A right to left shunt is an extreme example of a low ventilation to perfusion ratio where blood goes from the right to the left sides of the heart without going past an alveolus at all. Some of the blood returning from the bronchial or coronary circulations returns to the pulmonary veins or left side of the heart, so gets into arterial blood without being saturated with oxygen or giving off any carbon dioxide. These measurements show that the blood flow decreases from the base to the apex of the lung. The distribution of the ventilation in the lungs can be measured in a similar way by making patients inhale gas mixtures containing radioactive xenon. The increase in the ventilation as you go down the lungs is less than the increase in the perfusion, so the ventilation to perfusion ratio falls as you move from the apices to the bases of the lungs.
Many diseases can affect the ventilation to perfusion ratio.
The pulmonary hypoxic vasoconstriction Differences in the ventilation to perfusion ratio from one alveolus to another can be reduced by the pulmonary hypoxic vasoconstriction. Pulmonary arterioles constrict when the alveoli that they supply are hypoxic, contrary to what happens in hypoxic arterioles in the systemic circulation. The pulmonary hypoxic vasoconstriction increases the resistance of arterioles going to hypoxic alveoli, so reducing the blood flow to them. This will not only make the alveoli that the arterioles supply become less hypoxic but it will also divert the blood to other vessels supplying better ventilated alveoli, in both cases improving the oxygenation of the blood leaving the lungs. However, the hypoxic pulmonary vasoconstriction is not beneficial when the whole lung is hypoxic, such as at high altitudes, because there will be a generalised vasoconstriction throughout the pulmonary circulation, which will raise the pulmonary arterial pressure and increase the amount of work the right ventricle has to do.