Compensation: Restoring pH to normal

•Any primary disturbance immediately triggers a compensatory response.

Any respiratory disorder will be compensated for by the kidneys (process takes hours to days - 1 to 3 days).

Any metabolic disorder will be compensated for by the lungs (rapid process, occurs within minutes - 1 to 3 minutes).

Respiratory acidosis (hypoventilation)

Compensatory Response: Renal retention of HCO3 raises pH toward normal

Respiratory alkalosis

Compensatory Response: Renal elimination of HCO3  lowers pH toward normal

Metabolic acidosis

Compensatory Response: Hyperventilation ↓CO2, raising pH toward normal

Metabolic alkalosis

Compensatory Response: Hypoventilation ↑CO2, lowering pH toward normal

The CO2 hydration reaction's effect on [HCO3–]

A large portion of CO2 is transported as HCO3. So any thing that affects CO2 will also affect HCO3 levels, yet this is not part of the renal compensatory response - only a result of the hydrolysis reaction. As such, slight changes in the HCO3 in the blood are an expected consequence of elevated or decreased PCO2 and should not be interpreted as compensatory.

As CO2 increases, it also increases HCO3.

In general, the effect is an acute increase of ~1 mEq/L HCO3 for every 10 mm Hg acute increase in PaCO2 .

An increase in CO2 of 30 would increase HCO3 by ~3 mEq/L . This should not be seen as compensation.