English: Microscopic anatomy of an artery. The outermost layer is known as tunica adventitia, and is composed of connective tissue made up of collagen fibers. Inside this layer is the tunica media, or media, which is made up of smooth muscle cells and elastic tissue (also called connective tissue proper). The innermost layer, which is in direct contact with the flow of blood, is the tunica intima, commonly called the intima. This layer is mainly made up of endothelial cells. The hollow internal cavity in which the blood flows is called the lumen.