ALVEOLAR SEPTAL STRUCTURE IN DIFFERENT SPECIES

Because the retractive forces due to surface tension decrease with inc reasing radius of curvature, there should be a greater contribution to lung recoil attributable to the stress-bearing role of elastic elemen ts in the lung parenchyma of species with larger alveoli. To examine a lterations in lung structure that may relate to this stress-bearing ro le, the lungs of mice, hamsters, rats, rabbits, rhesus monkeys, baboon s, and humans were preserved by vascular perfusion of fixative. The nu mber of alveoli per lung, alveolar radius of curvature, surface area, and volume were measured by serial section reconstruction. Electron-mi croscopic determinations were made of the volume fraction and thicknes s of the epithelium, interstitium, and endothelium and of the connecti ve tissue fibers of the alveolar septa and the portions of alveolar se pta that form the alveolar ducts. The thickness of the alveolar septal interstitium increased linearly with the increase in radius of curvat ure of alveoli. The increase in interstitial thickness in lungs with l arger alveoli was paralleled by large increases in the volume of colla gen and elastin fibers present in this space. Comparable changes in th e thickness of connective tissue fibers in alveolar duct walls were al so found. This study demonstrates species-related changes in the struc ture of alveolar septa and in lung collagen and elastin fibers that ar e consistent with connective tissue fibers having a greater stress-bea ring role in both the alveolar septa and alveolar ducts of species wit h larger alveoli.