Arteriolar contribution to microcirculatory CO2/O2 exchange

The role of arterioles and capillaries in microcirculatory gas exchange was evaluated using a multicompartmental model for O2-CO2 transport in the rat skeletal microcirculation. Model predictions were examined to investigate the effects of model formulation and model parameter values. The factors in model formulation included radial blood diffusion resistance, the discrete nature of capillary blood, and the method of determining compartmental fluxes. A comparison with earlier models in the literature indicated that, by refining the method for determining compartmental fluxes, the CO2 flux contribution ratio of arterioles versus capillaries (Fa/Fc) increased by 52% during rest and diminished by 34% during moderate exercise. It also resulted in negative venular fluxes during exercise. Incorporating radial blood diffusion resistance into the model lead to a decrease of up to 43% in Fa/Fc. It also resulted in a decrease in central arteriole-venule shunt. Including the discrete nature of capillary blood into the model caused a small increase in Fa/Fc. Results indicated similar effects of these factors on oxygen Fa/Fc. Model parameters whose effects were investigated included metabolic rate (M), blood flow rate (Q), ratio of arteriolar diffusion conductance versus capillary diffusion conductance (Ea/Ec), the magnitude of arteriolar diffusion conductances (Ea), and the CO2/O2 respiratory quotient (Qu). Simulation results suggested that Q was a major factor responsible for the variations in Fa/Fc when the rest/exercise state of rat skeletal muscle changes. Ea and Qu were also responsible for differences in model predictions for different body organs or animal species. Our model predicts capillary dominance in both CO2 and O2 exchange and reveals the existence, under certain conditions, of negative venular flux contribution.