Fig. 1. Total lung capacity, the maximum amount of air the lungs can hold, is divided into four primary volumes: inspiratory reserve volume ( IRV ); tidal volume ( VT ); expiratory reserve volume ( ERV ); and residual volume ( RV ). Capacities are combinations of two or more lung volumes. They include inspiratory capacity ( IC ), functional residual capacity ( FRC ), and vital capacity ( VC ). (Shapiro BA, Harrison RA, Cane RD et al: Clinical Application of Respiratory Care, 4th ed. Chicago, Mosby-Year Book Medical Publishers, 1991.)