Calculate the percentage of oxygen carrying capacity at high altitudes where po2

Question

Calculate the percentage of oxygen carrying capacity at high altitudes where po2= 65 torr. Assume that pO2 in the tissue is 20 torr, p50 in hemoglobin = 30 torr, n=2.8. Compare that result with hemoglobin oxygen-carrying capacity at normal conditions.

I understand that YO2 = 89.7% when using 65 torr in the high altitudes, i understand yO2 = 24% when using 20 torr for the tissues and normal conditions being yO2=96.7%. What I dont understand is why the solution in my book is 96.7%-24% = 72.7 % and they completely neglect the 89.7%. The other sample problem my professor has for a question similar does the same thing with no explanations.. What am i missing to better understand this?

Explanation / Answer

Normaly oxygen rapidly combines with hemoglobin as partial pressure of oxygen increases. Beyond 65torr further increase in partial pressure of oxygen does not significantly increase oxygen hemoglobin bonding.

The hemoglobin is already 90% saturated at 65 torr, An increase in the Po2 from 60 to 100 torr increases the saturation of hemoglobin by only 7% (90% to 97%). because of that 89.7% is neglected in your text book

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