Geometric determinants of airway resistance in two isomorphic rodent species.

The flow resistance of the pulmonary airway tree (Raw) is disproportionately larger in large animals than in smaller ones. This is thought to be due to the fact that smaller animals have relatively wider central airways than larger animals. However, Raw is not determined solely by the diameter of the main bronchi or trachea. It depends on the dimensions of all the individual airways and how they are connected. We investigated how the degree of asymmetry of the airway tree, the number of airway orders, and dimensions of the individual airways combine to determine Raw. We performed this investigation using computer models of the airway trees of two different sized isomorphic rodents that differ in weight by more than two orders of magnitude-the harvest mouse and the giant pouched rat. We calculated airway resistance in these models under various conditions, and confirmed that the smaller species has a lower Raw relative to body size than the larger species. We also showed that these differences are due to a combination of differences both in relative airway diameters and in the degree of asymmetry of the airway trees.