AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 3 384-R388, Copyright © 1984 by American Physiological Society
Pulmonary transit time and diffusing capacity in mammals
S. L. Lindstedt
Allometry is used as a tool to explain the apparent mismatch of oxygen
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 oxygen pressure difference increases as body size
decreases. The time course of oxygen binding to hemoglobin may limit
maximum oxygen consumption in the smallest mammals. To assure carbon
dioxide 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 oxygen delivery in all mammals.
