Ventilation-perfusion relationships in the lung .Because the intrapleural pressure is more negative at the apex than at the base, alveoli at the base of the lung are less expanded than those of the apex prior to inhalation and receive a larger fraction of the inspired air. The base of the lung also receives a larger blood flow, because the outward transmural pressure across the pulmonary vessels is greater, increasing their diameter and decreasing their flow resistance.
Red arrow indicates blood flow, blue area indicates lung expansion during tidal volume breathing .