Figure 5-3 A, The rectangle represents a lung filled with 80% nitrous oxide plus 1% of a second gas. B, Uptake of one half of the nitrous oxide does not halve the concentration of nitrous oxide, and the reduction in volume thereby increases the concentration of the second gas. C, Restoration of the lung volume by addition of gas at the same concentration as that contained in A increases the nitrous oxide concentration and adds to the amount of the second gas present in the lung. (Adapted from Stoelting RK, Eger EI II: An additional explanation for the second gas effect: A concentrating effect. Anesthesiology 30:273–277, 1969.)