Microvascular and tissue oxygen distribution.

Understanding of oxygen delivery by the microcirculation has been dominated by the unitary component analysis of Krogh and Erlangen focussed on oxygen transport mediated by single capillaries, oxygenation of tissue as a whole being extrapolated from findings on oxygen exchange in these vessels. This analysis is under revision since capillaries are not sole sources of oxygen. It is increasingly apparent that arterioles are a significant equivalent source, while venules may serve as sinks for capillary and arteriolar oxygen. As a consequence detailed descriptions of the architecture of the microcirculation based on the tissue cylinder conceptualization does not yield new information given the non-exclusive role of capillaries as purveyors of oxygen to tissue. In the present study we investigate how tissue is oxygenated directly from the arteriolar supply on the basis of current results with newly developed optical techniques for the measurement of local intra- and extravascular pO2 by phosphorescence decay. This methodology shows that tissue regions between arterioles and venules have essentially uniform tissue pO2. The only experimentally detectable gradients in pO2 are those present in the immediate vicinity of arterioles. Findings on vascular longitudinal gradients are used to devise a model that links convective and diffusive processes, showing how blood viscosity, blood oxygen-carrying capacity and the slope of the oxygen dissociation curve are linked in determining intravascular and tissue pO2. The integrated approach provides a numerical basis for interpreting consequences of alterations in transport properties of blood applicable to the field of blood substitutes.