The volume of air present in the lungs after forceful expiration is known as residual volume which prevents the collapsing of alveoli even after forceful expiration. The tidal volume (TV), about 500 mL, is the amount of air inspired during normal, relaxed breathing. The inspiratory reserve volume (IRV), about 3,100 mL, is the additional air that can be forcibly inhaled after the inspiration of a normal tidal volume. The expiratory reserve volume (ERV), about 1,200 mL, is the additional air that can be forcibly exhaled after the expiration of a normal tidal volume.