Unequal distribution of blood flow in exercising muscle of the dog.

The incomplete O2 extraction in stimulated muscle preparations as well as in exercising muscles has been attributed to diffusion limitation. In particular, the constancy of the ratio maximum O2 uptake-venous PO2 with varied O2 supply conditions is in agreement with predictions on the basis of simple perfusion-diffusion models. But several methods (inert gas washout, local xenon clearance, microsphere embolization) have revealed presence of a considerable degree of inhomogeneity of muscle blood flow and shunting both in supramaximally stimulated and in naturally exercising muscle. This inhomogeneity must be taken into account when estimating diffusion limitation in O2 supply.