This layer needs its own blood supply because it is quite thick.
The blood vessels that supply the tunica adventitia are called
vasa vasorum (vessels of the vessels).
This consists of concentric layers of smooth muscle fibres
and elastin. Some small blood vessels lack muscle fibres and elastin.
This is the innermost coat. In blood vessels the simple squamous
lining cells (epithelium) is called the endothelium.
The tunica intima consists of the endothelium
and underlying basement membrane. A small amount
of subendothelial connective tissue and and internal elastic layer
(lamina), is sometimes present in some blood vessels.