Gas dilution is a method of determining those lung volumes that cannot be determined from simplespirometry. These include functional residual capacity, which is computed directly, and residualvolume and total lung capacity, which are computed from FRC. The subject is connected to aspirometer containing a known concentration of helium, or some other inert and insoluble gas.
After several minutes of breathing, the helium concentrations in the spirometer and lung becomethe same. From the law of conservation of matter, we know that the total amount of helium beforeand after is the same. Therefore we can set the fractional concentration times the volume beforeequal to the fractional concentration times the volume, because of the conservation law of matter.
We solve for the volume after (the volume of the lung and spirometer), subtract out the volume ofthe spirometer, and we get the volume of the lung.