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
−
]
