Muscle O2 gradients from hemoglobin to cytochrome: new concepts, new complexities.

The capillary is the principal barrier at high Vo2. Mathematical modelling indicates that at high flow and Vo2 the time required for release of O2 is greater than red cell transit time in some capillaries. This convective shunting appears to be particularly important in the myocardium. The Mb acts to buffer Po2 below 5 Torr during muscle contraction. This greatly increases the transcapillary O2 gradient and promotes O2 delivery. During voluntary movements, Mb should act as a major O2 source in parallel with capillaries. The Kroghian model of the capillary as a "low-resistance" point source of O2 supplying a spatially uniform sink appears to be the reverse of the actual geometry of O2 supply to working red muscle.