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Bicarbonate Buffer

Overview
  • Because of its high concentration in the extracellular fluid (ECF), bicarbonate acts as the dominant buffer of the ECF. Importantly, the body can independently modulate the concentrations of both the weak acid and weak base forms of the bicarbonate buffer. As a result, the modulation of the bicarbonate buffer is used as the primary mechanism for controlling the pH of the extracellular fluid.
Chemistry
  • Bicarbonate (HCO3-) can reversibly bind a free hydrogen ion and in doing so acts as a buffer against pH changes (Review Buffer Basics). The pK of the bicarbonate buffer is 6.1, providing excellent buffering capacity around the normal ECF pH of 7.4.
  • Importantly, the weak acid form of the bicarbonate buffer (H2CO3) is rapidly inter-converted by carbonic anhydrase into gaseous CO2 that remains disolved in the extracellular fluid. Given the extremely rapid inter-change of H2CO3 and CO2 within the ECF, the weak acid form of the bicarbonate buffer can for all intensive purposes be considered gaseous dissolved CO2.
Regulation
  • Overview
    • The major feature of the bicarbonate buffer is the capacity of the body to easily eliminate both the Weak Acid and the Weak Base form of the buffer in an independent fashion. Consequently, the body can gain exquisite control over the ECF pH simply by modulating elimination of either HCO3- (Weak Base) or CO2 (Weak Acid).
    • This concept may appear confusing to some students as buffers are typically presented in the following way: The pH of the fluid determines the ratio of the weak acid and weak base of the buffer. However, the body uses the converse principle to its advantage: By controlling the the ratio of the weak acid and weak base forms of the bicarbonate buffer, the body can determine the pH of the entire extracellular fluid.
  • Chemistry
    • Given the dominant role of the bicarbonate buffer in determining ECF pH, the Henderson-Hasselbalch Equation for the bicarbonate buffer essentially comes to represent that of the entire blood pH. The pK of the bicarbonate buffer is 6.1 and because the weak acid form of the buffer is for all intensive purposes CO2, it can be replaced by the partial pressure of arterial carbon dioxide. A correction factor of 0.03 is used to for unit conversion to partial pressure.
  • Regulatory Mechanism
    • As mentioned, the bicarbonate buffer represents a powerful tool for modulating total ECF pH because the Weak Acid and Weak Base forms of the buffer can be independently controlled. The ECF partial pressure of the Weak Acid form (PaCO2) can be controlled by changing rates of alveolar ventilation as explained in Respiratory Acid-Base Control. The ECF concentration of the Weak Base form (HCO3-) can be controlled by changing rate of renal bicarbonate excretion or by generating novel bicarbonate, as explained in Renal Acid-Base Control.
Significance
  • The significance of the bicarbonate buffer to the overall regulation of the extracellular fluid pH cannot be overstated. In essence the ratio of the Weak Acid (CO2) and Weak Base (HCO3-) forms of the bicarbonate buffer determine the overall pH of the ECF with the quantitative relationship given by the Henderson-Hasselbalch Equation discussed above. Consequently, the blood pH can be regulated by simply modulating the ratio of [HCO3-] to PaCO2. Once again, the HCO3- concentration is controlled by the kidneys whereas PaCO2 is controlled by the lungs.