Why capillary blood flow is Slow in longer-lived species.

Our friend @CPV_Physiology raises the interesting question of transport phenomena in nature. In this third post about capillary blood flow velocity I draw to your attention a nice review of the evolution of vascular systems in animals. It is a very reasonable hypothesis that the longest-lived animals have evolved an energy-efficient cardiovascular system that delivers just enough blood: neither too little (ischaemia) nor too much (high output heart failure). This would sit nicely with the idea that most mammals can manage a billion heart beats in a lifetime, and life span is determined by how energy-efficiently each species can deliver blood to satisfy metabolic requirements. Before we talk about capillary blood flow, I want to direct your thoughts once more to vascular compliance. If the arterial and venous vascular systems were entirely rigid, cardiac output would be Zero and life would be short. Very very short. Which is why I very much regret that medicine has passed up the opportunity to routinely measure arterial and venous pressure – volume relationships in order to promote good health and long life. But that’s another story. In earlier posts we have considered the remarkable aortic capacitance of whales, which contributes to their ability to sustain bradycardia during dives. Studies on other mammalian species confirm that normal capillary blood flow in vital organs like the brain is just enough to supply vital oxygen delivery. Neither too fast nor too slow, just enough to satisfy need. The slowest possible capillary blood flow velocity leads to longevity, which for species like humans is a significant reproductive advantage.

I also draw your attention to the fact that primitive vascular systems rely on the contractility of conducting vessels to propel their contents. Much like mammalian lymphatic systems…

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