What makes the blood move through the veins?

Venous blood is collected by the vein system into the atria. A number of factors serve for the unidirectional flow of blood through the veins. For example, the pressure in the chest cavity is below atmospheric. This makes the blood move to the heart. The pressure here drops even more during inhalation, and the venous blood rushes to the heart even faster. The contractions of the muscles constrict the veins, which directs the blood upward. In the veins, in addition to the vena cava and veins of the portal organ system, there are valves that create a unidirectional movement of blood through the vessels.