pH and CO2 concentration

[nashaiy]

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Hi all,

This is probably really easy for some people but I'm having trouble figuring it out. I know the CO2 combines with H2O to form carbonic acid which dissociates into hydrogen ions and bicarbonate ions. The thing I don't understand is why an increase in the partial pressure of CO2 creates an increase in the acidity of the blood. If both bicarbonate ions AND hydrogen ions are released, shouldn't the pH remain the same? For example, during acidosis, the body compensates by increasing breathing rate to expel CO2. By doing so, wouldn't it be getting rid of both bicarbonate and hydrogen, keeping the pH the same? I am very confused about this, could someone please help?

 

[Ineedhopenow]

There isn't complete dissociation. Carbonic acid is a weak acid.

 

[BB8730]

By doing so, wouldn't it be getting rid of both bicarbonate and hydrogen...

Yes

...keeping the pH the same?

No, they are not equal in terms of acid/base strength so they don't cancel each other out. The hydrogen ion is much more acidic than the bicarbonate ion is basic, so the key player is the hydrogen ion concentration. If that goes up, even if the bicarbonate concentration goes up in an equal amount, the pH will go down.

When looking at the equilibrium equation:
CO2 + H2O

H2CO3

HCO3− + H+

The H+ concentration will tell you where the pH is going. If CO2 concentration decreases, the equilibrium shifts left, H+ concentration falls, pH goes up. Vice versa.

 

[Temperature101]

Yes

No, they are not equal in terms of acid/base strength so they don't cancel each other out. The hydrogen ion is much more acidic than the bicarbonate ion is basic, so the key player is the hydrogen ion concentration. If that goes up, even if the bicarbonate concentration goes up in an equal amount, the pH will go down.

When looking at the equilibrium equation:
CO2 + H2O

H2CO3

HCO3− + H+

The H+ concentration will tell you where the pH is going. If CO2 concentration decreases, the equilibrium shifts left, H+ concentration falls, pH goes up. Vice versa.

Yes...Perfect explanation...

 

[nashaiy]

Yes

No, they are not equal in terms of acid/base strength so they don't cancel each other out. The hydrogen ion is much more acidic than the bicarbonate ion is basic, so the key player is the hydrogen ion concentration. If that goes up, even if the bicarbonate concentration goes up in an equal amount, the pH will go down.

When looking at the equilibrium equation:
CO2 + H2O

H2CO3

HCO3− + H+

The H+ concentration will tell you where the pH is going. If CO2 concentration decreases, the equilibrium shifts left, H+ concentration falls, pH goes up. Vice versa.

Thank you! That really is a great explanation! Just an added question, does all of this happen in the RBCs and tissue? Or does it happen in the plasma as well?

 

[BB8730]

Thank you! That really is a great explanation! Just an added question, does all of this happen in the RBCs and tissue? Or does it happen in the plasma as well?

It happens extracellularly - so in the blood plasma and interstitial fluid of tissues.