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Clinical Correlates of pH Levels
Problem Set

Problem 6: Compensation for chronic metabolic alkalosis

Tutorial to help answer this question

For chronic metabolic alkalosis, effective compensation by the body involves:
A. expelling H+ in the kidney.
B. retaining CO2 in the lung.
C. expelling HCO3- in the kidney.
D. retention of NH4+Cl- in the kidney.
E. expelling OH- in the kidney.


Compensation for chronic metabolic alkalosis

A slowing of the respiration rate compensates for chronic metabolic alkalosis by allowing blood CO2 to rise. By mass action, this increase in CO2 triggers an increase in the H2CO3 intermediate, which, in turn, dissociates to bicarbonate and H+, thus lowering pH.

As one would expect, pH is near normal in compensated cases of metabolic alkalosis. The causal defect for metabolic alkalosis, increase in [HCO3-], remains, and pCO2 is elevated as compensation.

respiratory metabolic
acidosis alkalosis acidosis alkalosis
pH decreased decreased decreased decreased
HCO3-:CO2 ratio decreased decreased decreased decreased
[HCO3-] increased decreased decreased decreased decreased decreased
pCO2 decreased decreased decreased decreased decreased increased
total CO2 decreased decreased decreased decreased decreased decreased
increased = increased
decreased = decreased
= no major change
U = uncompensated
C = compensated
increasedincreased = Red arrows indicate the primary defect.
increasedincreased = Green arrows indicate compensation mechanisms.

As seen in the table above, metabolic alkalosis always shows a rise in blood pH which is accompanied by a corresponding rise in blood HCO3-. Since compensation may or may not occur (but usually does), the pH itself is not the "primary" cue to a metabolic alkalosis, rather the elevated HCO3- is a better indication of what is going on.

To explain why this is so, one must consider two consequences of alkalosis:

  1. In the simplest case, ingestion of alkali will instantaneously change pH by consuming H+. The same thing can be accomplished, however, by removal of acid from the body, such as might occur during prolonged vomiting.
  2. A drop in [H+] will pull the bicarbonate equation to the right, thus raising [HCO3-]:
CO2 + H2O <--> [H2CO3] <--> H+ + HCO3-

The main compensatory response to this is respiratory (indicated by a green arrow in the table), and is perhaps is a bit confusing in that the body tries to further push the reaction to the right (in the same direction it is being pulled already by the loss of H+!). However, recall that what the body is trying to do here is correct pH, so by pushing the reaction further to the right by retaining CO2 in the lungs, the [H+] is raised, hence lowering pH back toward the normal range.

Further compensation
Further compensation (not shown in the table) can occur more slowly by adjusting H+ and HCO3- (trying to normalize each of them by retaining H+ and excreting HCO3-), but this is usually very incomplete.

[Problem 6] [Answer] [Problem 7]

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Department of Biochemistry and Molecular Biophysics

The University of Arizona
January 1999
Revised: October 2004
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