Muscle blood flow during exercise: the limits of reductionism

This paper attempts to integrate some important concepts about the various mechanisms that are thought to cause blood flow to rise during rhythmic exe rcise. Mechanisms including the muscle pump, substances released by skeleta l muscle, substances transported by blood, and factors released by nerves h ave been postulated to contribute to the rise in muscle blood flow during e xercise. Additionally, the factors that initiate the dilation may not be th ose which sustain it. Although there is normally a close relationship betwe en contractile activity, metabolic rate, and muscle blood flow, this relati onship can be disrupted under a variety of circumstances and the active ske letal muscle overperfused, This delinking of flow and metabolism raises imp ortant questions about the nature of the vasodilating substances responsibl e for the rise in blood flow during exercise. We propose that understanding the mechanisms responsible for the "delinking" of flow and metabolism alon g with a more synergistic view of current concepts. can provide new insight into the mechanisms which govern exercise hyperemia.