The second gas effect applies to the situation in anaesthetics where a gas such as nitrous oxide is given in a high enough concentration so that its subsequent uptake from the alveolus causes an increase in the concentration of the other gases. For example if breathing 20% oxygen and 80% N2O (nitrous oxide) in the alveoli say half the N20 was taken up into the bloodstream then of the original 100% of the mixture 20% oxygen will remain with 40% N20. the new concentration of oxygen will thus be increased with now 33.3% of the gas mixture comprising of oxygen.

This can also be applied to increase the alveolar concentration of a volatile anaesthetic. Imagine in the above example that there was 19% oxygen and 1% anaesthetic gas then after N20 uptake there would be 1.7% anaesthetic gas and 31.7% oxygen.