In the lung, the uptake of oxygen is in part determined by the ventilation-perfu

The ratio of ventilation to perfusion is an important determinant of oxygen saturation. If ventilation is restricted by an obstruction, then alveolar PO2 and PCO2 will no longer be maintained at 100 mm Hg and 40 mm Hg respectively, thereby decreasing the gradient for the movement of gases to and from the blood. There are inequalities in ventilation-perfusion ratios throughout the lung. This is especially true in the lower lung where the hydrostatic pressure of gravity increases blood flow compared to the upper regions. Thus, the ventilation-perfusion ratio is lower in the bottom of the lung than in upper portions. In addition, ventilation-perfusion is controlled locally by gases. Carbon dioxide controls the extent of bronchiole constriction. An increase in PCO2 will cause bronchioles to dilate, which affects ventilation. At the same time, an increase in PO2 will cause arterioles to constrict. Hyperventilation results in an increase in PO and a decrease in PCO2. Thus, both bronchioles and arterioles will constrict and thereby reduce both ventilation and perfusion. The ratio of ventilation-perfusion will be determined by the magnitude of the changes in ventilation relative to the changes in blood flow.