Abstract
The respiratory system of mammalians is made of two successive branched structures with different physiological functions. The upper structure, or bronchial tree, is a fluid transportation system made of approximately 15 generations of bifurcations leading to the order of 215 = 30,000 bronchioles with a diameter of order 0.5 mm in the human lung.1 The branching pattern continues up to generation 23 but the structure and function of each of the subsequent structures, called the acini, is different. Each acinus is made of a branched system of ducts surrounded by alveolae and play the role of a diffusion cell where oxygen and carbon dioxide are exchanged with blood across the alveolar membrane.2 We show in this paper that the bronchial tree presents simultaneously several optimal properties of totally different nature. It is first energy efficient;3-6 second, it is space filling;7 and third it is "rapid" as discussed here. It is this multi-optimality that is qualified here as magic. The multi-optimality physical characteristic suggests that, in the course of evolution, an organ selected against one criterion could have been later used for a totally different reason. For example, once energetic efficiency for the transport of a viscous fluid like blood has been selected, the same genetic material could have been used for its optimized rapidity. This would have allowed the emergence of mammalian respiration made of inspiration–expiration cycles. For this phenomenon to exist, the rapid character is essential, as fresh air has to reach the gas exchange organs, the pulmonary acini, before the start of expiration.
Published Version
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