The different sizes and types of blood vessels are involved in blood circulation. They range from thick-walled, wide vessels of 1 cm in diameter to fine capillaries with a diameter of a few microns. The three types of blood vessels are arteries, veins and capillaries; they are all connected to form one continuous closed system. The arteries have three main coats; the inner tunica intima, the middle tunica media, and the outer, tunica adventitia. The veins too have these three coats, but the middle and outer coats are thin, making the walls of the veins thinner than those of corresponding arteries. Therefore, veins can accommodate more blood than the arteries. Arteries branch out into arterioles and finally into capillaries. The latter are very thin blood vessels with just one layer of squamous cells.
Capillaries ultimately join to form venules and veins, which return blood to the heart. Thus, arteries take blood from the heart and supply it to various tissues via the capillaries and veins return blood from the tissues to the heart. Such a unidirectional flow is maintained use to the pressure exerted by heart, and the walls of blood vessel. Large veins have valves in their lumen to prevent backward flow of blood; arterial blood is rich in oxygen and is solved food, whereas venous blood carries carbon dioxide and waste materials. Pulmonary artery and pulmonary vein from exceptions to it. Pulmonary artery supplies CO2 rich blood to the lungs and pulmonary vein collects oxygen-rich blood from lungs and passes it to the heart.