What action assists in moving blood back to the heart via veins?

The assistance in moving blood back to the heart via veins is significantly provided by the milking action performed by working muscles. As muscles contract during physical activity, they compress the veins that run through them. This compression helps to propel blood towards the heart, leveraging the one-way valves present in the veins that prevent backflow.

When muscles contract, they create a positive pressure in the veins, which aids in overcoming gravity, especially in the lower extremities. This action is crucial, particularly during activities such as walking or exercising, as muscular contractions are among the primary mechanisms aiding venous return.

While gravity does play a role in blood movement, especially in the return of blood from the head and torso back to the heart, it is not sufficient alone, particularly in the lower body. The contraction of muscles is much more effective, especially during physical activity. Valvular contraction is not a mechanism that is actively controlled but rather a passive response to changes in pressure, thus does not contribute directly to moving blood. Blood reservoir expansion primarily pertains to the storage of blood in larger veins rather than its return to the heart.