Optimal Hematocrit: Theory, Regulation and Implications1

SYNOPSIS. Hematocrit is likely to be optimized because of its influences on oxygen transport. However, optimal hematocrit must also change because shear rates and blood vessel radii within circulations change temporally. Blood vessel endothelia regulate shear stress on their walls by changing their radius. Wall shear stress is dependent on shear rate and viscosity. Because there is regulation of vessel radius by the endothelium it is hypothesized that hematocrit may be regulated near optimal by changes in plasma volume. The implication of such regulation is that changes in vascular volume (blood vol-ume) would occur with alterations in red blood cell mass. Data are presented which indicate that regulation of optimal hematocrit normally occurs through changes in plasma volume. The regulation of optimal hematocrit has signifi-cant implications for processes that depend on oxygen transport (e.g., exer-cise) because of the effect of blood volume on cardiac output. Two major principles have been applied to the theoretical understanding of physi