Beyond the principle of similitude: renormalization in the bronchial tree.

The average dimensions (diameter, length, and volume) of the airways in the mammalian bronchial tree, long thought to be exponential functions of the generation number, are shown to be power laws in generation number modulated by a harmonic variation. These data are satisfactorily described by means of a functional scaling relation--renormalization group property--between successive generations for the average variable of interest. This type of scaling may provide a mechanism for the morphogenesis of complex but highly stable structures.