The convective diffusion of oxygen, carbon dioxide and inert gas in blood.

Abstract

In modern membrane-type artificial lungs, the major resistance to gas transfer is offered by the blood and not the membrane. Thus, a suitable theory for convective diffusion in blood must be available in order to design these devices. A general equation for the convective diffusion of gases in blood is presented in this work, and numerical solutions to this equation are compared with experimentally measured rates of oxygen, carbon dioxide, and krypton exchange with citrated blood in a boundary layer flow. The measurements were made in a new type of rotating disk apparatus which is especially suited to the study of gas transfer in complex fluids. Solutions of the general equation based on velocities derived from the Navier-Stokes equations are shown to predict the measured transfer rates, even though blood is known to be non-Newtonian at low shear rates. The desorption rate of oxygen from blood at low oxygen partial pressures was found to be up to four times the corresponding transfer rate of inert gas. This effect is described somewhat conservatively by a simple local equilibrium form of the general convective diffusion equation. The carbon dioxide transfer rate in blood near venous conditions was found to be about twice that of inert gas. This great an augmentation is not predicted by any simple form of the general convective diffusion equation. The behavior of the membrane lung system was studied numerically using these results, and the practical implications of this study are discussed.