Ventilation (V) refers to the flow of air into and out of the alveoli.
perfusion (Q) refers to the flow of blood to alveolar capillaries.
Gas exchange occurs in the lungs between alveolar air and blood of the pulmonary capillaries. For effective gas exchange to occur, alveoli must be ventilated and perfused.Gas exchange in the alveoli occurs primarily by diffusion. Traveling from the alveoli to capillary blood, gases must pass through alveolar surfactant, alveolar epithelium, basement membrane, and capillary endothelium.
The V/Q ratio evaluates the matching of ventilation to perfusion .There is regional variation in the V/Q ratio within the lung. Ventilation is 50% greater at the base of the lung than at the apex. The weight of fluid in the pleural cavity increases the intrapleural pressure at the base to a less negative value. As a result, alveoli are less expanded and have higher compliance at the base, resulting in a more substantial increase in volume on inspiration for increased ventilation. Perfusion is also greater at the base of the lung due to gravity pulling blood down towards the base. Overall, perfusion increases more than ventilation at the base of the lung, resulting in lower V/Q ratios in the base of the lung compared to the apex.
In a healthy individual, the V/Q ratio is 1 at the middle of the lung,with a minimal spread of V/Q ratios from 0.3 to 2.1 from base to apex.
High V/Q ratios, PO2 increases and PCO2 decreases as alveolar air more closely matches the larger volume of inspired air than perfused blood.
Low V/Q ratios result in a decreased PO2 and an increased PCO2.
V/Q mismatch leads to – respiratory failure, pneumonia, ARDS, copd & etc.