Expansion
Pleural pressure is more negative at the apex than at the base
Expansion
In the upright lung, the weight of the lung makes pleural pressure more negative at the apex (about -10 cmH2O) than at the base (about -2.5 cmH2O).
Consequences:
- Apical alveoli have a larger transpulmonary pressure, so they are more expanded at rest. They lie on the flat, less compliant part of the pressure-volume curve, so they expand less during inspiration
- Basal alveoli are less expanded at rest but sit on the steep, compliant part of the curve, so they receive more ventilation
So ventilation is greatest at the base, and since perfusion is also greatest there but increases more steeply, V/Q is lower at the base and higher at the apex.
Two clinical corollaries: tuberculosis favours the apex, where the high V/Q gives a high oxygen tension suiting an aerobic organism; and at very low lung volumes near residual volume the gradient reverses, so basal airways close and ventilation preferentially goes to the apex.