Muscle O2 Gradients from Hemoglobin to Cytochrome: New Concepts, New Complexities

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Abstract
The purpose of this paper is to reappraise the sequential barriers to O2 transport from red cells to mitochondria in light of a more accurate model of intracapillary O2 transport, and measurements of myoglobin (Mb) saturation in subcellular volumes. We find that transcapillary gradients are larger and tissue gradients smaller than predicted by existing models of O2 diffusion. During exercise the principal “resistance” to O2 transport resides in the capillary and extracellular space; the limiting variables are rate of O2 release and red cell transit time. Mb plays a major role in overcoming “resistance” at the capillary by buffering Po2 well below capillary Po2 (Pcapo2). These findings are incompatible with classical concepts of O2 delivery to red muscle. Alternatives to Kroghian tinking are proposed.