What are blood vessels?

The blood vessels are the part of the circulatory system that transport blood throughout the body. There are three not major types of blood vessels: the arteries, which carry the blood away from the heart, the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the back towards the heart.The arteries and veins have the same basic structure. There are three layers, from inside to outside while the capillaries have only one thick cell. Capillaries consist of little more than a layer of endothelium and occasional connective tissue.

References:

http://http//en.wikipedia.org/wiki/Blood_vessels
http://http//www.diabetes.org/diabetesdictionary.jsp

Glossary:

Endothelium- the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall.