Arterial blood gases during maximum metabolic demands: Patterns across the vertebrate spectrum

Elevations of metabolic rate, for example during physical activity, elicit immediate and coordinated respiratory and cardiovascular responses that ensure adequate diffusive and convective fluxes of O₂ from the environment (water or air) to the mitochondria where ATP is produced. The same physiological responses also provide for CO₂ to be removed in the opposite direction. There is significant variation in the morphology of the cardiovascular and respiratory structures among vertebrates, and a varying reliance on aerobic versus anaerobic metabolism to power activity. However, gas exchange in all vertebrates can be decribed as diffusive and convective steps in series, and we summarise data on the diffusive step across the respiratory surface of gills and lungs in this graphical review. Based on relatively constant arterial partial pressures of O₂ and CO₂ from rest to near maximal levels of physical activity, we conclude that under normoxic conditions, the diffusive step within the respiratory system exert no or small limitations for either O₂ or CO₂ exchange at or near maximal rate of oxygen consumption (VO₂max). However, there are exceptions, such as the exercise-induced arterial hypoxemia (EIAH) in racehorses, and elite human athletes. Our analysis also indicates that exercise-induced arterial hypercapnia (i.e. a rise in arterial PCO₂) at or near VO₂max is not common among vertebrates. Across the vertebrate spectrum, the diffusive and perfusive conductances (D/βQ) of water and air-breathing vertebrates are well-matched to maximal rates of gas exchange, and diffusion is not a limiting factor when aerobic metabolism increases.