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The constructal-law origin of the wheel, size, and skeleton in animal design
Adrian Bejan Citation: Am. J. Phys. 78, 692 (2010); doi: 10.1119/1.3431988 View online: http://dx.doi.org/10.1119/1.3431988 View Table of Contents: http://ajp.aapt.org/resource/1/AJPIAS/v78/i7 Published by the American Association of Physics Teachers

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The constructal-law origin of the wheel, size, and skeleton in animal design
Adrian Bejana
Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708-0300

Received 7 October 2009; accepted 30 April 2010 This paper shows that the emergence of body organs is predictable as an integral part of thedesign for moving animal mass more easily on Earth, in accord with the constructal law of design in nature: For a finite-size open system to persist in time to live , it must evolve such that it provides easier access to the imposed global currents that flow through it. Every organ destroys useful energy in two ways: Internally by thermodynamic irreversibilities and by having to be carried. Fromthe constructal law follows the necessity of characteristic-size organs and the emergence of solid columns legs to facilitate the flow of stresses. This natural “wheel” endows the body with rolling falling-forward locomotion, with predicted speeds that agree with the observed speeds in the body mass range of 10−6 – 103 kg. The constructal law also accounts for animal design features for changingspeeds. Skeletons bones and legs are solid organs that emerge in accordance with the constructal-law design of moving animal mass: More and stronger material emerges along the lines of highest stresses. A connection between animal wheel movement and swimming, water waves, and tsunamis is also made. © 2010 American Association of Physics Teachers. DOI: 10.1119/1.3431988 I. SIZE AND THERMODYNAMICSJudson1 has drawn attention to the importance and timeliness of the size in nature question. She noted that science has made much progress in explaining the relation between size and function. References 2 and 3, among many others, document this progress. But, she concluded that “we know rather little about how an animal’s body ‘knows’ what size it is suppose to be.” Thermodynamics teaches thatimperfection irreversibility and the destruction of exergy useful energy is due to flows that overcome resistances.4 Yet, blood vessels are not straight and perfectly round, and heart chambers and pulmonary airways are small and constrictive. Larger organs would have been more efficient. Why does nature not select the largest organs? A growing body of predictions based on the constructal law4–15 answersthese questions. Constructal theory implies that the occurrence of “design” features in nature is a universal phenomenon of physics. This phenomenon is summarized by the constructal law: “For a finite open system to persist in time to live , it must evolve such that it provides easier access to the imposed global currents that flow through it.”4 The constructal law implies that the sequence of flow...