P.J. Kilner, Guang-Zhong Yang, D.N. Firmin
Through cardiac looping during embryonic development, human and other vertebrate hearts adopt sinuous curvatures with marked changes in direction of flow at atrial, ventricular and arterial levels. We used magnetic resonance phase velocity mapping to study flow through the hearts of resting volunteers, and Doppler ultrasound to record changes with exercise. We found asymmetric recirculation of blood during filling phases of all four heart cavities, with blood redirected appropriately for onward passage to the next cavity. Doppler traces showed that biphasic ventricular filling became rapid and monophasic on strenuous exercise. We propose that looped curvatures of the heart have fluidic and dynamic advantages. Intra-cavity flow appears to be asymmetric in a manner that preserves stability, and allows momentum of inflowing streams to be redirected towards rather than away from the next cavity. Direction-change at ventricular level is such that recoil away from ejected blood is in a direction that can enhance rather than inhibit ventriculo-atrial coupling. These factors may combine to allow a reciprocating, sling-like, 'morphodynamic' mode of action become effective when heart rate and output increase with exercise. Dynamic efficiency of the looped heart may have favoured evolution of large, complex, active species characteristic of the vertebrate line.
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