Which of these data points is the most reliable?
point 1
point 2
point 3
point 4

There’s no picture how do I know which one?

Let's see what makes the amount of Bi-carbonate ion low in X!

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The question is a bit tricky so read the below points carefully.

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This is what happens inside the Red blood cells:-

At First Hydrogen ion reacts with bicarbonate ion thus forming carbonic acid .

Reaction :

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This bicarbonate in presence of Carbonic anhydrase, converts into carbon-di-oxide and water just before crossing the pulmonary capillary wall! The same happens with the carbaminoharmohlobin!

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Thus the PCO2 is high though the amount of Bi-carbonate ion is low in X!

And just the opposite happens when blood is travelling from alveoli to tissue