Acid-base physiology is about hydrogen ion concentration, written [H⁺], in body fluids. Hydrogen ion concentration in aqueous (water) solutions, e.g., blood plasma, is expressed in moles per liter (mol/L), or more specifically in fractions of a mole, e.g., a tenth of a mole can be written 0.1 moles/liter or 1 X 10^-1 moles/liter, or a hundred trillionth of a mole can be written 0.000000000000001 moles/liter or 1 X 10^-14 moles/liter. The “power of Hydrogen” of a solution, otherwise known as “pH,” tells you about hydrogen ion concentration, and does so by indicating the size of the negative exponent (e.g., 5 in10^-5) of the concentration, generally ranging from 10^-1 (pH = 1) to 10^-14 (pH = 14). Thus, as pH increases, e.g., from 10^-7 to 10^-8, hydrogen ion concentration decreases (less hydrogen ions per mole). And, as pH decreases, [H⁺] increases.

Mathematically speaking, pH is the negative logarithm of [H⁺], and is shorthand for comparing tiny negative numbers otherwise representing enormously significant differences in body chemistry. More accurately, however, pH is a measure of hydrogen ion activity (aH⁺), but in dilute solutions, such as in the case of most body fluids (aqueous solutions), [H+] and  aH⁺ are for all practical purposes equivalent.

When is an aqueous (water) solution, e.g., blood plasma, acidic or alkaline? The pH of distilled water is 7.0. In other words, hydrogen ion concentration is approximately 1 X 10^-7. Water also, however, contains an equivalent concentration of hydroxyl ions [OH‾], ions of opposite charge, which offsets the concentration of hydrogen ions [H⁺], and is thus said to be “neutral” (buffered). The pH can be changed by adding more hydrogen ions (e.g., lactic acid) and/or other kinds of ions of opposite charge (e.g., bicarbonates) which reduce hydrogen ion concentration. Aqueous solutions with a pH below 7.0 are acidic; [H⁺] is greater than [OH‾]. Solutions with pH levels above 7.0 are alkaline; [OH‾] is greater than [H⁺].

The range of extracellular fluid pH levels is very restricted. Blood plasma, for example, is a slightly alkaline aqueous (water) solution, with a normal pH range of 7.35 to 7.45. Plasma acidemia is a pH below 7.35 (although this is still alkaline), and plasma alkalemia is a pH above 7.45. Plasma pH levels below 6.9, and above 7.8, are fatal. Levels below 7.35 and above 7.45 can result in significant physical symptoms, psychological changes, and behavioral deficits.

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