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by scotradamus
2541 days ago
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It's true, and the microscopic theory was done by the same physicist who found the macroscopic relationship, Geoffrey West (one of my personal heroes an current director of the Santa Fe Institute). The theory also explains the upper and lower bounds for the possible size of an animal. If you want to read about it he wrote an amazing book called Scale. I _highly_ recommend it. |
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TL;DR – effective pumping of blood requires precise allowable branching network of capillaries in the circulatory system. At a certain size this network goes from AC to DC and sets the scale for metabolic rate and lifespan in all animals with a circulatory system.
The scale is set by impendance matching of the circulatory system. The heart pumps blood. It is AC. To prevent the AC wave of blood from reflecting back when the capillaries branch out and get smaller, the branching network of the capillaries requires the cross-sectional area of the mother branch to be equal to the sum of the cross-sectional areas of the daughter branches. But when capillaries get too small the heart loses this AC advantage and goes from AC blood wave to purely DC.
So the smallest mammal a shrew has 2 capillary branches, 1 AC, 1 DC. Humans have 8 branches 6 AC then 2 like the shrew 1AC, 1 DC. A blue whale has 15 branches 7 AC then 8 like a human (6 AC then 2 like the shrew 1AC, 1 DC). This branching network sets the smallest length scale of a mammal (actually for any animal that uses blood and mitochondria as its metabolic energy source).
Going back, a shrew has 2 branches 1AC, 1DC. A shrew is 4cm in length and its heart must beat 20 times a second, as nearly all the hearts energy for all animals is expended to push blood through the last small DC branch portion of the capillaries. The shrew must be only 4cm and pump blood with the same pressure and speed as a human (which is the same speed and pressure as a blue whale) through the shrew’s tiny little heart. That is, smaller the animal the larger the metabolic rate. The larger the metabolic rate, the more wear and tear and repair on cells, the shorter the life span.