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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 are epithelial cells that form a thin layer of simple squamous epithelium known as endothelium. Endothelium makes up the inner layer of vessels such as arteries, veins, and lymphatic vessels. In the smallest blood vessels, capillaries and sinusoids, endothelium comprises the majority of the vessel.