PCO2 is the partial pressure of carbon dioxide in the blood plasma.The pH of the extracellular fluids can thus be controlled by the regulation of 
  
    
      
        
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    {\displaystyle P_{{\mathrm {CO} }_{2}}}
   and the other metabolic acids.
pKa H2CO3 is the cologarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1. [HCO−3] is the molar concentration of bicarbonate in the blood plasma.