Peripheral venous disorders largely arise from the difficulties associated with the passage of venous blood from the dependent extremity toward the heart against gravity. Since venous return in the horizontal position presents no real problem, the crux of the matter lies in the adequacy of the mechanisms for combating the effect of gravity and providing for the continuous flow of blood from the dependent extremity. Muscle activity and tonus provide the principal force producing this flow. The direction of flow appears to be determined by the valves, the push of blood from the capillary bed, and the proximal emptying of blood from the venous reservoirs by passage through the right side of the heart and by the negative intrathoracic pressures produced by respiratory movements.
The assumption of an erect or semierect position through a sizable portion of each day imposes a substantial load on these relatively imperfect mechanisms for venous
MOORE TC. Functional Venography as an Aid in the Study of Peripheral Venous Disorders. AMA Arch Surg. 1956;72(1):122-135. doi:10.1001/archsurg.1956.01270190124014