Tracheobronchial airway structure as revealed by casting techniques.

Research quality tracheobronchial airway casts, prepared in the intact thorax, have proved to be useful in comparative mammalian anatomic studies. Grossly, casts trimmed free of alveoli are quite different in appearance for different species. Overall organ shape, tracheal length/diameter ratio, presence or absence of a tracheal bronchus, and degree of branching symmetry constitute the major gross characteristics. Detailed morphometric measurements performed on such casts reveal important species differences in branch shape, number of divisions in the tree, and variations in such structure as a function of airway generation number. Of the mammalian tracheobronchial casts examined to date, those of humans have several distinctive characteristics. Their overall shape is the most nearly spherical; most other mammals have lungs that are significantly longer along the tracheal axis in relation to their width or thickness. Human branches are typically relatively symmetric with respect to both daughter tube diameter ratio and daughter branch angle ratio. In short, of all of the studied mammalian lungs those of humans appear to be the least heterogeneous.