blood vessels - B2

• arteries have thick, muscular ,elastic walls that can contract and relax rhythmically in time with the heart and can withstand high pressure
• Arteries have a narrow lumen which keeps blood at high pressure
• veins have thinner walls which have some elasticity and muscle
• Veins have a wide lumen so blood is transported at low pressure
• Veins have valves which prevent back flow of blood and to insure blood flows in the right direction
• Veins carry blood towards the heart
• Arteries carry blood away from the heart
• Capillaries walls are one cell thick which creates a short diffusion pathway
• Capillary walls are only one cell thick
• Capillary walls are only one cell thick to allow diffusion across them