The concentration effect refers to the principle that the concentration of inspired anesthetic gas influences both (1) the alveolar concentration that may be attained and (2) the rate at which that concentration may be attained. The rate of rise of alveolar end tidal concentration is thus dependent upon and accelerated by a high initial inspired concentration of anesthetic gas. The concentration effect is only clinically relevant with nitrous oxide, although it may occur with other gases. The concentration effect helps explain why nitrous oxide has a more rapid uptake than other gases. If the inspired concentration is 100% of an anesthetic gas, the rate is solely dependent on the rate at which the ventilation washes gas into the lung, and uptake no longer limits the level to which FA/FI may rise. At 100% inspired concentration, replacement gas is drawn down into the trachea to replace the void of the gas that was taken up. Since this replacement gas has a concentration of 100%, its uptake does not change the alveolar concentration. This explains why the rise of nitrous oxide is more rapid than the rise of other gases such as desflurane, independent of their blood/gas partition coefficients.