2.小腸為多數pKa＞3之弱酸性藥物及pKa＜7.8之弱鹼性藥物的主要吸收位置的原因為：
(A)具有廣大之黏膜面積
(B)腸道蠕動的速度快
(C)藥物不受腸道酵素之影響
(D)腸道上皮細胞孔隙較大

Such a gradient would greatly favor the absorption of this weak acid across the gastrointestinal tract into plasma. This is the situation that exists for weak acids such as penicillin, aspirin, and phenylbutazone. In con- trast, a weak base would tend to be trapped in this environment and thus minimal absorption would occur. Examples of such weak bases are morphine, phenoth- iazine, and ketamine. Specific active transport systems may counter these predictions (e.g., β-lactam trans- porters in intestines), as well as the extreme surface area of the small intestines compared to gastric mucosa, which generally favors absorption of most drugs in the small intestines. With the weakly basic strychnine, pH- dependent absorption is toxicologically significant. If strychnine were placed into the strongly acidic stomach, no systemic toxicity would be observed. However, if the stomach were then infused with alkali, most of this base would become nonionized, readily absorbed, and lethal. In summary, weak acids are readily absorbed from an acid environment and sequestered in an alkaline medium. In contrast, weak bases are absorbed in an alkaline environ- ment and trapped in an acidic environment.

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紅本 Veterinary Pharmacology and Therapeutics p11.