Convection Effect of Plasma Flow on Oxygen Transport in Capillaries: An In-Depth Numerical Investigation

The convection effect of plasma flow on gas transport in the microcirculation has been a controversial topic in the literature. We aim to clarify this concern via thorough and rigorous analysis of the oxygen release process from red blood cells (RBCs) to the surrounding tissue.

We develop a comprehensive model that considers the plasma flow, RBC deformation, oxygen transport and oxygen-hemoglobin reaction kinetics. The boundary integral and lattice Boltzmann methods are employed in the numerical solutions. In particular, the oxygen fluxes due to plasma convection and mass diffusion are separately calculated along the capillary wall for further comparison.

Our results show that the most significant diffusive flux occurs in the narrow gap between the RBC side surface and the capillary wall and the diffusive flux is primarily directed outward, which favors oxygen release into the surrounding tissue. Furthermore, although the axial convective flux is the most profound in magnitude, it contributes little to the overall blood-to-tissue oxygen transport in the radial direction. The radial convective flux also has a larger magnitude compared to the diffusive oxygen flux, but is limited to two small areas and to opposite directions. This results in a negligible net effect of the plasma convection compared to the diffusive flux on the overall oxygen transport. This observation is further confirmed by comparing the oxygen distributions and diffusive fluxes from simulations with and without considering the plasma convection flow relative to RBCs. Moreover, we revisit the Peclet number definition and propose that different characteristic length scales should be adopted for oxygen diffusion and convection in capillaries. The revised Peclet number has a value three orders of magnitude lower than that from the classical Peclet number definition.

Our simulation results show that the influence of plasma convection on the overall oxygen transport can be neglected in typical microcirculation situations. This is consistent with the revised Peclet number value, suggesting that the revised Peclet number can better reflect the relative importance of convection and diffusion mechanisms in microvascular gas transport.