Functional Morphology of the Pulmonary Circulation

Abstract

The main bronchus enters the lung hilum, and by a series of dichotomous divisions, gives rise to about 25,000 terminal bronchioles, each of which supplies an acinus. At each division the diameters of the bronchi decrease, but at a rate which is less than the increase in numbers, so that the summed cross-sectional area at any given level increases down the tree (Fig 1). Within the acinus dichotomous division continues down to and including the first generation of alveolar ducts, but thereafter branching is more profuse and irregular. Reduction of diameter at each division is, however, less (Fig 2) so that there is a very rapid increase in summed cross-sectional area with respect to distance down the acinus.