What do you breath in?

You breath in (inhale) all the gases in the atmosphere. viz. Nitrogen , Oxygen, Carbon Dioxide, Water, the inert(Noble) gases.

However, only one gas is beneficial to animated life viz., oxygen. , which is absorbed into the animated organism.

The animated organism then exhales carbon dioxide, together with all the other atmosphereic gases, formerly inhaled.

In humans, carbon dioxide also regulates the breathing rate of approximately 40 breaths per minute. This can vary depending on the level of physical activity, e.g. sleeping, or running etc.,