Blood vessels

• Flow of blood


  1. arteries
  2. arterioles
  3. capillaries
  4. venule
  5. vein
• Arteries


  • carry oxygenated blood - move AWAY from the heart
  • HIGH blood pressure
  • no valves (except aorta and pulmonary artery)
• Arteries


  • thick outer collagen layer
  • elastic fibres stretch and recoil
  • thick smooth muscle that can contract and relax
  • single squamous layer (endothelium) to provide a smooth surface
• Capillaries


  • exchange between blood and vessels
  • very NARROW lumen
  • erythrocytes travel single file
  • rate of blood flow falls, allows more time for exchange
  • one cell thick (endothelium), short diffusion distance
  • LOWER PRESSURE
• Endothelium


  • smooth - minimises friction
  • thin - good exchange surface
  • gaps for filtration - leukocytes can pass
  • lines the circulatory system
  • exchange oof substances into tissue fluid
• Elastin


  • stretch and recoil
  • strong
  • flexible
  • insoluble
  • stable
• Collagen


  • inelastic
  • flexible
  • high tensile strength (strong)
• Veins


  • carry deoxygenated blood - TOWARDS THE HEART
  • LOWER blood pressure
  • SEMILUNAR valves - to prevent back flow
  • contraction of skeletal muscles squeeze veins
  • thin outer collagen layer
  • thinner smooth muscle and elastic fibre layer
  • single squamous cell layer (endothelium)
• tunica externa - collagen layer

  • provides structure and support
  • prevents over expansion of vessels under high pressure
  • supports large volumes of blood
• tunica media - elastic fibres, smooth muscle

  • enables expansion of lumen to withstand pressure during systole
  • elastic recoil maintains high pressure during diastole
  • contraction narrows lumen -> to control blood pressure and flow to tissue
• tunica intima - endothelial/squamos epithelial tissue


  • inner layers of flat, thin cells
• layers of blood vessels

  1. outer - tunica externa
  2. middle - tunica media
  3. inner - tunica intima