Please note that most students can answer the objectives fine without knowing the following material. However, some students are bothered by what seems to be a discrepancy based upon their knowledge of basic chemistry. For those students, I have added this explanation.

Why doesn’t the bicarbonate concentration change during respiratory alkalosis?  Also, why doesn’t the bicarbonate change after breathing into a paper bag to correct the alkalosis?  After all, if you look at the equation:

CO2 + H2O = H2CO3 = H+ + HCO3-,

If you remove CO2 from the left side of the equation, shouldn’t the HCO3- be lowered?

Also, if you breathe into a paper bag and increase the CO2, shouldn’t the HCO3- be increased as a result?  How can the teacher say bicarbonate remains the same?

Actually, you are correct, the HCO3- does change but the amount is so little that it is insignificant.   Consider the bicarbonate buffering system for a normal person:

pH = pKa + Log [HCO3-]/[H2CO3 ]

7.4 = 6.1 + Log [24mM]/[1.2mM]

Now have that person breath faster or deeper until the pH is increased to 7.7.

7.7 = 6.1 + Log [24mM]/[0.6mM]

Why did not the  [HCO3-] change?  After all, when one looks at the equilibrium equation,

CO2 + H2O = H2CO3 = H+ + HCO3-

If CO2 is lowered, then the equation should shift to the left and HCO3- should also be lowered?

Here is the answer!  If normal pH is 7.4 and increases to 7.7, the hydrogen ion concentration in the solution is halved from 40 nM to 20 nM.  If the H+ is decreased by 20 nM, then according to our equation, the concentration of HCO3- must also decrease by 20 nM.

We do the math:

24 mM – 0.000020 mM = 23.99998 mM,

the measurements we make in the laboratory cannot tell the difference between 24 mM and 23.999998 mM.  So, we ignore this insignificant change and pretend that the concentration of bicarbonate did not change.

The reverse happens when Percy breaths into a paper bag to correct his alkalosis.  Although he increases his CO2 from 0.6 mM to 1.2 mM, only 0.000020 mM of this added CO2 dissociated into H+ + HCO3-.  So the HCO3- increased by 0.000020 mM.  We can’t measure this difference, so we say it did not change.