There are two types of measurement to asses the physical and functional capacity of lungs. These are lung capacities which can be measured directly by an instrument called spirometer and lung volumes
which are measured indirectly by body plethysmography. TV is tidal volume which means the volume of air inhaled or exhaled with each breath in normal breathing(not forceful). IRV is Inspiratory Reserve Volume-volume of air that can be inhaled from end point of normal inhalation. ERV is Expiratory Reserve Volume- Vol of air that can be exhaled out from end expiratory position. RV is residual Volume- Amount of air remaining in the lungs after a maximum exhalation. VC is Vital capacity- Amount of air that is breathed out with maximum exahalation after a deep inhalation.