PH buffering: Bicarbonate

The human body has four native buffer systems – bicarbonate, hemoglobin, protein, and phosphate systems. An ideal buffering system has a pKa of ~ 7.4 (normal physiologic pH). Bicarbonate has a pKa of 10.3, which is NOT ideal in normal physiologic conditions. In fact, the pH range of effectiveness is probably ~ 5.1 – 7.1 for the bicarbonate buffer system. Bicarbonate is better described as a CO₂ transport mechanism and not as a buffer – protons combine with hydrogen ions which are at equilibrium with carbonic acid (H₂CO3), water, and CO₂.

According to Miller, “Volatile acid is principally buffered by hemoglobin. Deoxygenated hemoglobin is a strong base, and there would be a huge increase in the pH of venous blood if hemoglobin did not bind hydrogen ions that are produced by metabolism.” Thus, it seems that the primary function of bicarbonate is to accept protons which can be ultimately converted to water and CO₂ (and excreted as volatile acid), whereas the protons produced by accumulation of volatile acid that cannot be excreted (e.g. respiratory acidosis, in which CO₂ builds up, some of which is converted to carbonic acid, protons, and bicarbonate) are buffered by hemoglobin (and to some extent bicarbonate).

To gauge the relative importance of volatile versus fixed acids in the maintenance of acid-base equilibrium, it is helpful to know the relative contribution of each to total acid excretion. According to Miller, normal metabolic processes create the equivalent of 12,500 mEq of protons daily (which are exhaled). By contrast, the kidneys excrete between 20-70 mEq of strong anions (e.g. chloride) which effectively behave as acid.

That said, the bicarbonate system is still of vital importance. While it only makes up 13% of overall buffering, it accomplishes 92% of plasma buffering. Most importantly, however, is the ability of bicarbonate to convert protons to a readily excretable form (CO₂).