General Principles

The normal ECF pH is 7.40 ± 0.03. Perturbations in pH can occur with changes in the ratio of to partial pressure of carbon dioxide (pCO₂) as described by the Henderson–Hasselbalch equation:
Maintenance of pH is essential for normal cellular function. Three general mechanisms exist to keep it within a narrow window:
- Chemical buffering is mediated by in the ECF and by protein and phosphate buffers in the ICF. The normal is 24 ± 2 mEq/L.
- Alveolar ventilation minimizes variations in the pH by altering the pCO₂. The normal pCO₂ is 40 ± 5 mm Hg.
- Renal H⁺ handling allows the kidney to adapt to changes in acid–base status via reabsorption and excretion of titratable acid (e.g., ) and .
Acidemia and alkalemia refer to processes that lower and raise pH regardless of mechanism. They can be caused by metabolic or respiratory disturbances:
- Metabolic acidosis is characterized by a decrease in the plasma due to either loss or the accumulation of acid.
- Metabolic alkalosis is characterized by an elevation in the plasma due to either H⁺ loss or gain.
- Respiratory acidosis is characterized by an elevation in pCO₂ resulting from alveolar hypoventilation.
- Respiratory alkalosis is characterized by a decrease in pCO₂ resulting from hyperventilation.

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