What is the function of valves in veins

The main function of valves in veins is to prevent the blood from flowing in backward direction.

Explanation:

Valve are present only in veins which carries deoxygenated blood from the body cells to the heart. Deoxygenated blood contains high amount of carbondioxide which is harmful for the cells so for protecting the cells from deoxygenated blood, valves are present in vessel to stop the backward flow of blood. These valves present only in veins not arteries.

The main function of valves is to ensure the unidirectional flow of blood and also to prevent the back flow of blood in the veins.

Explanation:

Arteries and veins are the main two blood vessels that transport blood away from the heart and towards the heart respectively. the arteries transport the blood with great pressure which eventually maintains the unidirectional flow of blood, so the valves are not absent there. On the contrary, the veins transport the blood towards the heart which is against the force of gravity, hence the valves are required to carry the blood in only one direction.