Oxygen tension and content in the regulation of limb blood flow

During submaximal exercise, muscle blood flow increases when arterial oxyge n content (CaO2) is reduced. The increase in blood flow is brought about by elevating cardiac output (CO) and enhancing leg vascular conductance. Conv ersely, increased CaO2 elicits lower limb blood flow (LBF) and CO. During m aximal exercise, the influence of CaO2 on muscle blood flow is modulated de pending on the amount of muscle mass recruited. When a small muscle mass is activated and the pumping capacity of the heart is not limited, changes in CaO2 barely influence the level of blood flow attained at peak exercise. H owever, when a large muscle mass is engaged in the exercise, as occurs for example during cycling and running, muscle blood flow is decreased if maxim al CO is reduced, as happens during exercise in severe hypoxia. In contrast , maximal muscle blood flow and CO are maintained at peak exercise when CaO 2 is increased. As such, exercise intensity, muscle mass and CaO2 appear to be the critical factors determining muscle blood flow during exercise.