Sustainable energy, needed to power cell work such as muscle contraction, is generated by combustion of fuels in the mitochondria, the cells’powerhouses. This process depends on an adequate and continuous supply of oxygen along the network ofstructures constituting the pathway for oxygen: from the lung through the blood and heart to the cells. The principle of symmorphosispredicts that the structural design of this respiratory system is economic: “enough but not too much”to satisfy the functional needs. The test of the validity of such a hypothesis is based on a physico-mathematical model of the quantitative structure-function relations at the different levels of the network, exchange surfaces and distribution trees, and exploits the fact that the animal world shows great differences in energy needs, between large and small mammals, and between sedentary and athletic species.
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