A Concept of Mean Alveolar Air and the Venti- Lation -bloodflow Relationships during Pulmonary Gas Exchange’ Hermann Rahn

I N THE past many different methods have been employed for the direct sampling of alveolar air. A sound criticism of the validity of the technique has usually been futile since one was at loss to define or determine mean alveolar composition. The ease with which alveolar air composition can be altered by the slightest change in ventilation is very impressive and makes one aware of the possibility of unequal ventilation in various parts of the lung and the consequential regional variability of gas concentration. The concept of unequal bloodflow to the various alveoli has received less attention, although this factor is equally important in altering the gas composition. It is the purpose of this paper 1) to define the alveolar air composition in terms of alveolar ventilation and pulmonary bloodflow which allow one to define a concept of mean alveolar gas composition; 2) to discuss a method for the direct sampling of mean alveolar air; 3) to compare this with the Haldane technique of sampling alveolar air; 4) to predict on the basis of the ventilation-bloodflow equations the effect of unequal ventilation and bloodflow upon the alveolar-arterial oxygen gradient.