This equation describes the concentration of gases in the alveolus, and thus allows us to make educated guesses as to the effectiveness of gas exchange. One can use this to calculate the tension-based indices of oxygenation, such as A-a gradient or the a/A ratio (which is expressed as a percentage).
At sea level in room air, certain constants can be assumed:
PAO2 = (0.21 x (760 - 47)) - (PaCO2 x 1.25) PAO2 = (149 - (PaCO2 x 1.25)