The vertical V/Q gradient is not fixed anatomy; it is a consequence of gravity acting on a compliant lung, so it changes whenever the direction of gravity relative to the lung changes or whenever lung volume changes.
In the standing or sitting position, the base is dependent and receives the greatest perfusion, while the apex is non-dependent and receives the least. Lying supine rotates the vertical axis by 90 degrees: the posterior regions become dependent and receive the most blood flow, while the anterior regions receive less. The same total blood flow is redistributed, so the regional V/Q ratios shift with it. In the lateral decubitus position, the dependent lung receives more perfusion and also more ventilation, but perfusion increases more, so the dependent lung operates at a lower V/Q ratio than the non-dependent lung.
Lung volume changes the picture through the compliance curve. At high lung volumes, the whole lung is stiffer and the apex and base are both near the flat upper portion of the compliance curve, so the vertical ventilation gradient flattens. At low lung volumes, the base is closer to its closing volume and small airways may narrow or close during expiration, which reduces ventilation to the base and can even divert it toward the apex. Exercise adds a further change: increased pulmonary arterial pressure recruits previously closed apical capillaries and distends open ones, which flattens the perfusion gradient and makes the distribution of blood flow more uniform.
The practical consequence is that any statement about regional V/Q must specify the posture and the lung volume. A ratio measured or predicted for one posture does not transfer directly to another. The heterogeneity itself persists in every posture, because gravity always creates a dependent-to-non-dependent gradient in a fluid-filled, compliant organ.