Why do veins have valves?
When blood is pumped by the heart, it enters the systemic (body) circulation at a high pressure and is propelled forward by this pressure. When blood reaches the level of the capillaries however, there is a great decrease in pressure which allows for exchange of materials. When blood leaves the capillaries and enters veins to return to the heart, the pressure is so greatly reduced that backflow is possible. Valves are present at intervals along the veins to block the blood from flowing backwards once it has moved along the circulation towards the heart.