Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt.

The role of diffusion in tissue gas exchange is investigated using a simple mathematical model which incorporates both tissue-blood equilibration and gas transfer between arterial and venous vessels with counter-current flow, leading to 'diffusion shunt'. Both increasing the diffusion coefficient of the gas considered or decreasing the blood flow results in two antagonistic effects: (i) improvement of blood/tissue equilibration, (ii) increase in extent of diffusion shunt. The diffusion shunt retards inert gas wash-out (local tissue clearance) and leads thus, if not taken into account, to an underestimation of capillary blood flow calculated from the wash-out rate constant. For O2 (and CO2) the diffusion shunt reduces the efficacy of blood/tissue transfer, but its extent is expected to be only moderate because of the chemical combination of these gases in blood.