What keeps the blood from flowing backwards in the veins?
We have valves that prevent the backward flow.
Blood is a constant whereabout in our bodies. As of such a frequent heartbeat exists within our system. Which causes the blood within our bodies to flow forward, in lamense terms it produces a very large push for the blood to distribute it's nutrients throughout the body and back within our heart and out again. This constant push is what prevents blood from flowing backwards in your veins.
The blood moves around the body inside the circulatory system. This is made up of arteries, veins and capillaries. The blood keeps moving through these blood vessels because it is pumped by the heart. Valves prevent the blood from flowing backwards. The blood always circulates through the body in the same direction.
There are one-way valves (leaflet valves) in the veins which normally allow blood to flow only back to the heart. Muscle pressure is the main force pushing blood back from the extremities, especially against gravity in the legs. When the valves malfunction or are affected by high blood pressure, the vessels can form varicose (swollen) veins, blue veins, or spider veins.