Blood vessels

• • Arteries carry blood away from the heart
  • Capillaries exchange materials at tissues
  • Veins carry blood to the heart
• Arteries:

  • Strong and elastic walls - heart pumps blood at high pressure
  • Thick walls, small lumen
  • Thick layers of muscle for strength, elastic fibres to stretch
• Capillaries:

  • Small
  • Carry blood to cell walls in body to exchange substances
  • Permeable walls - substances can diffuse in and out
  • Walls are one cell thick - decreased distance, increased rate of diffusion
  • Very small lumen
• Veins:

  • Medium walls - blood is at low pressure
  • Thick lumen (thicker than arteries) to help blood flow despite low pressure
  • Valves to keep blood flowing in right direction
• rate of blood flow = volume of blood ÷ number of minutes