Influence of pH on the distribution of a weak acid between plasma and gastric juice separated by a lipid barrier. A. The dissociation of a weak acid, pKa =
4.4. B. Dissociation of the weak acid in plasma (pH 7.4) and gastric acid (pH 1.4). The uncharged form, HA, equilibrates across the membrane. Blue
numbers in brackets show relative concentrations of HA and A–. In this example, the ratio of nonionized to ionized drug in plasma is 1:1000; in gastric
juice, the ratio is 1:0.001, as given in brackets. The total concentration ratio between the plasma and the gastric juice therefore would be 1000:1 if such a
system came to a steady-state. For a weak base with a pKa of 4.4 (eg, chlordiazepoxide), the ratio would be reversed, as would the thick horizontal
arrows, which indicate the predominant species at each pH. Accordingly, at steady state, an acidic drug will accumulate on the more basic side of the
Source: Pharmacokinetics, Workbook and Casebook for Goodman and Gilman's The Pharmacological Basis of Therapeutics
membrane and a basic drug on the more acidic side.
Citation: Rollins DE, Blumenthal DK. Workbook and Casebook for Goodman and Gilman's The Pharmacological Basis of Therapeutics; 2016
Available at: http://mhmedical.com/ Accessed: July 28, 2017
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