Bicarbonate buffer system (BBS)

What is the bicarbonate buffer system (BBS)?

To minimize binding of H ⁺ to intracellular proteins, another process must prevent [H ⁺ ] from rising, despite the H ⁺ load. This occurs if added H ⁺ are forced to bind to bicarbonate (HCO ₃ ⁻ ), the predominant extracellular buffer, and this illustrates the BBS. The vast majority of HCO ₃ ⁻ is in skeletal muscle. If the BBS in skeletal muscle is not appropriately titrating, and therefore removing H ⁺ in a patient with metabolic acidosis, acidemia will be more pronounced, and more of the H ⁺ load will be titrated in vital organs (e.g., brain and heart). Critically, for the BBS to function there must be a low concentration of carbon dioxide (CO ₂ ) in capillaries around skeletal muscle; in other words, equation 3 must maintain its rightward direction.

How is the adequacy of the Bicarbonate buffer system assessed?

A high tension of capillary CO ₂ within skeletal muscle will lead to a failure to maintain rightward shift, thus extinguishing the BBS. CO ₂ tension within muscle capillaries can be assessed with a venous blood gas (VBG). This is because the PCO ₂ in capillaries of skeletal muscle reflects the PCO ₂ in its cells and interstitial space; further, there is no CO ₂ added once the blood leaves the capillaries and enters the veins. The secret here is that if the partial pressure of carbon dioxide in the venous effluent (PvCO ₂ ) is high, the BBS in skeletal muscle is less effective (i.e., the rightward direction of equation 3 is blunted). Acidemia will be more pronounced, as more of the H ⁺ load will be titrated in vital organs.