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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.
Its innermost layer is a simple squamous epithelium, called endothelium, which is continuous with the endocardial cells were regarded as little more than a passive barrier between the blood and the remainder of the vessel wall.hree tunics of blood vessel. The three structural layers of a generalized blood vessel from innermost to outermost are the tunica intima, tunica media and tunica adventitia.