1. If an artery is empty, it collapses.   True   False   2. Valves are present in arteries.   True   False   3. Veins are normally located just below the skin.   True   False   4. Veins have a narrow lumen (the central space through which blood flows).   True   False   5. Capillaries have muscular walls.   True   False   6. Arteries have thick walls.   True   False   7. Veins have non-elastic walls.   True   False