Intrinsic Factors affecting venous blod return:
The calf muscle pump
Abdomino-thoracic Pump
The heart
Venous gradient betwen the ankle and right side of the heart
Vein valve Integrity
Sympathetic tome
Extrinsic Factors affecting venous blood return:
Gravity
Native external compression (thickened skin, e.g. lipodermatosclerosis)
Passive mechanical external compression (compression stockings)
Active mechanical external compression (pneumatic pumops)