The Controller and Acid-Base Physiology: An Introduction to a Complex Process

Compensatory Mechanisms for Primary Acid-Base Disorders

The body must maintain the pH of the blood and cells within a narrow range. To accomplish this, the respiratory and renal systems provide compensatory mechanisms for the primary acid-base disturbances we have just discussed. If a problem occurs in either of these systems, the other system can adjust its function to offset the problem. In the simplest version of this process, a primary respiratory acidosis, for example, is offset by a secondary (compensatory) metabolic alkalosis. Conversely, a metabolic acidosis is offset by a secondary respiratory alkalosis. (Table 7-2).

With a primary respiratory acidosis, the P_aCO₂ increases and the pH decreases. As already noted, the protons in this disorder are initially buffered by intracellular proteins. This metabolic compensation results in the production of bicarbonate that diffuses back into the serum, resulting in an increase in the serum bicarbonate. If the respiratory acidosis persists, the kidneys begin to compensate by increasing the amount of hydrogen ions eliminated in the urine, a process that leads to the generation of bicarbonate that enters the blood. In the case of a primary respiratory alkalosis, the P_aCO₂ decreases, the pH increases, and the kidneys compensate by excreting more bicarbonate. In the example of a primary metabolic acidosis, a fixed acid accumulates, which causes the pH to decrease. The respiratory system compensates by increasing ventilation, and P_aCO₂ decreases. Finally, with a primary metabolic alkalosis, bicarbonate concentration increases, pH increases, and the respiratory system compensates by reducing ventilation, which results in an increase in P_aCO₂ (Fig. 7-6).