V/Q ratio is maximum in which part of lung -

Ans. is 'a' i.e., Apex Ventilation perfusion ratio (V/Q)o Considering that cardiac output is 5.0 L/min and alveolar ventilation is about 4.2 L/min, the overall ventilation : perfusion ratio is 0.8. Idealy, therefore, each alveolus should have a V/Q ratio of 0.8. However, that is not so even in normal lungs.o Due to gravity, the apical alveoli are both underventilated and underperfused while the basal alveoli are both overventilated and overperfused. However, gravity affects perfusion much more than it affects ventilation. Hence, apical alveoli are more underperfused than underventilated while the basal alveoli are more overperfused than overventilated. Therefore, V/Q is maximun at apex (about 3 0) and least at base (about 0-6).o Since ventilation is far in excess of perfusion at apex, comparatively little oxygen is transferred from the alveoli to the blood, and CO2 transferred to the alveoli is also less. Hence the gas tension at the apices are quiet close to those of inspired air, i.e., High PaO2and low PaCO2. On the other hand, at the base of lung perfusion is better than ventilation; Hence PaO2 and PaCO2 of basal alveoli are quiet close to those ofpulmonary artery, i.e., low PaO2 and high PaCO2 In simple words, ventilation-perfusion mismatch is responsible for high PO2 with low PCO2 at apex and Low PO2 with high PCO2 at base.