Respirometry depends on a "what goes in must come out" principle. Consider a closed system first. Imagine that we place a mouse into an air-tight container. The air sealed in the container initially contains the same composition and proportions of gases that were present in the room: 20.95% O, 0.04% CO, water vapor (the exact amount depends on air temperature, see dew point), 78% (approximately) N, 0.93% argon and a variety of trace gases making up the rest (see Earth's atmosphere). As time passes, the mouse in the chamber produces CO and water vapor, but extracts O from the air in proportion to its metabolic demands. Therefore, as long as we know the volume of the system, the difference between the concentrations of O and CO at the start when we sealed the mouse into the chamber (the baseline or reference conditions) compared to the amounts present after the mouse has breathed the air at a later time must be the amounts of CO/O produced/consumed by the mouse. Nitrogen and argon are inert gasses and therefore their fractional amounts are unchanged by the respiration of the mouse. In a closed system, the environment will eventually become hypoxic.