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

Abstract

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 ex tended 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 ( p 0 2) 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 p 0 2 drop in a surrounding typical tissue cylinder. This means that the impediment to 0 2 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 O z 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.

Origins of 1/f-like tissue oxygenation fluctuations in the murine cortex

“…The vast majority of oxygen in the blood is carried by RBCs, with the plasma carrying a small fraction of the total oxygen [78,83,85,86], which means heterogeneities in RBCs densities will cause changes in local oxygen supply [67,70,72,78,[83][84][85][86]. It has long been appreciated from theoretical models that the tissue oxygenation can vary with the passage of a single RBC, creating an erythrocyte-associated transient (EAT) in tissue oxygenation [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]. Recent high-resolution measurements of oxygenation with phosphorescent dyes have confirmed the existence of these transients [67,72,[83][84][85][86], but these measurements require aligning the signals to the passage of the RBCs and would not be able to assay any slow oxygenation change that drive 1/f-like dynamics.…”

“…Blood flow and arterial diameter show fluctuations in a similar frequency range as oxygen fluctuations [3]. Additionally, as oxygen is carried by red blood cells (RBCs), fluctuations in the flux of RBCs can drive erythrocyte-associated transients (EATs) in oxygen in the tissue [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82], and fluctuations in flux of these changes in local oxygenation in the cortex [83][84][85][86]. Stalls, brief stoppages in blood flow through capillaries happen sporadically and continuously in the cortex due to transient blockage of blood flow by leukocytes [87][88][89][90][91][92], which are known to greatly increase vascular resistance [93].…”