An examination of the contribution of red cell spacing to the uniformity of oxygen flux at the capillary wall

Abstract

It is generally assumed that capillary blood is homogeneous for O 2 supply and that red cells can provide a constant, uniform flux of O 2 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 O 2 at the capillary wall. The results suggest the existence of a critical red cell separation distance above which the flux of O 2 at the capillary wall between red cells cannot remain uniform and the capillary blood is no longer homogeneous for O 2 supply. In resting muscle the predicted critical separation distance is greater than four cell lengths. During maximal O 2 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 O 2 at the capillary wall.