Local regulation of effective blood flow in muscle.

• Blood flow through skeletal muscle is augmented characteristically during and for some time after exercise.-' Information on the basic mechanism of this functional hyperemia is still far from complete. It has been suggested that vasomotor nerves or local chemical imbalances are concerned in the development and maintenance of vasodilatation. Similar uncertainty exists concerning the mechanism of reactive hyperemia, the gross increase of measured blood flow immediately following an imposed deprivation of blood supply. In the case of skeletal muscle at least three alternative hypotheses have been offered to explain relaxation of smooth muscle in the local blood vessels: (a) effects of nerve impulses which originate centrally or locally; (b) direct response of smooth muscle in the resistance vessels themselves to lessened distention, i.e., the "myogenic response"; and (c) local accumulation of metabolites, or depletion of nutrients or of oxygen during the period of ischemia. Each of these theories has been questioned on the basis of experiment. For example, total chronic denervation does not significantly alter reactive hyperemia. That something in addition to the myogenic response must contribute to the total effect is suggested by evidence that a qualitatively similar hyperemia may be obtained after venous occlusion, when the distending force is augmented rather than diminished, during the occlusion. In its simplest form, any hypothesis based on local chemical alteration during ischemia implies