In mammals, blood velocity at the aortic root varies little with body mass, M, and so the kinetic work is a constant fraction of the total work of the heart, and also of the metabolic rate of the animal. This happens because body metabolism and the cross-sectional area of the aortic root vary to the same power of M. But if metabolism increased with M to a higher power than area, aortic velocity would increase and kinetic work would become a greater fraction of total work in a large animal. In larger animals it could exceed pressure work at rest and even more so in exercise. But there is a limit to how much an increase in aortic area can balance an increase in metabolism without the aorta becoming ridiculously large. Also aortic area should not exceed ventricular area. These factors could dictate how high the power of M is in relation to the metabolism of an animal.