Why do we have a bronchial tree with 23 levels of bifurcation?

A simple one-dimensional mass transfer model has been proposed for the oxygen transport through a bronchial tree to alveolar tissues as well as the carbon dioxide removal from the tissues in the human respiratory system. The proposed model mathematically describes the mass transfer between the airway inlet and the red blood cell interior in the pulmonary capillaries. The quasi steady one-dimensional analysis based on the model reveals that the bronchial tree is constructed such that it promotes the easiest access to the external air. Naturally, there exists the optimal number of the bifurcation levels, namely, 23, that yields the minimum overall mass transfer resistance for the mass transport from the external air to the red blood cells.