ANATOMY

Detailed explanation-1: -Veins have valves to prevent the backflow of blood because the blood in veins flow .

Detailed explanation-2: -Veins often pump blood against the flow of gravity and make use of valves to prevent the blood from flowing downwards.

Detailed explanation-3: -What are Vein Valves? Interspersed throughout your veins are small crescent-shaped flaps of tissue, known as valves, that help prevent a backflow of blood. These valves project from the innermost layer of the vein wall to the center of the vein and are angled towards the heart in the direction of venous blood flow.

Detailed explanation-4: -Final answer:The valve which prevents back flow of blood in the veins is Semilunar valve.