The tunica intima or interna is formed by epithelial and connective tissue layers. In particular the simple squamous epithelium forms a particularly thin layer called endothelium: this may release local chemicals, the endothelins, capable of constricting the vessel to increase blood pressure.Under the endothelium is the basal membrane connecting the epithelial component to the connective one: it provides strength keeping always a certain level of flexibility. In the larger vessels it is possible to find out an additional layer formed by elastic fibers above all, the internal elastic membrane. It is typical of the large arteries where it is possible to define an internal and an external elastic membrane, on the contrary veins do not usually present it.