ABSTRACT
Although it is well known, in any discussion relating to gravitational effects on pulmonary perfusion it is worth reiterating that pulmonary vascular pressures are very low in normal subjects. At rest in a normal adult, mean pulmonary artery pressure is only about 15 mmHg and pulmonary venous pressure about 5 mmHg (1). In units of cmHzO, the pulmonary arterial pressure is therefore -20 cmHzO. The pulmonary artery enters the lung at the hilum, and in an average-sized adult, the lung extends approximately 20 cm above this point. As a consequence, the hydrostatic pressure gradient that exists in the pulmonary vasculature is of a comparable magnitude to the vertical size of the human lung. This would lead one to believe that gravity must play an important role in the nature of pulmonary perfusion, at least at rest, in the upright human.
