Modeling O2 transport as an integrated system limiting V˙O2MAX

Maximal endurance exercise capacity is determined by a variety of factors, including maximal ability to transport O2 to the muscle mitochondria and to use this O2 for ATP generation (V˙O2MAX). This analysis combines the individually well-known O2 mass conservation equations for the four critical steps in the O2 transport pathway (ventilation, alveolar/capillary diffusion, circulation and muscle diffusion) into an analytical, closed form, model showing how V˙O2MAX depends on all four steps. It further shows how changes in any one step affect the function of the others. This analytical approach however requires approximating the O2Hb dissociation curve as linear. Removing this condition to allow for the real O2Hb curve requires numerical analysis best explained graphically. Incorporating maximal mitochondrial metabolic capacity to use O2 allows prediction of when V˙O2MAX is limited by transport or by metabolic capacity. This simple approach recapitulates in vivo behavior and clarifies the determinants of maximal exercise.