Effect of age on pulmonary structure and function of immature and adult animals and man.

Literature on the comparative effects of aging on lungs of immature and adult animals and man was reviewed and additional data for dogs were presented. The dog is the only animal for which comprehensive data have been reported. Human lungs grow primarily by addition of alveoli to 8 yr, then by increasing the size of respiratory airspaces. Alveolar sizes of most mammals are similar at birth but vary in proportion to body size in adults. Lung volumes of dogs and man increase in proportion to body growth until young adulthood, and then total lung volume remains constant. Maximal functional efficiencies are reached at approximately 20 yr in man and 1 yr in dogs. Lungs of dogs and man also undergo similar progressive age-related changes during adulthood. Alveoli enlarge and coalesce, resulting in losses of elasticity and surface area and an increase in the fixed lung volume at the expense of mobile volume. Gas mixing and alveolar--capillary gas exchange become less efficient. Static pressure--volume relationships of all adult mammals are shifted toward reduced elastic recoil with age. In contrast to findings in man and dogs, both excised and intact lung volumes of rodents continue to increase after young adulthood. The gas exchange capacity of rats increases in parallel to the volume increase. These species differences demonstrate the need for careful selection of animal models for the study of aging of the human lung.