Importance of mitochondrial PO2 in maximal O2 transport and utilization: A theoretical analysis

In previous calculations of how the O2 transport system limits V˙O2max, it was reasonably assumed that mitochondrial PO2 (PmO2) could be neglected (set to zero). However, in reality, PmO2 must exceed zero and the red cell to mitochondrion diffusion gradient may therefore be reduced, impairing diffusive transport of O2 and V˙O2max. Accordingly, we investigated the influence of PmO2 on these calculations by coupling previously used equations for O2 transport to one for mitochondrial respiration relating mitochondrial V˙O2 to PO2. This hyperbolic function, characterized by its P50 and V˙MAX, allowed PmO2 to become a model output (rather than set to zero as previously). Simulations using data from exercising normal subjects showed that at V˙O2max, PmO2 was usually <1 mm Hg, and that the effects on V˙O2max were minimal. However, when O2 transport capacity exceeded mitochondrial V˙MAX, or if P50 were elevated, PmO2 often reached double digit values, thereby reducing the diffusion gradient and significantly decreasing V˙O2max.