It is not currently known whether central hemodynamic or peripheral (vascular or metabolic) factors limit maximal oxygen uptake. By measuring the blood flow and oxygen uptake of exercising muscles when only a small fraction of the total muscle mass is engaged in exercise, it has been demonstrated that the skeletal muscle of man could accommodate a blood flow of at least 200 ml/100 g min, and consume 300 ml O2/100 g min at exhaustive exercise. Thus, in whole body exercise the limiting factor is the capacity of the heart to deliver oxygen, not the muscle. It has also been observed that at high perfusion of the muscle the arteriovenous O2 difference is small (14 to 15 vol %), and that the low extraction of oxygen is related to the mean transit time (MTT) of red blood cells passing through the capillaries. It has been concluded that the primary importance of enlargement of the capillary bed with endurance training is not to accommodate flow but to maintain or elongate MTT. It has also been concluded that, in whole body exercise, the capacity of the muscles to receive a flow exceeds by a factor of 2 to 3 the capacity of the heart to supply the flow. Thus, vasoconstrictor tone must also be present in the arteries that "feed" exercising muscles.