Which of the following regional differences characterizes a normal upright lung at the level of the functional residual capacity (FRC)?

In an upright lung, gravity causes pleural pressure to be more negative at the apex than at the base. Consequently, alveoli at the apex are larger (more distended) and less compliant at FRC, leading to lower ventilation per unit volume compared to the base. Perfusion is also highest at the base due to hydrostatic effects, but ventilation decreases from base to apex less steeply than perfusion. Therefore, the ventilation-perfusion (V/Q) ratio is highest at the apex (approx 3.3) and lowest at the base (approx 0.6).