The tunica externa (New Latin "outer coat"), also known as the tunica adventitia (or just adventitia), is the outermost tunica (layer) of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and, in arteries, is supported by external elastic lamina. The collagen serves to anchor the blood vessel to nearby organs, giving it stability.
The tunicae of blood vessels are 3 layers: an inner, middle, and outer layer that are called, respectively, the tunica intima, the tunica media, and the tunica externa (or tunica adventitia).
A common pathological disorder concerning the tunica externa is scurvy, also known as vitamin C deficiency. Scurvy occurs because vitamin C is essential for the synthesis of collagen, and without it, the faulty collagen cannot maintain the vein walls and rupture, leading to a multitude of problems.