ANATOMY OF MEMBRANOUS BRONCHIOLES IN NORMAL, SENILE, AND EMPHYSEMATOUS HUMAN LUNGS

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