We have the innermost layer, the tunica intima where tunica means layer and intimate means inner. We have the mid layer, the tunica media and we have the outer layer, the tunica extrema. Now what exactly is the function of each one of these layers? 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.