This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position.
In veins, the tunica externa is typically the thickest layer and may surpass the thickness of the tunica media in some larger arteries. The outer layers of this layer do not form clear boundaries but rather integrate with the surrounding connective tissue to help maintain the vessel's position relative to its environment.