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1. Oxyhaemoglobin

  • Red blood cells contain haemoglobin (Hb)
  • Hb is a large protein with a quaternary structure - made up of more than one polypeptide chain. Each chain has a haem group which contains iron and gives haemoglobin its red colour
  • Hb has a high affinity for oxygen -  each molecule can carry four oxygen molecules. In the lungs, oxygen joins to iron in haemoglobin to form oxyhaemoglobin. This is a reversible reaction - when oxygen leaves (dissociates) oxyhaemoglobin near the body cells, it turns back into Hb

2. Haemoglobin saturation

  • Partial pressure of oxygen (pO2) is a measure of oxygen concentration
  • The partial pressure of carobon dioxide (pCO2) is a measure of the concentration of carbon dioxide in a cell
  • Hb's high affinity for oxygen varies depending on the partial pressure of oxygen
  • Oxygen loads onto haemoglobin to form oxyhaemoglobin where there is a high pCO2. Oxyhaemoglobin unloads where there is a lower pCO2
  • Oxygen enters the blood capillaries at alveoli, they have a high pCO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin
  • when cells respire, they use up oxygen and lower pCO2. Red blood cells deliver oxyhaemoglobin to repsiring tissues, it then unloads the oxygen
  • Haemoglobin then returns to the lungs to pick up more oxygen

3. Dissociation Curves

  • A dissociation curve show how satured the haemoglobin is with oxygen at any given partial pressure
  • Where pCO2 is high (eg in the lungs) haemoglobin has high affinity for oxygen (ie. it will readily combine with oxygen) so it has a


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