Well notice this inside portion is the lumen. It's the internal cavity of the blood vessel and this is where the blood actually flows. And so the tunica intima is actually in direct contact with the blood plasma. And so what that means is it contains a layer of endothelial cells that are responsible for absorbing the nutrients that the blood vessels actually need to function correctly and efficiently. Now within the tunica intima we also have a network of protein elastin fibers and this basically gives the ability, gives our blood vessel the ability to actually be flexible, so have the ability to basically expand and stretch back. Now the middle layer, this red layer is our tunica media and this is where the smooth muscle of the blood vessel is actually found.