Chapter 7 - Mechanisms of Oxygen Transport in the Microcirculation: Effects of Cell-Free Oxygen Carriers

Abstract

The primary site of oxygen supply to tissues is in capillary networks, although some oxygen is released from blood in larger vessels. The primary function of both the micro- and the macro-circulation is undoubtedly the supply of oxygen to the tissues, a process whose energy requirement is assumed to be circumscribed mainly by the energy expenditure of the heart and lungs. Development of animal models in which PO2 and blood flow can be studied together in individual capillaries and systems of cascading vessels of diminishing size has been critical in understanding the details of how oxygen supply to tissue is regulated. The efficacy of blood substitutes as agents for delivering oxygen to the tissue can currently be evaluated due to the convergence of technologies that yield a quantitative assessment of oxygen transport and consumption in the microcirculation. The analysis shows that in normal conditions the oxygen and fluid transport properties of blood are matched to provide signals to the microcirculation that maintain a specific oxygen pattern, which leads to an optimal partition of oxygen between the microvessels and tissue. Changing the relative magnitude of these properties affects microvascular function, shifting the localities where oxygen is unloaded, as well the mechanical condition of the microcirculation. The availability of modified hemoglobin that has no apparent toxicities suggests that new combinations of transport parameters can be devised which, recognizing that not all decreases in oxygen-carrying capacity have the same origin, may therefore be tailored for specific therapeutic interventions in diverse pathologies such as trauma, exsaguination and sepsis.