Alveolar CO2 and O2 During Breath Holding, Expiration, and Inspiration

Abstract

Analyses are made of alveolar air samples obtained during the 1st 20 sec. of breath-holding. The results are plotted on a PO2-PCO2 diagram and an equation is derived for the exponential curve which results. The initial slope of this curve is less than the slope of the respiratory exchange or R line and from the difference it is possible to calculate the CO2-combining capacity of the lung tissue and its contained blood. Analyses were made of the alveolar samples obtained immediately after diluting the lung gases by a large inspiration of air. This process is interpreted in terms of dilution followed by a brief period of breath-holding. This curve deviates slightly from the R line because of the CO2 capacity of the lung tissue. Calculations of the variations in alveolar PO2 and PCO2 during a normal respiratory cycle indicate that the alveolar composition passes through a counterclockwise loop on the PO2-PCO2 diagram. The expiratory limb of this loop corresponds to a short period of breath-holding and the inspiratory limb corresponds to the dilution curve. This loop encloses but does not pass through the mean alveolar point. Mean alveolar PO2 and PCO2 are best represented by an alveolar sample shortly after mid-expiratory time. However no single sample can be expected to yield the exact mean of both gases at the same time.