The study of blood vessels is called angiology (Gk. angeion-vessel, logos-study). Blood vessels are of three types - arteries, veins and capillaries.
Arteries : Arteries carry blood away from the heart to different parts of the body. They carry oxygenated blood exception is pulmonary arteries, which carry deoxygenated blood. They are thick walled muscular, elastic and without valves. They are deeply situated in the body, having narrow lumen and showing high blood pressure for efficient circulation of blood. Arteries divide and redivide into arterioles. Which further devide and redivide into capillaries. Histologically the wall of artery is made up of three layers namely, outer tunica externa, middle tunica media and inner tunica interna. Artery is characterised by presence of thick and muscular tunica media.
Capillaries : Capillaries are the thinnest of blood vessels and formed by division and redivision of arteriole. The wall of capillary is made up of squamous epithelium or endothelium. It is permeable to water and dissolved substances. Thus exchange of respiratory gases, nutrients, excretory products between blood and tissue takes place through wall of capillary. Capillaries unite to form venules.
Veins : Veins carry blood from different parts of the body to the heart. They carry deoxygenated blood, exception is pulmonary veins, which carry oxygenated blood. They are thin walled with semilunar valves to prevent backward flow of blood. They are superficially situated, having broad lumen and showing low blood pressure.
Histologically - the wall of vein is also made up of three coats as that of artery, but the layers are comparatively very thin than that of artery.