What Causes Valves in Veins and Not in Arteries?

The presence of valves in veins is a direct adaptation to low pressure and the force of gravity.

• Why Arteries DON'T Have Valves: Arteries carry blood away from the heart. The blood is propelled by the powerful, high-pressure contraction of the heart's ventricles. This force is so strong that there is no chance of blood flowing backward. The pressure gradient always moves blood in one direction.
• Why Veins DO Have Valves: Veins carry blood toward the heart. By the time blood reaches the veins, the pressure from the heart has dropped to almost zero. In the limbs, this low-pressure blood must travel against gravity to get back to the heart.

Valves are the solution. They are one-way flaps that allow blood to flow toward the heart but snap shut if gravity tries to pull it back down. This, combined with the skeletal-muscle pump (when your leg muscles contract, they squeeze the veins and push blood upward past the valves), ensures that blood makes it back to the heart.