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Oxygen forms 21% of the atmosphere at all levels and so the partial pressure of oxygen at 700mb cabin pressure is 21% of 700mb or 147mb. 

The partial pressure of oxygen is important because it determines the amount of oxygen taken up by the blood as it passes through the lungs.  The oxygen carried is related to the partial pressure by an important curve called the oxygen dissociation curve.  The shape of this curve makes is possible for us to survive at moderate altitudes and also for us to go ski-ing. 

The next screen shows the oxygen dissociation curve and explains why it is necessary to pressurise our aircraft to about 8000ft.  The numbers on the X axis scale for partial pressure are different to those already mentioned because they are in mmHg rather than mb and also because they refer to the partial pressure of oxygen in the alveolus, deep in the lung, where it is affected by the presence of water vapour and carbon dioxide. 

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