Physiology 5th Ed.

pH OF BODY FLUIDS

The hydrogen ion (H+) concentration of the body fluids is extremely low. In arterial blood, the H+ concentration is 40 × 10−9 equivalents per liter (or 40 nEq/L), which is more than six orders of magnitude lower than the sodium (Na+) concentration. Because it is cumbersome to work with such small numbers, H+ concentration is routinely expressed as a logarithmic function called pH:

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The normal H+ concentration of 40 × 10−9 Eq/L is converted to pH as follows:

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When using pH instead of H+ concentration, there are two points of caution. First, because of the minus sign in the logarithmic expression, a mental reversal is necessary: As H+ concentration increases, pH decreases, and conversely. Second, the relationship between H+ concentration and pH is logarithmic, not linear. Thus, equal changes in pH do not reflect equal changes in H+ concentration. This lack of linearity is illustrated in Figure 7-1, in which the relationship between H+ concentration and pH is shown over the physiologic range in body fluids. An increase in pH from 7.4 to 7.6 (0.2 pH units) reflects a decrease in H+ concentration of 15 nEq/L; a decrease in pH from 7.4 to 7.2 (also 0.2 pH units) reflects a larger increase in H+ concentration of 23 nEq/L. In other words, a given change in pH in the acidic range (pH < 7.4) reflects a larger change in H+ concentration than the same change in pH in the alkaline range (pH > 7.4).

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Figure 7–1 Relationship between [H+] and pH.

The normal range of arterial pH is 7.37 to 7.42. When arterial pH is less than 7.37, it is called acidemia. When arterial pH is greater than 7.42, it is called alkalemia. The pH range compatible with life is 6.8 to 8.0.

The mechanisms that contribute to maintaining pH in the normal range include buffering of H+ in both extracellular fluid (ECF) and intracellular fluid (ICF), respiratory compensation, and renal compensation. The mechanisms for buffering and respiratory compensation occur rapidly, within minutes to hours. The mechanisms for renal compensation are slower, requiring hours to days.