In the upright lung, gravity pulls blood toward the dependent regions, so pulmonary blood flow is highest at the base and lowest at the apex. Perfusion falls by roughly a factor of 10 to 20 from base to apex, while ventilation falls by only about a factor of 2 to 3 over the same vertical distance. Because perfusion changes far more steeply than ventilation, the ratio of the two changes systematically with height.
At the apex, ventilation is relatively well preserved but perfusion is low, so the local V/Q ratio is high, often above 2 to 3. At the base, perfusion is abundant but ventilation increases less, so the local V/Q ratio is low, often below 0.6. The whole-lung average of about 0.8 is simply the flow-weighted mean of this continuous vertical spectrum.
The vertical perfusion gradient is conventionally described by West's three zones. In zone 1, at the very top, alveolar pressure exceeds pulmonary arterial pressure, so the capillary is compressed and flow is minimal or absent; this zone is small or absent in a normal upright person at rest but expands when alveolar pressure rises or arterial pressure falls. In zone 2, the middle region, arterial pressure exceeds alveolar pressure, which in turn exceeds venous pressure, so flow is driven by the arterial-to-alveolar pressure difference and increases down the zone. In zone 3, at the base, venous pressure exceeds alveolar pressure, so the capillary stays open and flow is governed by the arterial-to-venous pressure difference, reaching its highest values.
Ventilation follows a gentler vertical gradient because the pleural pressure surrounding the alveoli is less negative at the base than at the apex. Near the base, the lung is already partly expanded at end-expiration and sits on a stiffer portion of its compliance curve, so it changes volume less for a given pressure swing. Near the apex, the lung starts less expanded and on a more compliant portion of the curve, so the same pressure swing produces a larger volume change per unit of resting lung volume. The net effect is that ventilation per unit lung volume is greater at the base, but the difference is modest compared with the perfusion gradient.
Because the two gradients differ in steepness, the V/Q ratio is not uniform: it is high at the apex and low at the base. This heterogeneity matters because regions with very high or very low V/Q ratios exchange gas less efficiently than a region operating near a ratio of 1. The high-V/Q apex is relatively over-ventilated for the blood it receives, so it contributes little to oxygen uptake and tends to waste ventilation. The low-V/Q base is relatively over-perfused for the ventilation it receives, so its blood cannot be fully oxygenated. When blood from all regions mixes in the pulmonary veins, the poorly oxygenated contribution from low-V/Q regions pulls the arterial PO2 below what an ideal homogeneous lung with the same total V and Q would produce.