Bats are the only mammals capable of powered flight. In flight, bats consume up to 20 times more oxygen per hour than under basal conditions. This is twice the increase reported for running mammals of the same body sizes. Birds are the only other group of vertebrate capable of powered flight. By plotting morphological and physiological parameters against body mass for bats, terrestrial mammals, and birds, we can observe to what extent these parameters permit high rates of oxygen delivery necessary for flight. In this study we analyzed basal metabolic rate, heart mass, and hematocrit, and compared how differently they scale in the afore-mentioned groups. We found that larger heart masses are associated with the ability to fly; hematocrit values for birds change markedly with body size, while those for mammals and bats are nearly independent of body size; and that BMR scales differently in all three groups.