When we breathe, our lungs absorb a portion of the oxygen in the air, and replace it with some amount of carbon dioxide and water vapor. Typically, how much $O_2$ (in grams, milliliters, or moles for instance) is absorbed and how much $CO_2$ and $H_2O$ are released in one breath of a healthy adult?
Of course, the exact amounts will vary from person to person and based on how deeply the person is breathing, lung health, etc. I'm just looking for a ballpark figure.