In summary, evidence shows that the respiratory muscles demand a significant portion of the cardiac output during maximal exercise. Estimates of both animal and human blood flow and VO2 to the respiratory muscles during maximal exercise approximate 14-16% of the total cardiac output and VO2. During heavy exercise, this metabolic demand of the respiratory muscles affects the distribution of cardiac output between the respiratory muscles and the legs such that leg vascular conductance and blood flow increases with respiratory muscle unloading and decreases with respiratory loading. The reflex effects underlying this blood flow redistribution remain unknown; however, these data do clearly support the existence of a significant sympathetic effect output to working skeletal muscle in heavy exercise. These data also invite the exciting (although speculative) prospect of important chemo- or mechano-induced reflexes emanating from respiratory muscle under load.Finally, while not yet completely understood or investigated, it appears that respiratory muscle work during strenuous exercise affects exercise performance.