The systematic increase in VO2 uptake and O-2 extraction with increasing work rates conceals a substantial heterogeneity of O-2 delivery (QO(2))-to- VO2 matching across and within muscles and other organs. We hypothesize that whether increased/decreased QO(2)/VO2 heterogeneity can be judged as "good" or "bad," for example, after exercise training or in aged individuals or with disease (heart failure, diabetes) depends on the resultant effects on O-2 transport and contractile performance.