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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.
This strong outer layer is composed of connective tissue that allows the blood vessel to withstand forces acting on the vessel wall. It also contains strong collagen fibers that help anchor the blood vessel to surrounding tissues, and this gives the vessel some stability.