The effect of blood flow on oxygen extraction pressures calculated in a model of pointlike erythrocyte sources for rat heart.

A mathematical description of pericapillary oxygen gradients that takes into account the particulate nature of blood is possible in terms of erythrocytes as pointlike sources. The formulation in terms of quasi-stationary sources [1] is extended to account for moving erythrocytes. The extended model is semianalytical and can be used to estimate the extraction pressure (EP), which quantifies the effect on partial pressure of oxygen (pO2) in the tissue far from the erythrocytes. Simulations have been done for rat heart muscle tissue around a capillary. For low hematocrit (Hct; 20%) and low blood velocity EP is highest, higher than the pO2 drop in a surrounding typical tissue cylinder. This means that the impediment to O2 release close to the capillary can be larger than that to transport further into the tissue. Increasing the hematocrit decreases EP, that is, it facilitates O2 release. Increasing the blood velocity decreases EP at low Hct values but has the opposite effect at high Hct values (> 35%). For zero velocity, results are the same as with the quasi-stationary model.