How does blood return to the heart when muscles are not contracting?

Blood is returned to the heart when muscles are not contracting through a process called venous return. It is the pressure gradient between the peripheral circulation and the right atrium that drives venous return. When muscles relax, the pressure in the veins decreases, and the valves in the veins prevent backflow of blood. The surrounding tissue pressure also helps to push the blood back towards the heart. Additionally, the heart's own suction action, created by the contraction of the atria, also helps to draw blood back to the heart. These mechanisms ensure that even when muscles are not actively contracting, blood is continuously returned to the heart.