Recent research has developed a fluid mechanical model aimed at unraveling the complex relationship between heart rate and mammalian body size. This innovative study proposes the notion of pulsatile pressure gradients within the arterial network, which, by optimizing the flow of blood, correlates directly with the physiological heart rate of various species. The findings, published on March 1, 2025, elucidate how the size and elasticity of arteries are pivotal determinants of heart rates across seventy-five distinct mammal species, including everything from diminutive ferrets to colossal elephants.
This extensive analysis, carried out by researchers R.D.M. Travasso, C.A. Penick, and R.R. Dunn, utilized data on heart rates and body mass from 95 mammals, thereby providing comprehensive insights and substantiations of previously proposed theories on cardiovascular physiology. Earlier research suggested scaling laws relating heart rate with body size, but clear explanations for these phenomena remained elusive.
According to the study, traditional scaling laws indicated heart rates (C4) should follow the relationship C4 D7 W-0.25, with W representing body mass. Surprisingly, actual observed data revealed more evidence supporting the relationship C4 D7 W-0.16. To bridge this gap, the study hypothesized and then demonstrated through modeling, how phasic properties, namely vessel elasticity and geometry, critically influence blood flow.
Past work had attempted to explore heart rate frequency purely through static measurements, neglecting dynamic aspects of arterial structures. By developing their model with new insights focused on fluid dynamics, the authors were able to show how frequency differently fluctuates across various mammal sizes. This model operated on the principle of minimizing resistance to blood flow, asserting, as the authors articulated, "Our model provides a plausible explanation for the resting heart rate frequency in healthy mammals, as it minimizes flow resistance across the arterial network."
Gleaning from the substantial dataset, the researchers standardize data points to create reference curves showing heart rates against the radius of the aorta. Utilizing these insights, they constructed scaling laws which indicated wider arteries are correlated with lower heart rates—an intuitive yet previously unsupported assumption. Their explorations reveal heart rate exponents scaling with arterial radius yields enough predictability to be applicable for various species.
Notably, the relationship proposed also accounts for how larger mammals, equipped with broader arterial networks, manage to sustain lower resting heart rates; findings may hint at evolutionary adaptations favoring efficiency of cardiovascular systems. Further, they provided experimental support for their hypothesis through rheological data of human blood, situationally extending insights across even the largest-bodied terrestrial mammals.
Despite the alignment of model predictions with observed heart rates across species, the authors noted interesting discrepancies. It appears their model schedules predicted heart rates factor roughly 2.6 times above observed world values. This discrepancy points to the necessity for refining modeling parameters governing small vessel elasticity.
At the crux of their conclusion, Travasso et al. established the central theme, stating"the frequency maximizing the dynamic response -while minimizing resistance to flow- determines the heart rate, which governs blood flow in the arterial system." Such insights showcase the potential impact of future studies exploring variations across different species and contexts, like blood composition or environmental factors, thereby enriching knowledge of mammalian cardiovascular evolution.
The results suggest exciting avenues for research targeting cardiovascular health, where predicting heart rates and blood flow dynamics may offer clinical insights for both veterinary medicine and human health management. Given the wide-ranging features of the arterial size and network organization among species, this model not only paves the way for responses to specific ecological systems but also informs the working principles of heart functions across the mammalian lineage. Tackling the challenges of inaccuracies from direct heart rate measures may present opportunities to investigate the comprehensive role of arterial mechanics and blood rheology effectively.