Oxygen tension and content in the regulation of limb blood flow.

During submaximal exercise, muscle blood flow increases when arterial oxygen content (CaO2) is reduced. The increase in blood flow is brought about by elevating cardiac output (CO) and enhancing leg vascular conductance. Conversely, increased CaO2 elicits lower limb blood flow (LBF) and CO. During maximal exercise, the influence of CaO2 on muscle blood flow is modulated depending 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. However, when a large muscle mass is engaged in the exercise, as occurs for example during cycling and running, muscle blood flow is decreased if maximal CO is reduced, as happens during exercise in severe hypoxia. In contrast, maximal muscle blood flow and CO are maintained at peak exercise when CaO2 is increased. As such, exercise intensity, muscle mass and CaO2 appear to be the critical factors determining muscle blood flow during exercise.