Our atmosphere is mostly nitrogen already, so you probably wouldn't notice any difference. Since there would be a higher gas pressure of nitrogen in the air than in the blood, the lungs would absorb inhaled nitrogen, then because there would be a lower gas pressure of oxygen & CO2 in the air than in the blood the lungs would expel those gasses. The exchanging oxygen out part would happen in any oxygen-deficient atmosphere, like e.g. an argon purge or very thin air, like if an airplane's cabin depressurizes.