An Examination of the Contribution of Red Cell Spacing to the Uniformity of Oxygen Flux at the Capillary Wall

It is generally assumed that capillary blood is homogeneous for O2 supply and that red cells can provide a constant, uniform flux of O2 out of the capillary regardless of the spacing between cells. Using a simplified model of red cells moving through a capillary in skeletal muscle, an approximate analysis is developed to study the effect of red cell spacing on the ability of erythrocytes to provide a constant, uniform flux of O2 at the capillary wall. The results suggest the existence of a critical red cell separation distance above which the flux of O2 at the capillary wall between red cells cannot remain uniform and the capillary blood is no longer homogeneous for O2 supply. In resting muscle the predicted critical separation distance is greater than four cell lengths. During maximal O2 consumption, the critical separation distance predicted by the model is one cell length. These predictions agree closely with in vivo observations of red cell spacing. The total red cell flux through a capillary is determined not only by red cell spacing (hematocrit) but also by erythrocyte velocity; a simple example is given which suggests that changes in each of these variables are not equivalent in maintaining a constant and uniform flux of O2 at the capillary wall.