Ventilation-Perfusion Ratio


As has been seen, the rate of alveolar ventilation and the rate of transfer of oxygen and carbon dioxide through the respiratory membrane, determines the PO2 and PCO2 of the alveolar air. In this consideration it is assumed that all the alveoli are ventilated equally and that the blood flow is equal in all the alveolar capillaries. Both under normal conditions and especially under pathological conditions some alveoli might be fully ventilated but not perfused while others might be perfused but not ventilated. The ventilation-perfusion ratio (V/P ratio) is a concept that helps in understanding respiratory ventilation. In quantitative terms this is expressed as


Alveolar Ventilation Rate


Alveolar Capillary Blood Flow for the same alveolus


When the alveolar ventilation rate is normal and the alveolar capillary blood flow is also normal, then the V/P ratio is normal.


When the ratio is equal to 0 (zero), then the alveolus is not ventilated but it is still perfused. In this case the the gas mixture inside the alveolus comes in equilibrium with the venous blood and no gas exchange takes place. The amount of blood passing through these alveoli forms part of the shunted portion of blood flow through the lungs, i.e., blood that flows through the lung capillaries but is not oxygenated (the other major component of shunted blood is the blood supply to the conducting portion of the lungs)

When the alveolus is well ventilated but not perfused, then the V/P ratio is equal to infinity. Again in this case no air exchange is possible. As this portion of the respiratory volume (alveolar dead space) does not take part in gas exchange, it is added to the anatomical dead space and called the physiological dead space. Alveolar dead space can occur both normally as well as associated with certain lung diseases.