Question

Veins returning to the heart can close to prevent the backflow of blood. Which component in the veins contributes to this process?

a. Connective tissue in the tunica media
b. Smooth muscle in the tunica media
c. Endothelium in the tunica intima
d. Movable valves in the tunica intima

Answer 1

D. Movable valves

they help control the flow of blood to and from the heart,

For a second I thought C because Endothelium cells in the tunica intima do actually play an active role in maintaining homeostasis of blood flow but valves are the most direct force of change in blood flow, hope this helps 

Answer 2

Answer: The correct answer is option d.

Explanation:

Veins are the blood vessels which carry blood back to the heart. These are less muscular as compared to the arteries. The prevent back flow of the blood with the help of valves.

Valve is a membranous structure which allow the flow of blood in one direction only. They can fold or close in one direction only.

For example, with each pump or contraction when blood flows towards the heart the valves open. However, they close immediately which prevent the back flow.