== Introduction ==

Blood vessels are organs composed of different tissues (endothelium, connective tissue and smooth muscle) which are organized in a specific manner that is distinctive for a particular sized vessel. Although the arrangement of the various components is such that there are distinctive differences between arteries and veins and between different sized vessels in these categories, blood vessels as a whole have a basically similar structure. It is the amounts of connective tissue and muscle that vary according to the size and function of particular vessels.