Figure 17-2 Schematic diagram of the lung within the chest wall showing the tendency of the lung to assume a globular shape because of gravity and the lung's viscoelastic nature. The tendency of the top of the lung to collapse inward creates a relatively negative pressure at the apex of the lung, and the tendency of the bottom of the lung to spread outward creates a relatively positive pressure at the base of the lung. Thus, alveoli at the top of the lung tend to be held open and are larger at end-exhalation, whereas those at the bottom tend to be smaller and compressed at end-exhalation. Pleural pressure increases by 0.25 cm H₂ O per centimeter of lung dependency.