When a drug intended for systemic effect is taken by mouth, a proportion may remain in the bowel and be excreted in the faeces. Sometimes the objective of therapy is that drug should not be
21 Most drugs have a molecular weight less than 1000.
absorbed from the gut, e.g. neomycin. Drug in the blood may also diffuse passively into the gut lumen, depending on its pKa and the pH difference between blood and gut contents. The effectiveness of activated charcoal by mouth for drug overdose depends partly on its adsorption of such diffused drug, which is then eliminated in the faeces (see p. 155).
Biliary excretion. In the liver there is one active transport system for acids and one for bases, similar to those in the proximal renal tubule and, in addition, there is a system that transports un-ionised molecules, e.g. digoxin, into the bile. Small molecules tend to be reabsorbed by the bile canaliculi and in general only compounds that have a molecular weight greater than 300 are excreted in bile. (See also, enterohepatic circulation, p. 105)
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