Diffusive resistance to O2 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 microvascular 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 both the amount of capillary structure and the haemoglobin concentration in the blood. On the other hand, diffusional conductance appears unrelated to muscle 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 trained state. In fact, the calculated sensitivity of (V) over dot o(2max) to muscle diffusional conductance is about as great as that for muscle blood flow. While additional impairment of O-2 unloading from perfusion/metabolism heterogeneity cannot yet be excluded (for lack of methodology), it seems evident that the process of O-2 transport between the muscle microvasculature and the mitochondria is subject to significant limits, even in health, that have substantial effects on maximal (V) over dot o(2).