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).
TABLE 7-2 Primary Acid-Base Disorders and Compensatory Mechanism
| PRIMARY DISORDER | PRIMARY DISTURBANCE | COMPENSATORY MECHANISM | COMPENSATORY CHANGE |
|---|---|---|---|
| Respiratory acidosis | PaCO2 ↑ | Metabolic alkalosis | Serum bicarbonate ↑ |
| Respiratory alkalosis | PaCO2 ↓ | Metabolic acidosis | Serum bicarbonate ↓ |
| Metabolic acidosis | Serum bicarbonate ↓ | Respiratory alkalosis | PaCO2 ↓ |
| Metabolic alkalosis | Serum bicarbonate ↑ | Respiratory acidosis | PaCO2 ↑ |
With a primary respiratory acidosis, the PaCO2 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 PaCO2 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 PaCO2 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 PaCO2 (Fig. 7-6).
Use Animated Figure 7-6 to select a primary acid-base disturbance and observe the effect on the pH caused by the change in PCO2 or bicarbonate. Then choose the secondary (compensatory) process and note how it serves to move the pH back toward normal.
Animated Figure 7-6 (Work in progress)
The goal of these compensatory mechanisms is to restore the pH of the blood to normal or near-normal levels. However, the compensation never results in a pH that is all the way back to 7.40, with the exception of the metabolic compensation for a chronic respiratory alkalosis. If the pH is fully restored to 7.40 or beyond, you should look for the presence of two, simultaneous, primary acid-base disturbances (more on how to recognize this in the next section).
Respiratory compensation for metabolic acid-base disorders can occur within seconds to minutes. The sensitivity of the chemoreceptors and the timely response of the controller to changes in their output ensure this. Metabolic compensation for respiratory disorders, however, generally requires 2 to 5 days to be fully evident. The kidneys’ ability to adjust levels of hydrogen ion and bicarbonate is not as rapid as the respiratory response.