Acid-Base Balance

Acid-base balance is assessed most completely using blood gas analysis. However, certain aspects can also be evaluated on a patient’s biochemical panel by using the bicarbonate, anion gap, and electrolyte (particularly chloride) results. Interpretation of changes on the biochemical panel can provide important information regarding metabolic acid-base status, but does not allow us to determine respiratory acid-base status.

The body maintains blood pH within a narrow range necessary for normal biochemical reactions. Decreased pH of the blood is caused by an increased concentration of hydrogen ions (H⁺). Increased pH of the blood is caused by a decreased concentration of H⁺. H⁺ concentration is regulated through buffer systems, such as bicarbonate; the respiratory system, which removes acid through expiration of CO₂ (H₂CO₃ ↔ H₂O + CO₂); and the kidneys, which reabsorb NaHCO₃ in the proximal tubule, excrete weak acids in the distal tubule, and convert CO₂ to H⁺ and HCO₃^–.

When evaluating blood gases, venous blood is adequate for pH, HCO₃^–, and pCO₂; however, arterial blood is necessary for evaluating pO₂ in cases where knowledge of pulmonary oxygenation of blood is required, such as in suspected hypoxic states due to cardiopulmonary disease. Very specific requirements must be met for sample acquisition and handling for blood gas analysis, and the reference laboratory should be consulted before sample collection.