Fig. 1. A recording of gaseous washouts obtained during exercise in an animal rebreathing a mixture of He, CO and air. The initial fluctuations and sudden decrease in He represents breath-by-breath change in He as it is mixed with alveolar gas. Helium decreases to a stable concentration whereas CO continuously diffuses from alveolar gas to blood, causing a decrease of alveolar F_CO. The concentration of CO relative to He (rel-F_CO) changes exponentially with time such that a plot of ln(rel-F_CO) versus time yields a linear relationship, shown as the solid line fitted through the data after He has stabilized.