Anatomy of membranous bronchioles in normal, senile and emphysematous human lungs.

Serial reconstructions of the membranous bronchioles (MB) were performed in randomly selected tissue blocks cut parallel to the pleural surface in fixated human lungs. Two to four normal, senile, and emphysematous lungs were examined. Three (2 in senile lungs) orders of MB were observed with a dichotomous branching pattern. Emphysematous lungs are characterized by an overall decrease in airway diameter with localized stenoses. Comparison with mean airway diameter (d) and density [n (no. of MBs per cm2 of lung tissue)] of MBs obtained using standard morphometric techniques (random sections approximately 1 cm from the pleura) showed that the values of d and n are biased because of the inclusion in the measurements of a number of respiratory bronchioles and bronchi. When these misclassifications are corrected for, it appears that d corresponds quite well to the mean diameter of the terminal bronchioles (TB) and n to approximately twice the density of TBs. After correction, n is not significantly reduced in emphysematous lungs (the grossly destroyed areas being excluded) compared with normal lungs. The estimate of the number of TBs obtained from the present data is markedly less than that calculated by Weibel (Morphometry of the Human Lung. Berlin: Springer-Verlag, 1963), which suggests that the number of bronchi was overestimated by Weibel by at least one generation. Finally, values of peripheral airway resistance computed from the present anatomic data correspond quite well to direct measurement performed on the same lungs before fixation (Verbeken et al., J. Appl. Physiol. 72:2343-2353, 1992).