EXPERIMENTAL AND THEORETICAL ANALYSES OF OXYGEN EXCHANGE BY HUMAN ERYTHROCYTES
VANDEGRIFF HYSELL, KIM D.
Doctor of Philosophy
Oxygen uptake by human erythrocytes was examined both experimentally and theoretically in terms of the influence of unstirred solvent layers that are adjacent to the cell surface. A three dimensional cylindrical model was compared to less complex spherical and cylindrical and coordinate schemes. Although simpler and faster, the plane sheet and spherical algorithms are inadequate representations when unstirred solvent layers are considered. The cylindrical disk model most closely represents the physical geometry of red cells and is required for a quantitative analysis. In our stopped-flow, rapid mixing experiments the thickness of the unstirred layer expands with time as the residual turbulence decays. This phenomenon was quantitated using a formulation that is based on previously developed hydrodynamic theories. An initial 10('-4) cm unstirred layer is postulated to occur during mixing and to expand rapidly with time by a (t)('0.5) function when flow stops. Oxygen release time courses for human erythrocytes were also examined and analyzed by this three dimensional model. At high concentrations of external sodium dithionite, the effects of unstirred layers were less important but still necessary for a quantitative understanding of the reaction. In this case, the kinetics were influenced more by chemical reaction parameters, both for the extracellular O(,2)-dithionite reaction and for the intracellular O(,2)-hemoglobin reaction, and rigorous theories of these processes were required for fits to the observed data. A cooperative equilibrium scheme was developed which includes corrections for the Bohr effect in the physiological range of extracellular pH (pH 7.0-7.8). The relative importance of extracellular diffusion processes and chemical reaction parameters was examined in terms of red blood cell morphology and physiology. Effects on cell size were expressed primarily in oxygenation kinetics. Increasing internal heme concentration caused a proportional decrease in both uptake and release rates. Factors that alter the O(,2)-hemoglobin equilibrium influenced the rates of O(,2) release more than the rates of O(,2) uptake. For all of these experiments, the three dimensional cylinder scheme described quantitatively both the rates and overall shapes of the time courses.