In the upright individual the apex of the lung receives relatively little blood. This has often been explained by the low pulmonary arterial pressure which is said to be just sufficient to raise the blood to the apex. It is believed that pulmonary arterial pressure must overcome the pressure due to gravity.This misconception overlooks the fact that the siphon principle applies to the vascular system in which the gravitational pressure of venous blood counterbalances the gravitational pressure of blood in the arteries and vice versa. Accordingly, the perfusion or driving pressure (P1-P2) between arteries and veins at any horizontal level of the lung remains unchanged, irrespective of body position.Intravascular pressure at any point is the algebraic sum of dynamic pressure causing flow (cardiogenic) and the pressure of blood due to gravity which does not cause flow. In the upright position, since the dynamic pressure in the pulmonary circuit is low, the drop in gravitational pressure at the apex of the lung reduces significantly the intravascular and, consequently, the transmural pressure in these vessels. The pulmonary microvessels being highly compliant undergo collapse and increase their resistance to flow. The reduction in apical flow is, therefore, a consequence of increased vascular resistance and not a matter of raising the blood against gravity. Gravitational pressure of blood per se neither hinders upward flow nor favors downward flow.