The structure of arteries, veins and capillaries.

Arteries are the blood vessels that carry blood away from the heart, they repeatedly branch off getting smaller and smaller. The small arteries are known as arterioles.
Arteries are made up of three layers. The outside layer is made up of fibrous connective tissue which connects to surrounding tissues, this acts as an anchor to help keep the artery in place as blood is pumped through at high pressure.
The middle layer is made up of elastic connective tissue and smooth muscle. This layer also has two sets of nerves, one to make the muscle contract and the other to make the muscle relax.
The inner layer consists of flat epithelial cells, which makes the inner lining of the arteries smooth, reducing friction between blood and lining.
Arteries, with the exception of the pulmonary artery, always carry oxygenated blood away from the heart to the rest of the body.

Veins are responsible for carrying deoxygenated blood back to the heart and lungs in order for it to be reoxygenated. There is less pressure from the heart so the middle layer of muscle is much thinner than in arteries. Veins in fact are collapsible tubes if not filled with blood. They also differ from arteries because they have semi-lunar valves, which prevents blood from flowing backwards.

Capillaries are microscopically small tubes that connect arteries to veins. They from capillary beds in different tissues around the body. Their walls are only one cell thick, they don't have connective outer layers or a middle layer of smooth muscle, capillaries are only made up of the endothelial layer, and are so small that blood cells can only pass through them in single file. The capillary beds are the site of nutrient transfer between blood and tissue cells. This is also where oxygen diffuses from the blood into surrounding tissue to be used in metabolic processes, and where waste from metabolic processes, such as carbon dioxide, diffuse into the blood to be carried away.


References:
www.ehow.com/about_5381728_structure-blood-vessels.htm

www.bcbuwc.ac.za/SCI_ED/grade10/manphys/vessel.htm