Arteries are made of three layers in general: The inner intima, the media and the adventitia. The intima is made of a single layer of endothelial cells and the adjacent so-called sub endothelial tissue, and serves gaseous, liquid, and substance exchange between blood and surrounding tissues. The media is made of muscle and connective tissue. The adventitia is the tissue surrounding the vessel, it serves to fixate the vessel to its environment. There is an elastic layer between intima and media called internal elastic membrane (lat. Membrana elastic interna), between media and adventitia there is the external elastic membrane (Membrana elastica externa). Veins do not have those.