Influence of O2-Hb kinetics and the Fähraeus effect on the arteriolar role in gas exchange.

Abstract

The role of precapillary and postcapillary vessels in microcirculatory gas exchange have received considerable attention[1] ever since experimental findings indicated that significant amount of oxygen is exchanged between arterioles and the surrounding tissue[2–4]. Roth & Wade[5] simulated O2-CO2 coupled transport in the rat skeletal muscle microcirculation using a multicompartmental model. One of the noticeable results of their simulation was a dominant precapillary CO2 flux leading to the conclusion that virtually all of the CO2 exchange takes place in precapillary vessels during rest. They also concluded that, during exercise, 80% of CO2 is exchanged before the capillary bed. Our simulation[6] for cat cerebral O2-CO2 transport predicted that the arteriolar contribution to total CO2 flux was less than 20% and therefore suggested that capillaries were the prime site of CO2 exchange in the microcirculation. Our later work[7], using a multicompartmental model for O2-CO2 coupled transport in the rat skeletal microcirculation under various rest/exercise conditions, suggested that a number of factors in model formulation or in simulation parameters may affect model prediction regarding the roles of arterioles and capillaries in microcirculatory gas exchange, particularly in CO2 exchange. Simplifying assumption for compartmental concentration gradient, which was used by a number of earlier models, may significantly alter the result depending on the conditions. In addition, omission of radial blood diffusion resistance, which was also assumed by many investigators, may lead to an overestimation of arteriolar fluxes (Fa). Lower blood flow rate, higher arteriolar diffusion conductances, or higher CO2/O2 respiratory quotient may result in a higher contribution ratio of arterioles versus capillaries (Fa/Fc). These factors may contribute to the differences in predictions of Fa/Fc for different body organs or species of animals. In contrast with earlier models in the literature, our model usually predicts capillary dominance for O2 and CO2 exchange, and reveals the existence of negative venular flux contribution.