Determinants of blood pH in health and disease

Abstract

An advanced understanding of acid-base physiology is as central to the practice of critical care medicine, as are an understanding of cardiac and pulmonary physiology. Intensivists spend much of their time managing problems related to fluids, electrolytes, and blood pH. Recent advances in the understanding of acid-base physiology have occurred as the result of the application of basic physical-chemical principles of aqueous solutions to blood plasma. This analysis has revealed three independent variables that regulate pH in blood plasma. These variables are carbon dioxide, relative electrolyte concentrations, and total weak acid concentrations. All changes in blood pH, in health and in disease, occur through changes in these three variables. Clinical implications for these findings are also discussed.

Figure 1

Changes in the relationship between partial carbon dioxide tension (pCO₂) and H⁺ concentration as function of changes in 'buffer' strength. Individual curves are drawn for varying concentration of total nonvolatile buffers in mmol/l. Note that as the concentration of 'buffer' increases, the slope of the curve increases, making changes in H⁺ concentration more responsive to changes in CO₂.

Figure 2

Plot of pH versus strong ion difference (SID). For this plot, total weak acid concentration (A_TOT) and partial carbon dioxide tension (pCO₂) were held constant at 18 mmol/l and 40 mmHg, respectively. Assumes a water dissociation constant for blood of 4.4 × 10–14 (mol/l). Note how steep the pH curve becomes at SID <20 mmol/l. Adapted from Kellum and Kellum [10].