Blood Vessels

Comparative structure of blood vessels

There are five main types of blood vessels which are arteries, arterioles, capillaries, venules and veins. Arteries carry blood away from the heart to other part of body. Large and elastic arteries leave the heart and divide into small arteries called arterioles. When arterioles enter a tissue, they branch into numerous tiny vessels called capillaries. Capillaries have a thin cell wall and allow the substances exchange between the blood and body tissues. Group of capillaries within a tissue reunite to form small vein called venules. Venules in turn merge to form progressively larger blood vessels called veins. Veins are the blood vessels that convey blood from tissue back to the heart.

Arteries

Arteries were found empty at death, in ancient times they were thought to contain only air. The wall of an artery has the three layers of a typical blood vessel, but has a thick muscular-to-elastic tunica media. Due to their plentiful elastic fibers, arteries normally have hit compliance, which means that their walls stretch easilt or expand without tearing in response to a small increase in pressure

Arterioles 

Arterioles have a thin tunica interna with a thin, fenestrated internal elastic lamina that disappears at the terminal end. The tunica media consists of one to two layers of smooth muscle cells having a circular orientation in the vessel wall. The terminal end of the arteriole, the region called the metarteriole, tapers toward the capillary junction. At the metarteriole-capillary junction, the distal-most muscle cell forms the precapillary sphincter which monitors the blood flow into the capillary, the other muscle cells in the arteriole regulate the resistance to blood flow.

Capillaries

Capillaries , the smallest of blood vessels, that connect the arterial outflow to the venous return. Capillaries form an extensive network, approximately 20 billion in number, of short, branched, interconnecting vessels that course among the individual cells of the body. This network forms an enormous surface area to make contact with body's cells. The flow of blood from a matarteriole through capillaries and int o a postcapillary venule is called the microcirculation of the body. The primary function of capillaries is the exchange of substances between the blood and interstitial fluid. Because if this, these thin-walled vessels are referred to as exchange vessels.

Venules

Unlike their thick-walled arterial counteparts, venules and veins hava thin walls that do not readily maintain their shape. Venules drain the capillary blood and begin the return flow of blood back toward the heart.

Veins

While veins do show structural changes as they increase in size from small to medium to large, the structural changes are not as distinct as they are in arteries. Veins , in general, have very thin walls relative to their total diameter. They range in size from 0.5 mm in diameter for small veins to 3 cm in the large caval veins entering the heart. 

Source: http://www.youtube.com/watch?v=CjNKbL_-cwA