The effects of transmural transport in the microcirculation: a two gas species model.

Diffusion of oxygen and carbon dioxide across the walls of noncapillary vessels in the microcirculation has been suggested by several studies. The formulation and steady-state solutions to a nine-compartment mathematical model of the microcirculation of skeletal muscle with transmural gas diffusion in all vessels are presented. The simultaneous transport of oxygen and carbon dioxide between arterioles, capillaries, and venules, and connective and muscle tissue at rest and exercise are described. Special attention is paid to the interactions of these gases in blood. This model predicts a longitudinal intravascular gradient in oxygen tension from large to small vessel with the tension at the precapillary vessels relatively insensitive to changes in the input tension. At rest, there is significant small arteriolar oxygen flux. However, during exercise, the precapillary transmural flux of oxygen is only a small fraction of the total metabolic demand. The model predicts large noncapillary fluxes of carbon dioxide, and also that tissue PCO2 is dependent on input tensions. The model also predicts that Bohr shifts due to either changes in input PCO2 or increased precapillary PCO2 due to increased metabolism may cause physiologically significant changes in precapillary PO2. Countercurrent shunting was predicted by the model to be significant only for carbon dioxide.