The endothelium's the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. These cells reduce turbulence of the flow of blood allowing the fluid to be pumped farther. Endothelial tissue's a specialized type of epithelium tissue (one of the four types of biological tissue in animals). More spe… (More on Endothelial)