The TLC, FRC, and RV are absolute lung volumes and cannot be measured directly with spirometry. FRC is the amount of air remaining in the lungs after a normal, physiologic expiration (Figure 1A). TLC is the total volume of the lungs at maximal inspiration which is about 6 L on average, though true values are dependent on the same factors that affect residual volume. The residual volume is an important component of the total lung capacity (TLC) and the functional residual capacity (FRC). Reference values for residual volume are 1 to 1.2 L, but these values are dependent on factors including age, gender, height, weight, and physical activity levels. The residual volume remains unchanged regardless of the lung volume at which expiration was started. In other words, it is the volume of air that cannot be expelled from the lungs, thus causing the alveoli to remain open at all times. Residual volume (RV) is the volume of air remaining in the lungs after maximum forceful expiration.
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