Perfusion, diffusion and their heterogeneities limiting blood-tissue O-2 transfer in muscle

The relative roles of blood flow (perfusion) and diffusion in O-2 supply to exercising muscle can be estimated using a simple model consisting of an O -2-consuming tissue block in contact with blood (perfusion (Q) over dot, sl ope of O-2 equilibirum curve beta) through a resistance to O-2 diffusion (O -2-diffusing capacity D). The decisive variable is the 'equilibration index ' Y=D/((Q) over dot beta). With decreasing Y, diffusion limitation increase s and perfusion limitation decreases (Y > 3 indicates predominant perfusion limitation; 3 > Y > 0.1, combined perfusion and diffusion limitation, Y < 0.1, prevailing diffusion limitation). On the basis of literature data on h umans at maximum O-2 uptake, O-2 supply to muscle is shown to be always lim ited by both perfusion and diffusion. In nomoxia, perfusion limitation is p revalent, but in hypoxia diffusion limitation becomes predominant. The unde rlying model assumes perfect homogeneity of muscles with respect to O-2 req uirement, diffusion conditions and blood flow. In numerous studies on isola ted and in situ muscles a pronounced heterogeneity of blood flow has been f ound, also during exercise and at maximal O-2 uptake. It is shown that with unequal distribution of blood flow and/or O-2-diffusing capacity the effic iency of O-2 transfer is reduced with reference to the homogeneous model. T herefore, the diffusing capacity value calculated on the basis of the homog eneous model is an underestimate of the true diffusing capacity and diffusi on limitation is overestimated.