The oxygen dissociation curve

Initially, as altitude increases, (moving from top right to the left and downwards on the graph) and the partial pressure of oxygen decreases, the amount of oxygen carried by the blood remains high.  However, when you reach a cabin altitude of about 10,000ft, the amount of oxygen carried begins to fall off steeply and this condition is called hypoxia.  At this point many passengers will begin to get into trouble.  Passengers with heart or lung disease may become ill at much lower altitudes.  The most vulnerable are those whose lungs are in poor condition and are already failing to transfer oxygen to the blood.  Those who have heart problems may also be unable to transfer oxygen efficiently or may develop symptoms similar to a heart attack when they become short of oxygen.

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oxygen saturation curve