With homeostasis the pH of a biological solution is maintained at a constant value by adjusting the position of the equilibria
where  is the bicarbonate ion and  is carbonic acid.  
However, the solubility of carbonic acid in water may be exceeded. When this happens carbon dioxide gas is liberated and the following equation may be used instead.
 represents the carbon dioxide liberated as gas. In this equation, which is widely used in biochemistry,  is a mixed equilibrium constant relating to both chemical and solubility equilibria. It can be expressed as
where  is the molar concentration of bicarbonate in the blood plasma and  is the partial pressure of carbon dioxide in the supernatant gas.