The effect of changes in lung volume on the size and shape of alveoli.

1. A technique is described by which living anaesthetized guinea-pigs were artificially ventilated with a positive and negative pressure cycle, and rapidly frozen at the instant of cessation of ventilation at a preselected point on the respiratory cycle.2. The dimensions of alveoli and alveolar ducts were measured by histological morphometric methods, and related to the degree of lung inflation at the instant of freezing.3. The alveolar volume fraction increased from 0.52 at low lung volumes, up to 0.62 at lung volumes in the mid-inflation range, then decreased to 0.51 at high lung volumes. The alveolar duct volumetric fraction remained constant at all lung volumes.4. The total volume of alveoli increased linearly with increase in lung volume. The total alveolar duct volume increased little until the lungs were 40% inflated, above which it increased steeply.5. The total number of alveoli was linearly related to the body weight.6. The total alveolar surface area increased steeply between lungs of low lung volume and those which were up to 50% inflated, above which the increase levelled off. The absolute values of total alveolar surface depended on the body weight as well as the degree of lung inflation.7. The mean alveolar duct diameter was 40% greater, and the mean alveolar mouth diameter 35% greater in lungs which were fully inflated than in lungs which were nearly collapsed. The average geometrical shape of alveoli was not related to the degree of lung inflation.8. The harmonic mean thickness of the air-blood barrier was 33% less in lungs which were fully inflated than in lungs which were nearly collapsed. The alveolar surface membrane was smooth whether the lungs were fully inflated or collapsed.