Modeling oxygen availability to exercising muscle

To quantify the role of factors determining O2 availability to exercising muscle, a model previously devised for the analysis of O2 uptake in lungs is used. With the variables: (1) blood flow (Q); (2) slope of the blood O2 equilibrium curve (betaO2); and (3) muscle blood-tissue O2 diffusing capacity (D(O2)), the extent of diffusion and perfusion limitations are determined by the 'equilibration index', Y(O2) = D(O2)/(Q x betaO2). Application to recent literature values on maximal O2 uptake in humans reveals diffusion limitation to be less important than perfusion limitation in normoxia, but to be predominant in deep hypoxia. The validity of the model is restricted by several factors whose disregard leads to an underestimation of muscle D(O2), i.e. an overestimation of diffusion limitation.