Lung perfusion1 increases linearly from the top to the bottom of the lungs (Figure 15.1, lung perfusion line). The difference in perfusion at the top and bottom of the lung can be explained by the effect of gravity on the alveolar volume, which in turn determines the pulmonary capillary pressure. The difference in pulmonary capillary pressure between the lung apex and base is equivalent to the hydrostatic pressure exerted by a column of blood. The distance from apex to base is 30 cm, so the pressure difference is 30 cmH2O (equivalent to 22 mmHg). The pulmonary circulation is a low-pressure system: mean pulmonary artery pressure (MPAP) is typically just 15 mmHg. A pressure difference of 22 mmHg between the top and the bottom of the lungs is therefore potentially significant (this is discussed further in Chapter 16).