 While both ventilation and perfusion increase going from the apex to the base, perfusion increases to a greater degree than ventilation, lowering the V/Q ratio at the base of the lungs.The principal factor involved in the creation of this V/Q gradient between the apex and the base of the lung is gravity (this is why V/Q ratios change in positions other than the orthostatic position).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.