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. So veins do not need to be as strong as arteries.
Veins have a cross-sectional structure that is very similar to arteries.
One of the obvious differences is that they have a much wider lumen and thinner walls.
The other main difference is that veins have valves inside them, while arteries do not.
In these these veins you can see that the blood can flow upwards but the valves stop it from flowing downwards.
The valves occur occasionally along their length and ensure that blood can only travel in one direction. You wouldn't want all your blood slopping around in your feet, would you?
Having valves in arteries would not be much use as it would slow the blood down and stop it reaching those important bits of you, like your head.
Capillaries are tiny, thin-walled vessels. They carry blood close to all the body's cells in its tissues and organs. They may not be the most glamorous of the vessels but they are perhaps the most important.
Capillaries are made up of a single layer of endothelial cells around a very small lumen.
Molecules can easily move into and out of the capillaries by diffusion. This allows food, gas and waste molecules to be taken to and from every cell in the body.
The table below is a summary of the types of blood vessels and their structures:
|Large lumen along which blood can flow in one direction due to presence of valves.||Vein|
|Vessel that allows easy diffusion across its walls The wall is one endothelial cell thick.||Capillary|
|Thick walls of elastic fibres and smooth muscle to resist high pressure of blood.||Artery|