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 increased. This results in effects known as the "concentration effect" and the "second gas effect". These effects occur because of the contraction in volume associated with the uptake of the nitrous oxide. Previous explanations by Eger and Stoelting have appealed to an extra-inspired tidal volume due to a potential negative intrapulmonary pressure associated with the uptake of the nitrous oxide.
In the first reference below, it is shown that there are two extreme breathing patterns and that the extra-inspired tidal volume is an artificial construct associated with one of these patterns. Thus it is the volume change that actually causes the effects.
When a constant concentration of an anesthetic like halothane is inspired, the increase in alveolar concentration is accelerated by concomitant administration of nitrous oxide, because alveolar uptake of the latter creates a potential subatmospheric intrapulmonary pressure that leads to increased tracheal inflow.
Errata British Journal of Anaesthesia 1997;79:268
Korman B & Mapleson WW. Concentration and second-gas effects in the water analogue. British Journal of Anaesthesia 1998;81,837-843