Fluid, Electrolyte, and Acid-Base Balance

Acid-base homeostasis is maintained by three interconnected systems: chemical buffers (bicarbonate-carbonic acid, phosphate, protein), the respiratory system (adjusting PaCO2 via alveolar ventilation within minutes), and the renal system (excreting or reabsorbing hydrogen ions and regenerating bicarbonate over hours to days). The Henderson-Hasselbalch equation (pH = 6.1 + log[HCO3-] / [0.03 x PaCO2]) defines the relationship between the metabolic (HCO3-) and respiratory (PaCO2) components. Metabolic acidosis (low pH, low HCO3-) triggers compensatory hyperventilation (Kussmaul respirations) to lower PaCO2; respiratory acidosis (low pH, high PaCO2) triggers renal bicarbonate retention. The nurse systematically interprets ABGs by assessing pH direction, identifying the primary disorder, evaluating compensation adequacy, and calculating the anion gap (Na+ - [Cl- + HCO3-], normal 8-12 mEq/L) to narrow the differential diagnosis in metabolic acidosis -- elevated anion gap indicates an accumulation of unmeasured acids such as lactate, ketoacids, uremic toxins, or ingested toxins (MUDPILES mnemonic).