Role of vascular mechanics in circulatory dynamics.

Here is a paper that has to go in the next edition of Fluid Physiology:

Extreme bradycardia and tachycardia in the world’s largest animal

“The biomechanical properties and dimensions of the aortic wall in fin whales (Balaenoptera physalus) have led to the hypothesis that, at heart rates ≤10 beats min−1 (bpm) during dives, the highly compliant aortic arch acts as a windkessel that maintains blood flow during the long diastolic periods between heart beats and reduces pulsatility of blood inflow into the rigid distal aorta “

First to say I love this sort of research, I used to queue to listen to Warren Zapol enthral critical care audiences  with tales of his Antarctic expeditions to study the diving mechanisms and adaptations of the Weddell seal

It can be hard to explain to non-scientist clinicians the essentiality of vascular compliance to cardiac output generation. Sheldon Magder published a nice diagram and explanation of the fact that blood could not flow without compliance in the blood vessels supplied by the pump. The extreme bradycardia response of blue whales illustrates how “a large-diameter, highly compliant, elastic aortic arch allows the aorta to accommodate blood ejected by the heart and maintain blood flow during the long and variable pauses between heartbeats.” Brilliant stuff, but after reading Fluid Physiology you will appreciate that the aorta is only one part of this story. The venules contain far more blood than the aorta and their role is to accommodate blood leaving the inelastic microvasculature and maintain blood flow on the venous side of the circulation. Although the intravenular pressure is less than the intraaortic, the intravenular volume of blood receiving energy from vascular elasticity is much greater than the intraaortic. More than that, I suggest to clinicians that the fictitious Mean Circulatory Pressure (and other equally fictitious Guytonian inferred calculations) are just approximations of the venular filling pressure which is real, and exists in the heart-beating condition we all try diligently to preserve!

So feedback requested friends; does this analogy improve your appreciation of the role of vascular mechanical properties?

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