The pumping of the blood by the heart creates a high pressure in the arteries that pushes the blood forward. In the veins, however, the pressure is lower, and so there are special flaps called venous valves that allow the blood to pass only in one direction. This is especially necessary for lower parts of the body to prevent accumulation of blood there due to gravity. The heart itself has four valves (at the entrances and exits of both ventricles) to make sure it pumps blood only forward.
In your arteries, the blood going through is moving with lots of support from your pumping heart. The high pressure keeps your blood from flowing in the opposite direction (the same way that water pumping through a hose doesn’t flow back into the hose when you hold the nozzle upright). In your veins, special valves keep your blood flowing in only one direction.
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