Arteries, Veins and capillaries

• Arteries carry very high pressure blood from the heart to the organs in the bady.
• Arteries have very thick muscular walls. This allows then to withstand the very high pressure of the blood.

  
• Blood travels through the arteries in surges ever time the heart beats.
• Elastic fibres in arteries stretch when the surge blood passes through and then recoil in between surges, which keeps the blood moving.
• Capillaries - when the blood passes through capillaries, substances such as glucose and oxygen diffuse from the blood cells.

  
• In capillaries, carbon dioxide diffuses from the cells back to the blood.
• Capillaries have very thin walls, so the diffusion path is very short.
• Thin walls in capillaries allow substances to diffuse rapidly between the blood and the body cells.
• Once the blood has passed through the organs, it now makes its way back to the heart in veins.
• After the organs, the blood is not travelling slowly and at low pressure. That means that is could top or even go backwards.
• Veins have thin walls, The blood pressure is low so the wall does not need to be thick.

  
• Many veins contain valves. The job of valves is to stop blood flowing backwards.

  
• When the blood is flowing n the correct direction, then the valves open to allow the blood to flow through.
• When the blood starts to flow backwards in veins, the valves shut