Blood moves through veins when it is pushed by muscle contractions. Unlike arteries, which carry blood away from the heart in response to the rhythmic contractions of the left ventricle, veins are a low-pressure system that pushes blood toward the heart by means of a series of smooth muscle contractions known as peristalsis.
Blood traveling through the veins has previously passed through the capillaries and lost much of the systolic pressure from the heart that drove it through the thicker, narrower arteries. To keep the venous blood from pooling near the capillaries, or leaching into the space between cells, veins have smooth muscle tissue in their walls that gently contracts to push blood through the circulatory system. The smooth muscles in the walls of the veins manage the contractions, as does all smooth muscle tissue, as the autonomic nervous system is not under conscious control.
Another factor that tends to drive venous blood back into the heart is the contraction of skeletal muscle. The squeezing and flexing of these muscles puts pressure on the blood vessels and locally raises blood pressure enough to push the blood along its path. A series of valves that close in response to back pressure prevents the blood in the veins from flowing backwards.