Pharmaceutics Xenobiotics Across Bio Membrane Practice Test

Which type tends to be more in stomach than plasma?

Weak bases Weak acids Both equally Ethanol

Correct Answer:

Weak bases

Explanation:

Ion trapping driven by pH differences across compartments explains where a drug tends to accumulate. The stomach is highly acidic (low pH) while plasma is near neutral (pH ~7.4). For a weak base, the acidic environment protonates the molecule to form BH+, which is positively charged and crosses membranes poorly. As a result, the weak base remains largely in the stomach rather than moving into the bloodstream, so its concentration tends to be higher in the stomach than in plasma. Ethanol, by contrast, is not ionized and can freely cross membranes, so its distribution between stomach and plasma is more even. Weak acids become more unionized in acid and can cross into plasma, so they don’t concentrate in the stomach the same way.

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