Figure 25-9.

Open-circuit nitrogen method to measure functional residual capacity.

Dots represent nitrogen (N₂) molecules. A, Initially all the N₂ molecules are in the lungs (as 80% N₂). B, When N₂-free oxygen (“pure O₂”) is breathed, the N₂ molecules are washed out of the lungs and collected with the O₂ as expired gas in the spirometer. The spirometer contains 40,000 mL of mixed expired gas with a N₂ concentration of 5%. Thus the spirometer contains 0.05 × 40,000 = 2000 mL of N₂; the remaining 38,000 mL of gas is mainly O₂ used to wash the nitrogen out of the lungs, plus some carbon dioxide. The 2000 mL of N₂ was distributed within the lungs at a concentration of 80% N₂ when the washout began; therefore the alveolar volume in which the N₂ was distributed was 2000/0.8 mL = 2500 mL. Corrections must be made for the small amount of N₂ washed out of the blood and tissue when O₂ is breathed and for the small amounts of N₂ in “pure O₂.”