Pure blood flows in the arteries and impure blood flows in the veins. Pure blood flows at a higher pressure due to which there is less chance of flow of blood in the backward direction. However, impure blood flows in the veins which has lower pressure. As a result, it may flow in the backward direction.

To eliminate the possibility of backward flow of blood in veins, they contain valves. On the other hand, arteries do not face the possibility of backward flow of blood; hence, they do not have valves.