Figure 2. Coronary blood flow plotted against myocardial oxygen consumption in exercising dogs.

Note the linear relationship between the rise in coronary blood flow and myocardial oxygen consumption. Blockade of nitric oxide synthase, K_ATP channels, and adenosine receptors had no influence on the magnitude of coronary vasodilatation during exercise. The fact that all three of these powerful vasodilating mechanisms could be blocked and yet there was a normal hyperaemic response to exercise is a classic example of redundancy in physiology. If a physiological response, such as a rise in coronary blood flow during exercise, is critical to survival, typically multiple mechanisms operate to ensure a normal or nearly normal physiological response when one or more mechanism is blocked or inhibited in some way. (Reproduced with permission from Tune et al. 2001; for further discussion see Duncker & Bache, 2008.)