Morphometric study of human alveolar ducts based on serial sections

Interactive computerized morphometry was used for the quantitative study of the terminal airway branches (alveolar ducts) that followed the last bronchioles in three human acini. Two normal adult human lungs from the autopsy service were fixed by instillation and serial sections were prepared; three tissues blocks showing a central bronchiole were selected. Primary and secondary alveolar walls were traced and the following parameters were measured: volume, surface area (of primary and secondary septa), curvature (in equivalent radius) for branches of individual generations, and cumulative values starting with the first alveolar duct and moving peripherally. Although branching was dichotomous, we noticed considerable asymmetry in the pattern of branching and number of side branches. The branching trees of alveolar ducts that we studied comprised 6,7, and 10 generations. The average volume of ducts was 0.04-0.13 mm3, the surface area of primary walls ranged from 0.3616 to 0.7931 mm2 and of secondary septal walls from 0.0100 to 0.0647 mm2. The equivalent radius of curvature was between 22.7 and 38.1 microns. Cumulative increases of volume and surface area revealed similarity in the first five generations. Secondary walls represented only 4% (or 8% if 2 sides are considered) of the primary surface area, strengthening the view that alveoli are incompletely developed side chambers secondary to the alveolar ducts.