Blood-brain transfer of oxygen is a fundamental function of regional oxygen consumption. This function yielded a novel description of the mechanism of flow-metabolism coupling. The description revealed that constant oxygen consumption is not maintained by saturation of cytochrome oxidase but by adjustment of the enzyme's affinity towards oxygen. Interactions of oxygen and a factor that could be nitric oxide, at both cytochrome oxidase and nitric oxide synthase, match the affinities of both enzymes towards their respective substrates to the oxygen requirement of the tissue and, in doing so, explain several properties of flow-metabolism coupling.