blood vessels

• Arteries 


  Carry blood away from the heart
• capillaries 


  involved in the exchange of materials
• veins 


  carry blood to the heart
• Blood is pumped through arteries by contraction of ventricles of the heart.
• The blood pumps blood at high pressure so the artery walls are strong and elastic
• Capillaries have thin walls, allowing easy diffusion between cells and capillary lumen (blood)
• Arteries have thick walls to withstand high pressure
• Veins have thinner walls than arteries as they do not need to withstand such high pressures
• Veins contain valves which prevent backflow of blood
• Arteries have thick muscle to maintain force on blood
• capillaries are tiny blood vessels that link veins and arteries
• arteries branch into capillaries
• capillaries carry blood close to every cell in the body to exchange substances with them.
• capillaries have permeable walls, so substances can diffuse in and out
• capillaries supply food and oxygen, and take away waste like CO2
• capillaries eventually join up to form veins
• Veins have a larger lumen than arteries to help blood flow despite the lower pressure
• lumen 


   the hollow opening or the space inside the blood vessel
• capillaries have thin walls (one cell thick)


  short diffusion distance allowing efficient exchange of substances