Conclusions
Application of a cost minimization approach has proved useful in the analysis of arterial networks, although it is not without limitations and a number of issues remain unresolved. Although progress has been made we need to know more about the stimuli which shape the network, how their effects are mediated, and to what extent they synergise. There is some evidence that shear stress and pressure interact,55 and a range of interactions between diverse stimuli might permit a degree of ‘weighting’ or contextualization. For instance, it has been argued the circulation might be designed to attain highest efficiency during exercise,44,102 and by implication that it is slightly suboptimal under resting conditions e effectively allowing a reserve capacity. This is consistent with some evidence, at least in terms of minimization of wave reflection.103 If this were true, the system would need to optimize its design in response to an intermittent physiological state, albeit one that may be critical for survival. How is this intermittent state sensed? One possibility might be that shear stress could have more effect on remodelling in the context of the stimuli that accompany exercise. Finally, is optimal design desirable? It has recently been proposed (for the bronchial tree) that an optimal network is dangerous because of its vulnerability to imperfections or vessel constriction and that a ‘safety margin’ is required in design.104 This seems difficult to reconcile with the comparative cost-insensitivity of arterial networks to small deviations from optimum, but it does raise important questions about the appropriateness of optimality as an overriding factor in circulatory design e perhaps Murray’s law should be viewed as ‘more what you’d call guidelines than actual rules.
