Distribution of Blood Flow (Perfusion) and Alveolar Ventilation and the Ventilation-to-Perfusion Ratio (Va/Q) As a Function of the Distance from the Base of the Lung (to the Left in the Figure) to the Apex (to the Right)

(Reproduced with permission from Slinger P. Principles and Practice of Anesthesia for Thoracic Surgery. New York, NY: Springer; 2011.)

In the upright position, both ventilation and blood flow are greater at the base of the lung than at the apex. However, the gradient is steeper for blood flow than ventilation. Thus, the VA/Q ratio is higher at the apex than in the mid- or dependent lung regions.