Abstract:
A prevailing assumption in the cardiovascular field is that the
metabolic rate of the heart is a constant proportion of a mammal’s
whole-body aerobic metabolic rate. In this Commentary, we assemble
previously published cardiovascular, metabolic and body mass data
from matched terrestrial mammalian species, at rest and during heavy
exercise, and reveal scaling relationships that challenge this
assumption. Our analyses indicate that the fractional metabolic cost
of systemic perfusion compared with whole-body metabolic rate
increases significantly with body size among resting mammals, from
∼2.5% in a mouse to ∼10% in an elephant. We propose that two
significant body size-dependent effects contribute to this conclusion;
namely, that larger species generate higher mean systemic arterial
blood pressure and that their myocardium operates with lower
external mechanical efficiencies compared with those of smaller
species. We discuss potential physiological and mechanical
explanations, including the additional energy needed to support the
arterial blood column above the heart in larger species, especially
those with long necks, as well as the possible sources of greater
internal energy losses from the heart of larger species. Thus, we
present an updated view of how increasing blood pressure and
decreasing efficiency of the myocardium result in an increasing
fractional metabolic cost of perfusion as body size increases among
resting mammals.
