High rate of O₂ consumption in exercising foxes: large P_o2 difference drives diffusion across the lung

The fox has one of the highest mass specific rates of maximal oxygen consumption ( V ̇_o2 max/M_b) that has been measured, yet its specific pulmonary diffusing capacity (Dl_O2 /M_b, measured morphometrically) is similar to that of most mammalian species. It achieves a high O₂ flux per unit Dl_O2 with a large partial pressure difference driving O₂ diffusion from alveolar gas to capillary blood (Pa_O2 Pb_O2 ). This paper explores the mechanisms that the fox utilizes to achieve this large pressure difference and the extent to which it exploits its structural diffusing capacity. Foxes were exercised on a treadmill at maximal rates of O₂ uptake. The following parameters were measured or calculated: arterial and mixed venous P_O2 , P_CO2 , pH and O₂ concentration of the blood, cardiac output, hemoglobin concentration and O₂ equilibrium curve of the blood, and morphometric estimates of pulmonary capillary volume and pulmonary diffusing capacity for O₂. These data were used to calculate pulmonary capillary transit time and the time course of the change in O₂ concentration and P_O2 of the blood as it transits the lung. The fox has a morphometric pulmonary diffusing capacity of 0.098 ml O₂ · sec^-1 · mm Hg^-1 · kg^-1. At V̇_O2 max (3.6 ml O₂ · sec^-1 · kg^-1) the fox hyperventilates, resulting in a high Pa_O2 (124 mm Hg); it also maintains a low Pb_O2 (88 mm Hg) by having a short transit time (0.13 sec) due to a high specific cardiac output (25 ml · sec^-1 · kg^-1). Our calculations indicate that at V̇_O2 max the ifox uses almost all of the pulmonary capillary transit time for O₂ equilibration, in contrast to other species.