Pressure–volume relationship for the chest wall, lungs, and their combination (respiratory system) in normal (A) and obese (B) subjects in supine positions. (A) The shape of the respiratory system pressure–volume curve reflects the balance of the forces from the chest wall and lung parenchyma. In normal subjects, functional residual capacity (FRC) is reduced as compared to the upright position but large enough to locate an operating range of the respiratory system pressure–volume curve within a region of high compliance (dashed yellow rectangle). (B) In obese subjects, increased weight of the chest wall and abdomen shift the chest wall pressure–volume curve to the right at similar chest wall compliances. Combined with the substantial reduction of functional residual capacity, the operating range of the respiratory system pressure–volume curve moves to a region of low compliance (dashed yellow rectangle). This occurs even in the presence of the same normal pressure–volume curve of the lungs. C_RS, compliance of the respiratory system; ΔP, change in pressure; ΔV, change in volume. Modified from Behazin et al.¹⁰⁴