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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 is the underlying layer of areolar connective tissue. It contains blood vessels, nerve endings, lymphatic vessels, and cells of the immune system. It provides mechanical and metabolic support for the epithelium and protection as part of the immune system.he outer boundary of the mucosa is often marked by a band of smooth muscle arranged in two thin concentric layers, an inner circular layer and outer longitudinal layer. Contraction of the muscularis mucosae agitates the epithelium and alters its shape.