Question

Veins consists of valves and valves are absent in arteries.why

Answer 1

Arteries move blood away from the heart and veins bring the blood back to heart. reason for valves in the veins is because it assists in the oneway upward movement of blood from the lower portion of the body, because nothing is forcing it to return to the heart except for body movements and the valves. 
There is no need for valves in the arteries once it gets out of the heart because the blood pressure created by the heart pumping is what causes its movement.

Answer 2

The reason  is the blood  pressure  in veins if there is no blood  flow in arteries they can retain but whereas  in veins they collapse