Multiple choice Why are semilunar valves present in the veins? To allow the movement of leucocytes through them To allow the diffusion of oxygen into the intercellular fluids To prevent the loss of nutrients form the blood To prevent the backward flow of blood To prevent the entry of pathogens into the veins Reveal answer Fill a bubble to check yourself D Correct answer Explanation Veins are provided with semilunar valves, which prevent the backward flow of blood.