Clinical Correlates of pH Levels
The major buffering systems in the body are proteins, particularly those with the amino acids histidine and cysteine exposed to the outside environment, phosphate, and bicarbonate. All three of these are weak acids with pKa values lower than physiological pH. As a consequence, buffering capacity increases as the pH is lowered from the physiological range. This meets the needs of most organisms because physiological pH excursions generally occur in the acid direction. Hence, the low pKa of these buffering systems is poised to respond to metabolic acidosis.
Of these three, only the bicarbonate system, which is critical for buffering extracellular fluids such as blood, is in steady-state between production and removal. Thus, pH changes via this dynamic bicarbonate system are taking place on a background provided by the more static protein and phosphate systems.
Production of Bicarbonate
CO2 produced by metabolism is normally balanced by CO2 expired from
the lungs, resulting in no net production of H2CO3. However, as detailed in the table below, certain
medically significant circumstances can throw the equation out of balance.
For respiratory problems caused by alterations in CO2, the best treatment involves ventilation. If bicarbonate is used to raise the pH in cases of respiratory acidosis, the result can be fatal, since compensation is also working to increase the blood bicarbonate concentration.
For metabolic problems that involve HCO3-, the best treatment is either bicarbonate infusion (for acidosis) or NH4Cl infusion (for alkalosis). NH4Cl dissociates into NH4+ and Cl-. The NH4+ (ammonium ion) is in equilibrium with NH3 (ammonia) and H+. Because ammonia is volatile, it is respired through the lungs, leaving behind H+ and Cl- or hydrochloric acid, which lowers the pH. Often, metabolic acidosis is found in combination with respiratory alkalosis (e.g. compensated). This is a fragile situation because the buffering power is significantly reduced.
The Biology Project
Department of Biochemistry and Molecular Biophysics
The University of Arizona
Revised: October 2004 & June 2006
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