Answer:
all of those can be included
Explanation:
I’m pretty sure it’s physical
A relatively small blood pressure gradient is generally insufficient to move blood through the veins under given conditions, thus venous return must be facilitated by valves within veins and two "pumps." ... As skeletal muscles contract, veins are squeezed to help propel the blood toward the heart.
Some energy is always lost as heat
hope that helps! :))
