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The innermost layer of a blood vessel wall is the tunica intima (too'ni-ka in-ti'ma; intimus = inmost), or tunica interna. It is composed of an endothelium (a simple squamous epithelium lining the blood vessel lumen) and a subendothelial layer made up of a thin layer of areolar connective tissue. The tunica media (me'de-a; medius = middle) is the middle layer of the vessel wall. It is composed of circularly arranged layers of smooth muscle cells. Sympathetic innervation causes the smooth muscle to contract, resulting in vasoconstriction (va'so-kon-strik'shun), or narrowing of the blood vessel lumen.
Endothelial Cells. Endothelial cells are a lining layer of cells or tissue (endothelium), especially the interior of the blood vessels. In fact, the endothelium lines the entire circulatory system including the one and only heart and all types of blood vessels.ndothelial Cells. Endothelial cells are a lining layer of cells or tissue (endothelium), especially the interior of the blood vessels. In fact, the endothelium lines the entire circulatory system including the one and only heart and all types of blood vessels.