What is the role of the HCO3-/H2 CO3 buffering system in the pathogenesis of metabolic acidosis?

Updated: Oct 10, 2018
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

The major extracellular buffering system is HCO3-/H2 CO3; its function is illustrated by the following reactions:

H2 O + CO2 ↔H2 CO3 ↔H+ + HCO3-

One of the major factors that makes this system very effective is the ability to control PaCO2 by changes in ventilation. As can be noted from this reaction, increased carbon dioxide (CO2) concentration drives the reaction to the right, whereas a decrease in CO2 concentration drives it to the left. Put simply, adding an acid load to the body fluids results in consumption of HCO3- by the added H+, and the formation of carbonic acid; the carbonic acid, in turn, forms water and CO2. CO2 concentration is maintained within a narrow range via the respiratory drive, which eliminates accumulating CO2. The kidneys regenerate the HCO3- consumed during this reaction.

This reaction continues to move to the left as long as CO2 is constantly eliminated or until HCO3- is significantly depleted, making less HCO3- available to bind H+. That HCO3- and PaCO2 can be managed independently (kidneys and lungs, respectively) makes this a very effective buffering system. At equilibrium, the relationship between the 3 reactants in the reaction is expressed by the Henderson-Hasselbalch equation, which relates the concentration of dissolved CO2 (ie, H2 CO3) to the partial pressure of CO2 (0.03 x PaCO2) in the following way:

pH = 6.10 + log ([HCO3-]/0.03 x PaCO2)

Alternatively, [H+] = 24 x PaCO2/[HCO3-]

Note that changes in pH or [H+] are a result of relative changes in the ratio of PaCO2 to [HCO3-] rather than to absolute change in either one. In other words, if both PaCO2 and [HCO3-] change in the same direction, the ratio stays the same and the pH or [H+] remains relatively stable. To diminish the alteration in pH that occurs when either HCO3- or PaCO2 changes, the body, within certain limits, changes the other variable in the same direction.


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