What is acid-base chemical balance?

Acid-Base Chemical Balance

Acid-base balance refers to the precise regulation of hydrogen ion (H+) concentration in body fluids, maintained through the interplay of buffer systems, respiratory control of carbon dioxide, and renal regulation of bicarbonate and other electrolytes.

Fundamental Mechanisms

The body maintains acid-base homeostasis through two primary conceptual frameworks that are complementary rather than mutually exclusive 1, 2:

Traditional Physiological Approach

• The carbonic acid/bicarbonate buffer system is the primary framework, where blood pH is determined by the ratio of carbonic acid (reflected by PaCO2) to bicarbonate (HCO3-) 1, 3
• Acids function as hydrogen ion donors while bases act as hydrogen ion acceptors, with acid-base status reflecting the interaction of net hydrogen ion balance with body buffers 1, 4
• Normal arterial blood values include pH 7.35-7.45, PaCO2 35-45 mmHg, and bicarbonate 22-26 mmol/L 4

Physicochemical (Stewart) Approach

• Three independent variables mechanistically determine both hydrogen ion and bicarbonate concentrations 1, 3:

  • Strong ion difference (SID): the charge difference between fully dissociated cations (Na+, K+, Ca2+, Mg2+) and anions (Cl-, lactate) 5, 1
  • Total concentration of weak acids (primarily albumin and phosphate) 1, 6
  • Partial pressure of carbon dioxide (PaCO2) 1, 6

• The SID directly correlates with bicarbonate concentration, with a simplified calculation of (Na+ - Cl-) representing the primary determinant since sodium and chloride are the major strong ions 5

• An increase in plasma chloride relative to sodium decreases the SID and lowers both pH and bicarbonate, while decreased chloride increases SID and raises bicarbonate 5, 7

Respiratory Component

• Carbon dioxide elimination through the lungs provides rapid adjustment of acid-base status, with alveolar ventilation controlling PaCO2 levels 8
• Respiratory acidosis occurs when CO2 retention develops (PaCO2 >45 mmHg), while respiratory alkalosis results from excessive CO2 elimination (PaCO2 <35 mmHg) 4
• Permissive hypercapnia with pH maintained above 7.2 is well tolerated and reduces mortality in conditions like ARDS, avoiding ventilator-induced lung injury from excessive pressures 8

Metabolic Component

• Metabolic acidosis is identified by pH <7.36, PaCO2 <35 mmHg (compensatory), and bicarbonate <18 mmol/L, classified by anion gap 4
• High anion gap acidosis results from acid generation (lactate, ketoacids, uremic acids), while normal anion gap acidosis reflects bicarbonate loss 4
• Metabolic alkalosis shows pH >7.44, PaCO2 >45 mmHg (compensatory), and bicarbonate >32 mmol/L, requiring both generation and maintenance factors since kidneys normally excrete excess bicarbonate readily 4

Renal Regulation

• The kidneys provide slower but more complete compensation through bicarbonate reabsorption, acid excretion, and ammonia production 8, 4
• Ammonia exists in equilibrium as NH3 + H2O ⇄ NH4+ + OH-, aiding acid-base homeostasis through the urea cycle and glutamine metabolism 8
• Renal tubular reabsorption conserves 60-70% of filtered chloride, with chloride balance occurring independently from sodium, mainly in equilibrium with bicarbonate status 7

Clinical Assessment Considerations

• Arterial blood gas analysis alone may be misleading for assessing tissue-level acid-base status, as arterial and mixed venous samples may not reflect myocardial or cerebral intracellular conditions 8
• During cardiopulmonary arrest with effective basic life support, severe acidosis does not develop rapidly in previously healthy individuals, making routine bicarbonate administration unnecessary 8
• Sodium bicarbonate use should be limited to severe acidosis (arterial pH <7.1, base deficit <10) or specific circumstances like hyperkalemia or tricyclic overdose, as it produces adverse effects including alkalemia, hyperosmolarity, and CO2 generation 8

Common Clinical Pitfalls

• Do not assume chloride changes parallel sodium changes—chloride varies independently, particularly in acid-base disorders, affecting the strong ion difference 5, 7
• Avoid excessive 0.9% saline administration, which causes hyperchloremic metabolic acidosis by decreasing the SID and may worsen outcomes in trauma and critical illness 7
• Balanced crystalloid solutions with physiological chloride concentrations are preferred over 0.9% saline to prevent hyperchloremic complications 7
• Recognize that acid-base and electrolyte balance are interconnected at both cellular and clinical levels, requiring integrated assessment 2