Tunica externa (vessels)
The tunica externa, also known as the tunica adventitia (or adventitia), is the outermost layer of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and is supported by external elastic lamina. The collagen serves to anchor the blood vessel to nearby organs, giving it stability.
Pathology
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 they rupture, leading to a wide multitude of problems.
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This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.