Because air rises, alveoli in the apex of each lung are always partially inflated and thus don’t accommodate much further ventilation. The lung bases, on the other hand, have a much lower alveolar pressure and ventilate more readily than the apices. The low pulmonary arterial pressure is just enough to pump blood to the top of the lung. Although all parts of the lung receive some perfusion, hydrostatic pressure in the vasculature directs most of the blood flow through the lower
(dependent) portion of the lung. So when a person sits or stands, blood flow to the lung bases exceeds blood flow to the apices.