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blood vessels (veins (very low blood pressure compared to arteries (medium…

- - - - superior vena cava
        
        blood from head and upper body
      - inferior vena cava
        
        blood from lower parts of body
  - - - flaps or infoldings of inner lining of vein
      - blood flows in direction of heart - valves open
      - blood flows backwards - valves close
    - - e.g. arms and legs
      - muscles contract - squeeze veins and force blood towards heart
      - valves prevent back flow when muscles relax
    - - pressure changes and squeezing actions move blood in veins of chest and abdomen towards heart
- - - - maintains limits of stretch of arteries
    - - enable them to withstand force of blood pumped out of heart and stretch to take larger blood volume
      - elastic fibres recoil between heart contractions and return to original length
        
        evens out surges of blood pumped from heart to give continuous flow
  - - - VASOCONSTRICTION - smooth muscle contracts - constricts vessel and prevents blood flowing into capillary bed
      - VASODILATION - smooth muscle relaxes - blood flows through into capillary bed
- - - - substances pass out of capillaries into fluid surrounding cells
  - - - thin layer for diffusion
    - - rate of blood flow falls
        
        slow movement of blood through capillaries gives more time for exchange of materials by diffusion between blood and cells
- - - - provide vessel walls with flexibility
  - - - changes size of lumen