In this equation, the concentration of CO2 has been substituted for the weak acid H2CO3 because it can be determined from the arterial PCO2reported by the clinical laboratory (see later discussion). In the pH range of 7.0-7.7, more than 99% of the total CO2 in the plasma is in the form of dissolved CO2 rather than H2CO3. The pKa value defines the pH at which the ratio [A—]/[HA] (or [HCO—]/[CO2]) equals 1.0. At this pH, the buffering capacity is maximal. However, the physiologic pH of the extracellular fluid (7.4) is far from 6.1 (the pKa value of the HCO—/CO2 buffer system) and, in fact, represents a [HCO—]/[CO2] ratio of 20:1. Although the normal extracellular pH lies far from the pKa of this buffer system, its unique effectiveness in reducing fluctuations in pH derives from the fact that [CO2] is rapidly regulated by pulmonary ventilation.

Practically speaking, the concentrations of CO2 or H2CO3 are not reported directly from the laboratory but must be computed from the measured partial pressure of CO2 in arterial blood (Pco2). Under physiologic conditions, the solubility of CO2 in plasma is a linear function of the arterial Pco2 such that:

where a is the solubility coefficient for CO2 (0.03 mmol/L per mm Hg at body temperature). Thus, as discussed in Chapter 20, the working form of the Henderson-Hasselbalch equation becomes:

A simpler formulation of the relationship among the variables pH (or H+ concentration), Pco2, and [HCO—] is the Henderson equation, which does not involve the logarithm function. The Henderson equation expresses the hydrogen ion concentration in nanomoles per liter (i.e., 10—9 mol/L) rather than as the pH. This requires one to think either in terms of the absolute H+ concentration ([H+]) or to convert [H+] to pH, remembering that pH = —log [H+]. The Henderson equation is:

where the constant 24 includes the pKa of the HCO-/CO2 system and the solubility coefficient of CO2 (a) that were separate terms in the Henderson-Hasselbalch formulation. Equation [8] shows that for a normal Pco2 of 40 mm Hg and a plasma [HCO-] of 24 mmol/L the H+ concentration is 40 nmol/L, which is equivalent to a pH of 7.4. The equilibrium concentrations for normal acid-base conditions in the human body are as follows:

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