Compensatory Responses: summary and take home points
1. Compensatory responses never return the ph to normal or overshoot.
2. The basis of compensatory responses is to maintain the PCO2/[HCO3-] ratio.
3. Therefore, the direction of the compensatory response is always the same as that of the initial change.
4. Compensatory response to respiratory disorders is two-fold; a fast response due to cell buffering and a significantly slower response due to renal adaptation.
5. Compensatory response to metabolic disorders involves only an alteration in alveolar ventilation.
6. Metabolic responses cannot be defined as acute or chronic in terms of respiratory compensation because the extent of compensation is the same in each case.
Below is table summarizing compensatory responses and their mechanisms.
| Primary disorder | Initial chemical change | Compensatory response | Compensatory Mechanism | Expected level of compensation |
|---|---|---|---|---|
| Metabolic Acidosis | ↓HCO3- |
↓PCO2 | Hyperventilation | PCO2 = (1.5 × [HCO3-]) + 8 ± 2 |
↓PCO2 = 1.2 ×∆ [HCO3-] |
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| PCO2 = last 2 digits of pH | ||||
| Metabolic Alkalosis | ↑HCO3- | ↑PCO2 | Hypoventilation | PCO2 = (0.9 × [HCO3-]) + 16 ± 2 |
↑PCO2 = 0.7 × ∆ [HCO3-] |
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| Respiratory Acidosis | ↑PCO2 | ↑HCO3- |
|
|
| Acute | Intracellular Buffering (hemoglobin, intracellular proteins) | ↑[HCO3-] = 1 mEq/L for every 10 mm Hg ∆PCO2 | ||
| Chronic | Generation of new HCO3- due to the increased excretion of ammonium. | ↑[HCO3-] = 3.5 mEq/L for every 10 mm Hg ∆PCO2 | ||
| Respiratory Alkalosis | ↓PCO2 | ↓HCO3- |
|
|
| Acute | Intracellular Buffering | ↓[HCO3-] = 2 mEq/L for every 10 mm Hg ∆PCO2 | ||
| Chronic | Decreased reabsorption of HCO3-, decreased excretion of ammonium | ↓[HCO3-] =4 mEq/L for every 10 mm Hg ∆PCO2 |
Also: In acute respiratory acidosis, ↓pH = 0.008 × ∆ PCO2
In chronic respiratory acidosis, ↓pH = 0.003 × ∆ PCO2.