Concentration effect
During induction of general anesthesia when a large volume of nitrous oxide is taken up from alveoli into pulmonary capillary blood, the concentration of gases remaining in the alveoli is altered. This results in effects known as the "concentration effect" and the "second gas effect".
The concentration effect is the effect on the alveolar concentration of the gas which is the one taken up in such large volumes. In practice almost the only non-toxic gas capable of causing a measurable concentration effect is nitrous oxide.