals. To assure CO2 diffusion equilibrium, levels of carbonic anhydrase are much higher in small than in large mammals. Because of the differences in transit time, the pulmonary diffusing capacity must scale linearly to body mass to assure adequate O2 delivery in all mammals.

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