The heart pumps the blood along a series of tubes that are collectively called blood vessels. But they are more than just simple pipes
Arteries carry blood away from the heart.
Each time the heart beats it fires blood into the arteries at a high pressure, so they need to be tough so that they don't burst. They are also quite thick with only a small space, known as the lumen, down the centre.
Fortunately arteries are provided with a tough outer layer and another layer inside this that can cope with the stretching the pulses of blood. This elastic layer is made up of elastic fibres and smooth muscle which contracts and helps to keep the blood moving along.
The third layer that lines the lumen of the artery is called the endothelium. This is made up of special endothelial lining cells which give a smooth surface to the lumen.
Veins carry blood back to the heart.
The blood returning from the body is at a much lower pressure than that fired out by the beating heart…