A shunt is a region where perfusion continues but ventilation has stopped. Because no fresh gas reaches the alveolus, the local V/Q ratio is zero. The blood passing through that capillary bed cannot exchange oxygen or carbon dioxide with alveolar gas, so it returns to the left atrium with essentially the same composition it had in the pulmonary artery, that is, mixed-venous blood. When this blood mixes with fully oxygenated blood from normal regions, the resulting arterial PO2 falls. The size of the fall depends on the fraction of cardiac output passing through the shunt. A shunt of 20 percent of cardiac output is enough to lower PaO2 substantially, and the effect is not corrected by raising the inspired oxygen fraction, because the shunted blood never comes into contact with the higher alveolar PO2.
Dead space is the mirror image. Here ventilation continues but perfusion has stopped, so the local V/Q ratio approaches infinity. The alveolus receives fresh gas and then exhales it without any gas exchange having occurred. This is wasted ventilation. Anatomical dead space is the volume of the conducting airways, roughly 150 mL in a typical adult, where no gas exchange occurs even in health. Physiological dead space includes the anatomical dead space plus any alveoli that are ventilated but not perfused. Because dead space does not add CO2 to the blood and does not remove CO2 from it, its main consequence is that the remaining perfused alveoli must eliminate the entire CO2 production of the body. If they cannot increase their ventilation enough, PaCO2 rises. Supplemental oxygen does not worsen dead space and can correct the hypoxemia that often accompanies it, because the problem is wasted ventilation, not blocked diffusion.
Between these two extremes lies the ordinary lung. Most regions have V/Q ratios somewhere between zero and infinity, and the arterial blood gas is the flow-weighted result of mixing blood from all of them. A region with a low but nonzero V/Q ratio behaves like a partial shunt: it adds relatively deoxygenated blood to the arterial mixture. A region with a high but finite V/Q ratio behaves like a partial dead space: it wastes some ventilation. The clinical picture depends on how much blood flow passes through low-ratio regions and how much ventilation is wasted in high-ratio regions.