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VENTILATION-PERFUSION RATIO
In the ideal lung, inspired air reaches all the alveoli and all the alveoli have the same blood supply. Actually, neither alveolar ventilation nor capillary blood flow is really uniform. The supply of air and blood is never perfectly matched even in healthy men. Because of gravity, the lower parts of the lungs have greater blood flow than the upper parts (see Fig. 10). Distribution of alveolar ventilation from the top to the bottom of the lungs is also uneven. Since gravity affects blood flow more than ventilation, the ratio of ventilation (VA) and blood flow (QC) even in normal subjects is equal to 0.8. Under these circumstances PAO2 is about 100 mmHg and PACO2 is about 40 mmHg. Changes in VA/QC result in impaired gas exchange.

There are hundreds of thousands of gas-exchanging units, but for the purpose of simplification it is convenient to use a two-compartment model of lung to illustrate the effects of alterations of the VA/QC relationship in gas exchange (Fig. 15).
 

Fig.15
Fig. 15.  Examples of mismatching alveolar ventilation and pulmonary blood flow. A: Due to the airway obstraction, the right lung (R) is not ventilated but blood flow is normal ("wasted" blood flow); VA/QC is equal 0. B: The right lung has normal alveolar ventilation but has no blood flow ("wasted" ventilation=alveolar dead space), VA/QC is infinity. C: Both lungs are properly ventilated, but blood flow to right lung is sygnificantly decreased, VA/QC is increased, some air in this lung does not contribute to gas exchange.

Abnormalities in the distribution of ventilation can result from bronchial narrowing that causes one lung unit to receive only a fraction of the ventilation of the other unit. When pulmonary blood flow is evenly distributed, the ventilation/perfusion ratio of the poorly ventilated but well perfused lung unit is low as compared to the normal lung unit. The poorly ventilated compartment will have a lower alveolar and capillary PO2 and a higher PCO2 than the unit with a normal VA/QC (in the poorly ventilated unit only a little amount of oxygen flows in with each inspiration, and only a little amount of CO2 can be exhaled). If the level of ventilation to the abnormal lung unit were to fall to zero (Fig. 15A), the capillary PO2 and PCO2 would approximate those in mixed venous blood (there is no oxygen delivered during inspiration, and no CO2 can be removed from the alveoli). Therefore, the blood would pass unchanged from the right heart throughout the lungs to the left heart: right to left shunt. From the gas exchange point of view, this blood flow is "wasted". Under this condition arterial PO2 decreases if the patient is breathing air. (A simultaneous increase in PCO2 can be compensated by the reflex increase in alveolar ventilation.)

The other examples of abnormal VA/QC ratios are depicted in Figure 15 where the VA/QC  ratio is equal to infinity (B) or it is increased (C). These alveoli will be ventilated normally while receiving no or decreased blood flow. This alveolar ventilation, would be ineffective in gas exchange ("wasted" ventilation). It is the "alveolar dead space". In the case of increased VA/Qc ratio (Fig. 15B - right panel), blood is fully oxygenated and CO2 may diffuse to the alveoli. However, because of decreased perfusion, part of this ventilation is not used for gas exchange, representing "waisted" ventilation (or physiological dead space)

At a VA/QC ratio of infinity, there is no blood flow to remove O2 from or add CO2 to the inspired gas. Therefore, PO2 and PCO2 remain unchanged in these alveoli and will approximate those in the inspired air; these alveoli functionally behave as the dead space.