Blood

• red blood cells

  biconcave discs - increased surface area to volume ratio

  packed with haemoglobin - red pigment binds to oxygen

  no nucleus - more space for haemoglobin
• white blood cells

  much bigger than RBC's

  part of the body's defence system

  antibodies + antitoxins ( lymphocytes )

  engulf and digest ( phagocytes )
• platelets

  small fragments of cells

  no nucleus

  clot the blood at site of wound

  dries and hardens to form a scab

  stops bacteria entering body
• plasma


  yellow liquid

  transports RBCs and other substances around body :

  waste carbon dioxide

  urea

  small soluble products of digestion