ABSTRACT
Now it is important to consider how the air that has been drawn into the lung is distributed throughout the airways and alveoli. Just like the entire body, the lung is under the influence of gravity, especially when in the upright position. This causes the intrapleural pressure to be less subatmospheric at the base of the lung than at the apex. Accordingly, alveoli at the base of the lung are relatively smaller at FRC than are those at the apex but are situated on a steeper part of the pressure-volume curve for the whole lung compared to those at the apex (Fig. 8). Therefore, basilar alveoli expand more on inspiration than apical alveoli, causing more air to flow to the base than the apex. This situation helps optimize gas exchange, since more blood flow goes to the base than to the apex, allowing better matching of ventilation and perfusion (see Chap. 4).
