8.2.2

Oxygen Dissociation

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Dissociation Curves

The relationship between the percentage saturation of haemoglobin and oxygen partial pressure of the surrounding tissues can be shown in a dissociation curve.

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Low pO2

  • When partial pressure is low, haemoglobin has a low affinity for oxygen.
  • The percentage saturation of haemoglobin is low because oxygen dissociates from the haemoglobin.
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Increasing pO2

  • As pO2 increases, affinity of haemoglobin for oxygen increases slightly.
  • When the first molecule of O2 binds to haemoglobin, the protein undergoes a conformational change.
  • The change in shape allows the other O2 molecules to bind to haemoglobin more easily.
  • The percentage saturation of haemoglobin increases quickly.
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Plateau in percentage saturation

  • As more molecules of O2 bind to haemoglobin, it becomes more difficult for more O2 molecules to bind.
  • The percentage saturation of haemoglobin begins to plateau.
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S-shaped curve

  • The increasing affinity of haemoglobin with increasing pO2 in this way creates an S-shaped curve.
  • The S-shaped curve is called the dissociation curve.
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Higher altitudes

  • The partial pressure of oxygen is lower at higher altitudes. This makes it difficult to saturate haemaglobin with oxygen. It can be difficult to deliver enough oxygen to tissues.
  • To compensate for this, people who live at higher altitudes have a higher red blood cell count. This increases the amount of haemoglobin in blood.
    • The hormone erythropoietin stimulates red blood cell production at low partial pressures of oxygen. (You do not need to remember the name of this hormone).
  • This process is called acclimation.

The Bohr Effect

The partial pressure of carbon dioxide (pCO2) also influences the affinity of haemoglobin for oxygen. This is called the Bohr effect.

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High pCO2

  • Respiring cells use oxygen in respiration and produce carbon dioxide.
  • The respiring cells have low pO2 and high pCO2.
  • When pCO2 is high, the rate of oxygen dissociation increases.
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Bohr effect

  • The increased dissociation of oxygen causes a shift in the oxyhaemoglobin dissociation curve to the right.
  • The shift in the dissociation curve means that oxygen will dissociate from haemoglobin at a lower pO2 than normal.
  • This is called the Bohr effect.

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