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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.
Introduction to Endothelial Cells. Endothelial cells (Fig.1) line the inside of every blood vessel in the body. They form a one-cell-thick layer called the endothelium, which is also found on the inner walls of the heart chambers and lymphatic vessels (which carry excess blood plasma around the body).The endothelium is very extensive.n the larger vessels such as the veins and arteries, the endothelium forms the blood vessel wall along with much thicker layers of muscle cells and elastic fibers (Fig. 2). In the capillaries, however, the endothelium makes up the entirety of the blood vessel wall (Fig.