Aortic stretch and recoil create wave-pumping effect: the second heart in the systemic circulation.

Wave propagation in the heart tube is key to establishing an early pumping mechanism, as explained by impedance pump theory in zebrafish. Though initially proposed for embryonic blood circulation, the role of impedance-like behaviour in the mature cardiovascular system remains unclear. This study focuses on the understudied physiological mechanism of longitudinal displacement in the adult aorta caused by the long-axis motion of the heart. Using magnetic resonance imaging on 159 individuals, we compared aortic displacement profiles between a control group and those with heart failure, revealing a significant difference in aortic stretch between the two groups. Building on this clinical evidence, we conducted experiments to isolate the effects of longitudinal aortic wave pumping by eliminating the pumping action of the heart. We identified three biomechanical properties of stretch-related longitudinal wave pumping that exhibit characteristics like impedance pump: (i) a nonlinear flow-frequency relationship, (ii) bidirectional flow, and (iii) the potential for both positive and negative flow at a fixed frequency, contingent upon the aorta's wave speed dictating the wave state. Our results demonstrate for the first time that this mechanism generates a significant flow, potentially providing a supplementary pumping mechanism for the heart.