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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.
Similar in structure to veins, lymph vessel walls consist of the tunica intima, tunica media, and tunica adventitia. 1  Tunica Intima - lymph vessel inner layer composed of smooth endothelium (type of epithelial tissue). 2  Tunica Media - lymph vessel middle layer composed of smooth muscle and elastic fibers.