Alveolar populations contributing to expired gas tension plateaus.

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Abstract
Examination of expired argon, oxygen, and CO2 tension plateaus, following a single inspiration of an argon-air mixture, constantly showed 2 components. Transition from one component to the other occurred at a point 8-22% (mean 13%) of a vital capacity above residual volume. By various respiratory maneuvers, it was shown that the earlier longer component was consistent with the admixture, in constant proportions, of gas emerging from the upper and lower zones of the lung arranged in parallel; and that the late shorter component was consistent with an increasing contribution from the upper zone of the lung. The slopes of the expired gas tension plateaus were examined in the light of the hypothesis that they reflected emptying of alveolar populations arranged in parallel within each zone ("parallel ventilation of lung units"). Argon tensions at 2 fixed points in expiration and the mean argon tension of gas remaining in the lungs following different inspired test-breath volumes [humans] were compared. Within the limits of the assumptions made, the data indicated that parallel ventilation of lung units did not satisfactorily account for the slopes seen in expired gas tension plateaus.

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