Why we inhale oxygen only not any other gas like nitrogen?

When we inhale air,all the gases in it also enter our lungs. We have externally no selective mechanism to inhale only oxygen. In the alveoli ,exchange of gases takes place. The haemoglobin in red blood cells has an affinity for this oxygen present in the inhaled air and it is immediately absorbed. The rest are inert gases which are sent out during exhalation. Oxygen is vital to our body functions.