towards the heart. Diagram taken (then edited) from: http://appliedsciencehelp.co.uk/Circulatory%20and%20respiratory%20systems.htm Tunica intima (Endothelial cells) – The tunica intima is the innermost layer of an artery or vein . It is made up of one layer of endothelial cells and is supported by an internal elastic lamina. The endothelial cells are in direct