On the other side Ventilation-perfusion mismatch is the term used when the ventilation and the perfusion of a gas exchanging unit are not matched.
Gravity and the weight of the lung act on ventilation by increasing pleural pressure at the base (making it less negative) and thus reducing the alveolar volume.
In the dependent region smaller alveolar volumes mean the alveoli are more compliant (more distensible) and so capable of more oxygen exchange.
The apex, though showing a higher oxygen partial pressure, ventilates less efficiently since its compliance is lower and so smaller volumes are exchanged.
The impact of gravity on pulmonary perfusion expresses itself as the hydrostatic pressure of the blood passing through the branches of the pulmonary artery in order to reach the apical and basal areas of the lungs, acting synergistically with the pressure developed by the right ventricle.