The Physics of Oxygen Delivery: Facts and Controversies

At the microvascular level, the radial oxygen gradient is greater in arterioles than in any other vascular segment and thus drives the oxygen from the blood (high concentration, source) into the perivascular tissue (low concentration, sink). Thus, arterioles appear to be the main suppliers of oxygen to the tissue, in contrast to the capillaries, where the oxygen gradient is only a few millimeters of mercury. However, longitudinal oxygen loss from arteriolar blood is higher than can be solely accounted for by diffusion. This discrepancy becomes evident when determining how oxygen is distributed in the microvascular network, an approach that requires confirmation of the data in terms of mass balance and thermodynamic considerations. A fundamental difficulty is that measuring tissue Po2 is complicated by methods, exposure of tissue, interpretation, and resolution. The literature reports mean tissue Po2 as low as 5 and up to 50 mm Hg. This large variability is due to the differences in techniques, species, tissue, handling, and interpretation of signals used to resolve Po2 levels. Improving measurement accuracy and physiological interpretation of the emerging Po2 data is ongoing. We present an analysis of our current understanding of how tissue is supplied by oxygen at the microscopic level in terms of present results from laboratories using differing methods. Antioxid. Redox Signal. 12, 683–691