Arteries take oxygenated blood from heart to rest of body, branch to become smaller distally.
Veins return blood from the tissues to the heart, merge to become larger as they approach heart.
Some small vessels may lack this layer. Large Elastic Arteries 2. Medium-sized Veins 3. Anchoring filaments extend from outer surface into surrounding CT to hold vessels open - Lymph vessels with valves to prevent backflow in amniotes. Lymph movement is facilitated by skeletal muscle contraction.