Although different mechanisms of oral drug absorption have been shown in small intestinal regions, under physiological conditions, several routes may contribute to drug absorption at the same time. Usually, the fastest route dominates the absorption of a particular compound (Burton et al., 2002; Cao et al., 2005). In general, passive diffusion is the main mechanism for absorption of many lipophilic compounds, while the carrier-mediated process governs the absorption of transporter substrates. In some cases, paracellular junction is the route for the absorption of some small hydrophilic compounds with molecular weight less than 300.
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