Fig. 1. Changes in blood oxygen concentration reduce the arterial oxygen supply to both the skeletal muscles and the heart. If the heart is the slave to the muscles, as predicted by the classical theory, then the output of the heart must increase during hypoxia to ensure that oxygen delivery to the skeletal muscles is maintained. In contrast, the central governor theory predicts that cardiac output will fall during hypoxia in proportion to the reduction in oxygen delivery to the heart, specifically to prevent hypoxic damage to the heart or other vital organs including, perhaps, the brain and respiratory muscles.