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Heart and circulation

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The blood vessels

The blood pumped out of the heart is carried around the body in a 75,000 mile long transport network of blood vessels.

There are three main types of blood vessels - the arteries, the veins and the capillaries:


Arteries Veins Capillaries
  • carry blood away from the heart
  • in most arteries blood is bright red and oxygenated
  • stretch as the blood is forced through them and go back into shape afterwards (have a pulse)
  • have thick walls to withstand the pressure of the blood
  • carry blood towards the heart
  • in most veins the blood is deep purple red and deoxygenated
  • no pulse
  • often have valves to prevent backflow of blood
  • much thinner walls as much lower pressure
  • huge network of tiny vessels linking arteries with veins
  • narrow - often one red blood cell wide
  • very thin walls one cell thick to allow diffusion of substances into and out of the blood


The structure of an artery, a vein and a capillary. As a diagram, the colours might not be accurate, and the scale is relative.


Finding an artery

In some places in the body the arteries are relatively near the surface.

In these places you can feel the bulge as blood is forced out of the heart into the arteries followed by the return to normal shape.

This is known as the pulse.

Pulse Points (1)


Veins and valves

Valve System

The blood travelling in the veins back to the heart is not under any great pressure. It is often squeezed in the right direction by the action of your muscles. A system of valves stops the blood flowing in the wrong direction. You can see how well the valves in your veins work if you experiment with the veins in your arm or hands.

Capillaries - linking arteries and veins

It is in the capillaries that the exchange of substances takes place between the blood and the cells. The capillaries provide a massive surface area and thin walls for easy diffusion. Dissolved food and oxygen move out of the blood into the cells down a concentration gradient by diffusion. Carbon dioxide and urea move out of the cells into the blood in the same way.

Capilalries Network