What is the function of valves in the veins?

Valves in the veins have the critical function of preventing the backflow of blood towards the heart. This is particularly important because veins carry deoxygenated blood from various parts of the body back to the heart under lower pressure compared to arteries. The valves act as one-way gates, ensuring that blood flows in one direction—towards the heart—thereby preventing any reversal of flow that could occur due to factors like gravity or changes in pressure during movement.

The other choices do not accurately describe the role of venous valves. For instance, valves do not prevent oxygen exchange in the lungs; their function is specific to blood flow regulation in the venous system. Additionally, while venous return contributes to blood pressure, the primary role of the veins' valves is not to control blood pressure levels directly. They also do not directly facilitate nutrient absorption in tissues, which is primarily the role of capillaries and not veins.