Blood Vessels

• Arteries - Carry the blood away from the heart
• Capillaries - Exchange substances between the blood and body cells
• Veins - Carry the blood towards the heart
• Arteries carry blood under pressure
• The heart pumps the blood out at a high pressure so the artery walls are strong and elastic
• Artery walls are thick compared to the lumen
• They contain thick layers of muscle to make them strong, and elastic fibres to allow them to stretch and spring back to their original shape
• Arteries branch into capillaries
• Capillaries walls are only one cell thick
• Capillaries have permeable walls so they can exchange substances with the blood
• Capillaries walls are only one cell thick, this increases the rate of diffusion by decreasing the distance over which it occurs
• Capillaries join up to form veins
• The veins carry blood at a lower pressure so the walls arent as thick as artery walls
• Veins have a bigger lumen to help blood flow, despite the lower pressure
• Veins have valves to prevent back flow