The pH range of the body is kept within a narrow range. Normal is considered to range from 7.36 to 7.44. Clinically the safe range is between 7.30 to 7.52. The blood pH is buffered by the bicarbonate system (H₂CO₃ and HCO₃^-) and the nonbicarbonate system (proteins and phosphates). Acid imbalance has two sources, namely, respiratory and metabolic. The pressure of the CO₂ (pCO₂) and the Base Excess (Metabolic Acid) are used as indicators of repiratory and metabolic contributions to the acid base balance in the bloodstream. A change in the pCO₂ of 12 mmHg or a change of 6 mEq/L in the metabolic acid level can produce a 0.1 change in pH. Clinicians often use the CO₂, HCO₃^- and pH measurements to make assessments about patient care.

Fundamentals of acid-base balance.

The Henderson-Hasselbalch [local] equation is the primary source and is slightly modified to accomodate the pCO₂ term as follows: The 0.03 term converts mm pressure generated by CO₂ to mmoles/L, and the pK_a is 6.1. The pK_a is different than the pK_a found in standard tables because it has been measured in patient's blood which is maintained at body temperature rather than standard conditions.