Figure 25-10.

Closed-circuit helium method to measure functional residual capacity.

Dots represent molecules of helium (He). A, Initially, all He molecules are in the spirometer (as 10% He), and no molecules are in the lungs. If the spirometer contains 2000 mL of gas, of which 10% is He, then 2000 mL × 0.1, or 200 mL, of He is present in the spirometer before rebreathing. B, Rebreathing results in redistribution of the He molecules until equilibrium develops, at which time lung volume can be calculated. At the end of the test, the same amount of He (200 mL) must be redistributed in the lungs, tubing, and spirometer, assuming that He is inert and not soluble in blood or tissues.