The relative influence of hematocrit and red blood cell velocity on oxygen transport from capillaries to tissue

Objective Oxygen transport to parenchymal cells occurs mainly at the microvascular level, and depends on convective red blood cell (RBC) flux, which is proportional in an individual capillary to the product of capillary hematocrit and red blood cell velocity. This study investigates the relative influence of these two factors on tissue oxygen partial pressure (PO2). Methods A simple analytical model is used to quantify the respective influences of hematocrit, RBC velocity and flow on tissue oxygenation around capillaries. Predicted tissue PO2 levels are compared with a detailed computational model. Results Hematocrit is shown to have a larger influence on tissue PO2 than RBC velocity. The effect of RBC velocity increases with distance from the arterioles. Good agreement between analytical and numerical results is obtained and the discrepancies are explained. Significant dependence of mass transfer coefficients on RBC velocity at low hematocrit is demonstrated. Conclusions For a given RBC flux in a capillary, the PO2 in the surrounding tissue increases with increasing hematocrit, as a consequence of decreasing intravascular resistance to diffusive oxygen transport from RBCs to tissue. These results contribute to understanding the effects of blood flow changes on oxygen transport, such as occur in functional hyperemia in the brain. This article is protected by copyright. All rights reserved.