Determinants of maximal oxygen consumption in vertebrates.

Across vertebrates, the maximal rate of oxygen consumption (V˙O2max) defines the capacity of the respiratory and cardiovascular systems to transport oxygen from the environment to the cells. While V˙O2max depends on multiple conductance steps in the oxygen transport pathway - ventilation, respiratory diffusion, circulatory convection, and tissue diffusion - the relative importance of each step differs across species with varying gas exchange organs, cardiac morphologies, and aerobic capacities. I analyzed the determinants of V˙O2max across vertebrates by conducting a sensitivity analysis of V˙O2max limitations in a fish, and synthesizing published sensitivity analyses for an amphibian, a bird, the Thoroughbred racehorse, and humans of average and elite athletic ability. I also compared the effects of hypoxia on conductance contributions using data from birds and humans. To compare models, I calculated fractional limitations from modelled changes in V˙O2max following perturbations to each conductance step. Results reveal similar patterns: circulatory conductance (cardiac output) dominates in species with lower aerobic capacity, while diffusive and ventilatory conductance become more influential in athletic species and in hypoxia. Athletic phenotypes appear to operate at the functional limits of the lung - a consequence, at least in part, of high cardiac output. These findings reinforce the importance of viewing oxygen transport as an integrative system with multiple components that can individually or jointly constrain V˙O2max. Ultimately, the entire system serves to deliver oxygenated red bloods to the muscle capillaries, where the cumulative contributions of all upstream processes influence oxygen transfer from red blood cells to mitochondria.