Arteries are blood vessels that carry blood away from the heart, they vary in size depending on where they are in the body and how far away from the heart they are, the structure of the artery is three layers of tissue
Tunica Adventria - outer fiberous layer
Tunica Media - middle layer of smooth muscle and tissue
Tunica Intima - inner lining of endothelium
In the large arteries you will find tunica media consists of more elastic tissue and less smooth muscle, to allow them to stretch more each time the heart pumps blood into them, they then return to their original shape, they push the flow of blood when the ventricles are relaxed and the heart is refilling, the smaller the arteries get the more smooth muscle they contain they cannot stretch as much as the larger arteries, the smaller arteries are known as arterioles.
Veins
Veins are blood vessels that carry deoxygenated blood back to the heart, the walls are thinner than the arteries, but they still have the same three layers, they have less elastic tissue and muscle, some veins have valves (these can be found in the legs) to ensure the flow of blood travels to the heart and not backwards, the smaller veins that are furthest away from the heart are called venules, the blood pressure is lower in the veins compared to that in the arteries so no pulse can be felt.
Capillaries
Capillaries are the smallest of the blood vessels in the body, the structure consists of one layer of endotheliac cells, water and other small molecules can push through this wall, capillaries act as a link between the arteries and veins, they have no valves or pulse.
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