Arteries, Veins and Capillaries

• Arteries carry very high pressure blood from the heart to the organs in the body.
• The first adaptation is that arteries have very thick muscular walls and this allows them to withstand the very high pressure of the blood.
• Blood travels through the arteries in surges every time the heart beats.
• Elastic fibres stretch when the surge of blood passes through and then recoil in between surges, which keeps the blood moving.
• Blood travels to the organs in arteries, but once it’s in the organs the blood flows through capillaries.
• When the blood passes through capillaries, substances such as glucose and oxygen diffuse from the blood to the cells.
• Carbon dioxide diffuses from the cells back to the blood.
• This allows substances to diffuse rapidly between the blood and the body cells.
• Capillaries have very thin walls, so the diffusion path is very short.
• There is a problem here which is that blood is now travelling slowly and at low pressure. That means that it could stop or even go backwards.
• Once the blood has passed through the organs, it now makes its way back to the heart in veins.
• Veins have a thin wall and this is because 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 in the correct direction, then the valves open to allow the blood to flow through.
• However when the blood starts to flow backwards, the valves shut.