The blood vessels which carry the blood away from the heart to the tissues are called arteries.
The blood in these blood vessels flow at high pressure which is created by the peristaltic movements of the muscles of the arteries.
The arteries divided into thinner arterioles which branch into extremely thin and small metarterioles, these divide to form capillaries.
The wall of an artery consists of three layers, an outermost tunica externa or adventitia of connective tissue having longitudinal elastic and collagen fibres ; a thick middle tunica media made of circular smooth muscles and elastic fibres and an inner most tunica interna or intima made of endothelial cells and an elastic membrane.
In between the tunica intima and tunica media there is a sheath of elastic tissue called internal elastic lamina, it completely surrounds the artery.
There is also a similar membrane in between tunica media and tunica external and is called external elastic lamina. The elasticity of the artery is mainly due to the presence of elastic fibres in its wall.
Veins are the blood vessels which carry blood towards the heart. These collect the blood from the capillaries in the tissues and empty into the heart.
These are thin-walled structures with little musculature. The veins like arteries are made up of the same three layers but the tunica media poorly developed and hence the veins are thin-walled.
Tunica externa or adventitia is relatively better developed in veins than the arteries. The external elastic lamina is less developed.
In larger veins the tunica externa, some connective tissue and elastic fibres form valves which are crescentic pockets arranged in groups of three so that they may prevent a back flow of blood.
The larger arteries and veins have their own small blood vessels in their coats called vasa-vasorum which supply nourishment and oxygen to these blood vessels.
Functionally capillaries are the most important part of the circulatory system. These are thin-walled and smallest of vessels which anastomose and form a very fine network of minute tubes.
These are supplied with fine nerve ending and are so minute that they can be seen only under the microscope.
Their walls are made up of a single layer of endothelial cells. The lumen of capillaries varies from 7.5µ x to 0-0075 mm in diameter.
The capillaries are formed as a result of the branching of the arterioles. The capillaries reunite to form venules and venules form veins.