Endothelium is a layer of cells that lines the interior surface of blood vessels and lymphatic vessels. A word that is commonly used in relation to this innermost layer of blood vessels is "endothelially," which describes something that pertains to the endothelium. However, there are several synonyms that can be used in place of the word endothelially. Some of these words include: vasculature, blood vessel lining, vessel endothelium, vascular endothelial layer, and blood vessel endothelium. These synonyms are all useful in describing the innermost layer of blood vessels and the role it plays in the circulatory system.