Acid–base homeostasis

Acid–base homeostasis is the regulation of body-fluid pH within narrow limits through chemical buffers, respiratory control of carbon dioxide, and renal handling of acids and bases.

Bicarbonate buffer system: A reversible carbon dioxide–bicarbonate reaction that buffers extracellular fluid by binding or releasing hydrogen ions. It is the main extracellular buffer and links pH directly to carbon dioxide levels.
pH: A logarithmic measure of hydrogen-ion activity in a solution. Homeostasis means keeping this measure within a narrow physiological range.
Arterial blood gas: A test measuring arterial pH, carbon dioxide tension, and oxygen tension in blood. It provides the central measurements used to assess respiratory and metabolic pH disturbances.
Stewart approach: A quantitative acid–base framework that treats carbon dioxide, strong ion difference, and total weak acids as independent determinants of pH. It contrasts with explanations that treat bicarbonate as the primary independent regulator.
Respiratory compensation: The adjustment of ventilation that changes blood carbon dioxide and thereby alters pH. Breathing changes carbon dioxide within minutes, providing rapid physiological control of pH.
Henderson–Hasselbalch equation: An equation relating solution pH to the ratio of a weak base concentration to its conjugate acid concentration. It expresses how bicarbonate and dissolved carbon dioxide jointly determine blood pH.
Metabolic acidosis: A primary decrease in bicarbonate concentration that lowers blood pH unless compensated. It is a common disturbance in which bicarbonate loss or acid accumulation overwhelms regulation.
Base excess: A calculated estimate of the metabolic component of an acid–base disturbance, independent of respiratory carbon dioxide changes. It separates metabolic effects from the respiratory component emphasized by pH and carbon dioxide.
Renal acid excretion: The kidney removal of nonvolatile acid through hydrogen-ion secretion and urinary buffering. The kidneys eliminate fixed acids that ventilation cannot remove.
Buffer solution: A solution that resists pH change when small amounts of acid or base are added. Body-fluid buffers blunt rapid pH shifts before lungs and kidneys can respond.

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