Lung capacity or total lung capacity (TLC) is the volume of air in the lungs upon the maximum effort of inspiration. Among healthy adults, the average lung capacity is about 6 liters. Age, gender, body composition, and ethnicity are factors affecting the different ranges of lung capacity among individuals. TLC rapid increases from birth to adolescence and plateaus at around 25 years old. Males tend to have a greater TLC than females, while individuals with tall stature tend to have greater TLC than those with short stature, and individuals with a high waist-to-hip ratio generally have a lower TLC. Individuals of African descent have a lower TLC compared to individuals of European descent. Additional factors that affect an individual's lung capacity include the level of physical activity, chest wall deformities, and respiratory diseases.
Clinicians can measure lung capacity by plethysmography, dilutional helium gas method, nitrogen gas washout method, or radiographically by a relatively new technique using by computed tomography (CT). Methodically, the TLC is calculated by measuring the lung capacities: inspiratory capacity (IC), functional residual capacity (FRC), and the vital capacity (VC). Illustrated in Figure 1, Panel A, the lung capacities can be further divided into the following lung volumes: tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and the residual volume (RV). This review article will not delve into definitions of all of the lung capacities and lung volumes but instead will outline the methods in which lung capacity is measured and discuss the clinical significance of TLC (see image).