blood vessels

• Blood vessels


  • Arteries
  • Veins
  • Capillaries
• Blood transport


  1. Pumped from heart in arteries

  2. Returned to heart in veins

  3. Capillaries connect arteries and veins

  4. Molecules exchanged between blood and cells across capillary walls
• Capillaries


  Connect the smallest branches of arteries and veins
• Capillaries


  • Walls are just one cell thick
  • Allow the exchange of molecules between the blood and the body's cells
  • Molecules can diffuse across their walls
• Exchange of molecules is not possible across the walls of other types of blood vessel
• Exchange of molecules


  1. Oxygen diffuses through the capillary wall, into the tissue fluid, and the cells

  2. Carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma

  3. Glucose diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells

  4. The waste product urea diffuses from the cells of the liver, to the tissue fluid, and then across the capillary walls into the blood plasma
• Arteries


  • Always carry blood away from the heart
  • Carry oxygenated blood, except for the pulmonary artery
  • Carry blood under high pressure
  • Have thick muscular and elastic walls to pump and accommodate blood
  • A type of supporting tissue called connective tissue provides strength
  • The channel in the blood vessel that carries blood - the lumen - is narrow
• Veins


  • Always carry blood to the heart
  • Always carry deoxygenated blood except for the pulmonary vein
  • Carry blood under low or negative pressure
  • Have thin walls - have less muscular tissue than arteries
  • Have less connective tissue than the arteries
  • Have a wide lumen