KROSNIUNAS, Egle H.; GERSTNER, Geoffrey G.; Univ. of Wisconsin, Barron County; Univ. of Michigan, Ann Arbor: Resting Metabolic Rate Determines the Size of Blood Flow Responses Required During Activity: Why Aerobic Factorial Scope is Around Ten in Vertebrates

How levels of resting metabolism affect activity metabolism is not well understood. The aerobic capacity model proposes that factorial aerobic scope is around 10 in most vertebrates, no matter the physiological state, because of an unspecified constraint that restricts how much resting rate can be increased, with maximal oxygen uptake limited to around an order of magnitude above resting levels. We have developed a mathematical model that argues there is no such restriction. Our results suggest that rates of resting metabolism in vertebrates are maintained at levels that are optimal with respect to two key opposing variables: (1) the energetics of metabolic systems, which favor the lowest possible resting rates to conserve energy expenditure and capacity, and (2) the speed of cardiovascular response to activity, which would select for an increased resting metabolism. The latter point is based on our theoretical finding, obtained from the Fick equation for convective transport, that changes in oxygen consumption require increments of blood flow that vary as a nonlinear function of initial rate. Thus, the size of response required by the cardiovascular system to meet the added oxygen demands of activity decreases hyperbolically as resting rate increases. By maintaining metabolic levels at rest to around 10% of VO2max, therefore, vertebrates strike an optimal balance in the design of their metabolic systems: providing the greatest reduction in the incremental blood convection requirement during activity for the least energetic allocation to resting metabolic rate.