Why doesn't the superior vena cava have valves?

The superior vena cava does not have valves because it carries deoxygenated blood directly from the upper body to the heart's right atrium under relatively low pressure. Valves are typically found in veins that need to prevent backflow, especially in the limbs where blood must travel against gravity. Since the superior vena cava is positioned above the heart, the gravitational pull helps facilitate blood flow without the need for valves. Additionally, the pressure in the superior vena cava is generally sufficient to ensure unidirectional flow into the heart.