How is blood in the veins returned to the heart?

This question requires understanding about how blood flows in the veins. The body contains three types of blood vessel: artery, vein and capillaries, each with different characteristics. Veins carry deoxygenated blood to the heart and are characterised by thin, elastic, non-muscular walls and a low pressure. Blood in veins in areas above the head can move by gravity. But how does the blood move against gravity from the limbs to the heart without muscle to pump the blood? The veins utilise surrounding muscles in limbs to help move the blood. A specific feature of veins is also semi-lunar valves, which prevent backflow of blood if the muscles are not pushing it.