Ventilation of terminal air units

¹ Cardiovascular Research Institute, University of California Medical Center, San Francisco, California

We have used a rapid-freeze method to stop mechanical changes in the living lung in 0–10 sec and have adapted histological techniques to prepare thick three-dimensional sections from which diameters of alveoli and alveolar ducts were determined for lungs at two volumes, functional residual capacity and high lung volume, during positive-pressure ventilation. Diameters of both alveoli and ducts increase 30% from the lower to the higher lung volume. The calculated surface area of alveoli increases 70%. The volumes of alveoli and ducts were calculated to increase about twofold. The ventilation unit of the lung includes the alveolar ducts with their associated alveoli stemming from one respiratory bronchiole. The distribution of inspired gas to the respiratory surface of the lung is not determined by hundreds of millions of individual alveoli but by these larger ventilation units.

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