The ventilation-perfusion (V/Q) ratio is the ratio of alveolar ventilation to pulmonary blood flow; the normal value is 0.8.
Ventilation
In the erect posture, resting ventilation per unit lung volume is greater at the lung bases than at the apices.
Uniformity of distribution of inspired air is tested by the nitrogen washout method.
After 7 minutes of breathing oxygen, a normal alveolar gas sample contains less than 2.5% nitrogen; a higher alveolar nitrogen percentage indicates greater non-uniformity of distribution of inspired gas.
Perfusion
Pulmonary perfusion is also non-uniform in the erect posture, due to the effect of gravity.