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Why are organic bases generally absorbed more easily in the small intestine than in the stomach?

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Organic bases are absorbed more easily in the small intestine than in the stomach primarily due to differences in pH and surface area.1

Chemicals generally diffuse across cell membranes in their nonionized, lipid-soluble form. Organic bases dissociate into ionized forms based on their environment; they remain nonionized in basic (alkaline) environments, which facilitates their diffusion across membranes. Because the mucosal surface of the small intestine is alkaline, organic bases tend to exist in their nonionized form, allowing for efficient absorption.1

In contrast, the stomach is a strongly acidic environment with a pH typically between 1 and 2. In this acidic setting, organic bases become ionized, which significantly hinders their ability to diffuse across the stomach lining. Furthermore, even if some absorption were possible, the stomach possesses a much smaller surface area compared to the small intestine, which has a vast surface area optimized for absorption due to its numerous mucosal projections. Consequently, most ingested substances, including organic bases, are primarily absorbed in the small intestine.1