Theoretical predictions of end-capillary PO₂ in muscles of athletic and nonathletic animals at V̇O(2max)

Characterizing the resistances to O₂ transport from the erythrocyte to the mitochondrion is important in understanding potential transport limitations. A steady-state model of this process was developed to predict the minimum (critical) end-capillary PO₂ required to prevent hypoxia at maximal O₂ consumption (V̇O(2max)) in a circular region of tissue surrounding the venular end of a capillary. Capillary density was used as a measure of O₂ delivery, and mitochondrial density was used as a measure of O₂ consumption. The effects of oxyhemoglobin dissociation kinetics and diffusion facilitation by hemoglobin in the erythrocytes and facilitation by myoglobin in the tissue were taken into account. Calculations made for selected skeletal muscles, diaphragm, and myocardium in three adaptive animal pairs (dog and goat, horse and cow, and pony and calf) yielded values of end- capillary PO₂ that were consistent with measured values of mixed venous PO₂ in maximally working animals. Values of end-capillary PO₂ were found to be uncorrelated with values of V̇O(2max) in different muscles. No significant difference in end-capillary PO₂ was found between similar muscles of athletic versus nonathletic animals. Predicted intracapillary O₂ transport resistance ranged from 18 to 54% of the total transport resistance in the O₂ pathway. Further investigation is required to explore the extent to which spatial and temporal heterogeneities in O₂ delivery and consumption play a role in O₂ transport.