CIE AS Biology (9700) exams from 2022

Revision Notes

8.2.4 The Oxygen Dissociation Curve

The Oxygen Dissociation Curve

  • The oxygen dissociation curve describes the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin
  • It’s often easier to understand the curve by starting in the top right-hand corner of the graph. This area shows what happens in the lungs
  • The partial pressure of oxygen is high and so haemoglobin picks up oxygen rapidly, forming oxyhaemoglobin
  • The bottom left-hand portion of the graph shows what happens next
  • As oxygen reaches where it’s needed in respiring tissues, the partial pressure of oxygen in the tissues is comparatively low
  • As a result, oxygen diffuses out into the body cells down a concentration gradient to the tissues

Explaining the Oxygen Dissociation Curve

  • A small change in the partial pressure of oxygen can have a very large impact on the percentage saturation of haemoglobin
  • This is because haemoglobin has such a high affinity for oxygen
  • The partial pressure of oxygen in the lungs is high, so haemoglobin picks up oxygen rapidly
  • In respiring tissues, the partial pressure of oxygen is low, so oxygen is dropped off rapidly
  • This ensures that oxygen is picked up where there’s plenty of it and delivered to where it is needed in respiring tissues

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