1. Respiratory Pump : During inspiration and expiration a pressure difference is formed between the thoracic capacity and abdominal capacity forcing blood back to the heart
2. Gravity : drains head and neck; increases when limbs are flat or elevated, helps blood in the upper body return to the heart
3. Muscle Pump: Contracts during exercise, compressing veins located between them
4. Smooth Muscle: Layer within the vein wall that venoconstricts to aid in the movement of blood
Hope this helps!
