Direct measurement of intracellular O2 gradients; role of convection and myoglobin.

During steady phasic exercise in a red muscle the entire O2 gradient between capillary and mitochondria occurs as a step over less than 5 m. The magnitude of this step is determined by VO2 and capillary PO2, and is independent of distance from a capillary, or local capillary density. The above cannot be explained by ordinary and/or facilitated diffusion, according to a classical Krogh model. A step gradient can be produced by intracellular convection and Mb, acting in concert. A uniformly low cell PO2 maximizes the trans-capillary O2 gradient, and hence the O2 flux. Since the O2 affinity of Mb is about 50 times less than that cytochrome a, a3, mitochondria can respire maximally at tensions well below the Mb P50. It seems likely that the principal function of Mb during steady, phasic exercise is to compensate for short capillary transit times by accelerating O2 release from Hb.