Arteries, Veins and Capillaries

• Arteries carry very high pressure blood from the heart to the organs in the body
• Arteries have very thick muscular walls which allow 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.
• 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
• Capillaries have very thin walls so the diffusion path is very short which allows substances to rapidly diffuse between the blood and the body cells
• Once the blood has passed through the organs, it makes its way back to the heart in veins. The blood now travels slowly and at low pressure, meaning that it could stop or even go backwards.
• Veins have a thin wall because the blood pressure is low and they also contain valves to prevent blood flowing backwards. The valves open when blood is flowing in the correct direction.