Structure of the blood vessel wall

The walls of arterial and venous vessels are composed of three layers:

Tunica adventitia/externa:
Outermost layer composed of loose connective tissue and largely protective in function.

Tunica media:
The middle and thickest layer composed of varying amounts of vascular smooth muscle cells, collagen and elastin. The proportions of smooth muscle and elastin confer different properties on vessels. A high proportion of smooth muscle and collagen will make the vessel wall more rigid, a high proportion of elastin will make it more distensible.

Tunica intima:
Inner layer common to all vessels including capillaries. Comprises a single layer of endothelial cells supported by a layer of sub-endothelial connective tissue.