Blood vessels have three distinct layers of tissues, or tunics. The tunica intima is the innermost layer of all blood vessels and the only layer in capillaries. It consists of simple squamous epithelium (endothelium) with an underlying basement membrane.
The tunica media is the middle layer of smooth muscle cells in blood vessels. It is thick in arteries and very thin in veins. Arteries have an internal elastic lamina (elastic connective tissue) between the tunica intima and media. Contraction of smooth muscles cells in the tunica media narrows the blood vessel lumen (vasoconstriction). Relaxation of the tunica media expands the vessel lumen (vasodilation).
The tunica externa (adventitia) is the outer, fibrous connective tissue covering of blood vessels. When it blends with surrounding tissues in smaller vessels it called adventitia.
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