Introduction

Under normal circumstances, the body works to maintain the pH of body fluids within a narrow range.  This is carried out through control of the partial pressure of carbon dioxide in the blood (pCO₂) and the bicarbonate concentration.  These three variables are related through the Henderson-Hasselbach equation:

                [H⁺] = 24 x ( pCO₂ / [HCO₃^-] )

[H⁺] in nmol/l; pCO₂ in mmHg; [HCO₃^-] in mmol/l

The [H⁺] can be seen to be dependent on the ratio of pCO₂ and [HCO₃^-].

pCO₂ is regulated by the lungs and [HCO₃^-] by the kidneys.

The lungs can control [H⁺] on a minute-to-minute basis through changes in ventilation and hence pCO₂, while the kidneys maintain acid-base equilibrium on a day-to-day basis through excretion or reabsorption of bicarbonate.

The following topics will be discussed:

• Measurements and normal values

• Normal regulation of acid-base balance

• Diagnosing acid-base abnormalities

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