Biochemistry at The Biology Project

Clinical Correlates of pH Levels
Problem Set

Problem 4: Additional rapid response to acidosis

Tutorial to help answer this question

Continuing with the previous question, an additional rapid response to metabolic acidosis kicks in at or below pH 7.14, which is:
A. retention of OH- in the kidney.
B. retaining HCO3- in the kidney.
C. excreting H+ in the kidney.
D. direct buffering action of the H2CO3 central intermediate.
E. retaining CO2 in the lung.


Buffering action of H2CO3

Mild acidosis causes a mass action shift of the bicarbonate equilibrium towards the production of CO2, which is blown off in the lungs to raise blood pH. Should acidosis continue in spite of this, another mechanism that kicks in below pH 7.14 is the buffering action of H2CO3.

Bicarbonate Buffering System
CO2 + H2O <--> [H2CO3] <--> H+ + HCO3-
from metabolism (volatile) always available unstable intermediate the pH term major species in blood

Even though H2CO3 is an unstable intermediate (see equation above), it can still serve as the conduit for the conversion of HCO3- and excess H+ to CO2 and water, a process which becomes accelerated as blood pH gets closer to its pKa of 6.14.

The further pH falls toward the pKa of 6.14, the more powerfully these pH changes are resisted by the buffering capacity of the bicarbonate system. Thus, the pKa of carbonic acid can itself impose staunch resistance to a lowered value of blood pH below 7.

The problem is that, unlike mild acidosis, the consumption of HCO3- in acute metabolic acidosis can outstrip the body's metabolic replenishment of this vital blood buffer, such that, if the acidotic incident is prolonged, the buffering capacity of the blood can become seriously depleted. Thus, with severe metabolic acidosis (e.g. ingestion of large amounts of acid) infusion of bicarbonate may be necessary.

[Problem 4] [Answer] [Problem 5]

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January 1999
Revised: October 2004
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