A spirometer is used to plot breathing patterns
Vital Capacity: The maximum amount of air a person can exhale after inhaling the maximum possible volume of air
Tidal Volume: The volume of air inhaled & exhaled in one breath
Basal Metabolic Rate: The rate of respiration
The spirometer can be used to plot VC and TV directly. BMR can be worked out if a CO2 scrubber is used. The spirometer has fixed volume and is filled with 100% O2 before the experiment begins. As the person respires, O2 is replaced proportionally with CO2. The total volume should stay constant. However, if CO2 is removed, the total volume will slowly fall as O2 is used. The rate at which the volume decreases is proportionaly to BMR.
You are not expected to know how the spirometer works… although its not very difficult to understand.