Diffusive resistance to O-2 transport in muscle

Although it has been generally well-accepted since the work of August Krogh at the start of the 20th century that O-2 travels between muscle microvasc ular red cells and muscle mitochondria by diffusion, the importance of this process to maximal O-2 flux remains in question. This review presents data that suggest maximal rates of diffusion of O-2 in muscle are limited by bo th the amount of capillary structure and the haemoglobin concentration in t he blood. On the other hand, diffusional conductance appears unrelated to m uscle fibre size and thus to distance. Functional data further suggest that the limited rate of diffusion acts to constrain O-2 unloading from muscle capillaries and thus also maximal (V) over dot o(2), at least in the traine d state. In fact, the calculated sensitivity of (V) over dot o(2max) to mus cle diffusional conductance is about as great as that for muscle blood flow . While additional impairment of O-2 unloading from perfusion/metabolism he terogeneity cannot yet be excluded (for lack of methodology), it seems evid ent that the process of O-2 transport between the muscle microvasculature a nd the mitochondria is subject to significant limits, even in health, that have substantial effects on maximal (V) over dot o(2).