ABSTRACT
The amount of air flowing into and out of the lungs with each breath is called the tidal volume (TV). In a typical adult this amounts to about 500 ml during quiet breathing. The respiratory system is capable of moving much more air than the tidal volume. Starting at the resting expiratory level (REL in Figure 60.1), it is possible to inhale a volume amounting to about seven times the tidal volume; this volume is called the inspiratory capacity (IC). A measure of the ability to inspire more than the tidal volume is the inspiratory reserve volume (IRV), which is also shown in Figure 60.1. Starting from REL, it is possible to forcibly exhale a volume amounting to about twice the tidal volume; this volume is called the expiratory reserve volume (ERV). However, even with the most forcible expiration, it is not possible to exhale all the air from the lungs; a residual volume (RV) about equal to the expiratory reserve volume remains. The sum of the expiratory reserve volume and the residual volume is designated the functional residual capacity (FRC). The volume of air exhaled from a maximum inspiration to a maximum expiration is called the vital capacity (VC). The total lung capacity (TLC) is the total air within the lungs, that is, that which can be moved in a vital-capacity maneuver plus the residual volume. All except the residual volume can be determined with a volume-measuring instrument such as a spirometer connected to the airway.
