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Carbon Dioxide

Along with strong ions and weak bases, ECF contains carbon dioxide. The concentration of carbon dioxide in ECF is determined by tissue production and alveolar ventilation. Carbon dioxide, when in solution, exists in four forms: carbon dioxide, denoted CO2 (d); carbonic acid (H2 CO3 ); bicarbonate ions (HCO3 - ); and carbonate ions (CO3 2- ).

The concentration of CO2 (d) is determined by the solubility coefficient of carbon dioxide (SCO2 ), which depends on body temperature, PCO2 , and other factors. Several equilibria equations can be derived from the hydration of CO2 .

[CO2 (d)] = [SCO2 ] × PCO2 .

The tendency for CO2 to hydrate to H2 CO3 and subsequently dissociate to H+ and HCO3 - is reflected in the following equation:

[CO2 (d)] × [OH ] = K1 × [HCO3 ]

These equations can be combined together and with water equilibrium to produce the following formula:

[H+ ] × [HCO3 ] = Kc × PCO2 .

Bicarbonate also dissociates to release hydrogen ions and carbonate, the equilibrium reaction for which follows:

[H+ ] × [CO3 2− ] = K3 × [HCO3 ]

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