The Kidney as Excretory Organ

Most drugs are eliniinated in urine either chemically unchanged or as metabolites. The kidney permits elimination because the vascular wall structure in the region of the glomerular capillaries (B) allows unimpeded passage of blood solutes having molecular weights (MW) 

Apart from glomerular filtration (B), drugs present in blood may pass into urine by active secretion. Certain cations and anions are secreted by the epithelium of the proximal tubules into the tubular fluid via special, energyconsuming transport systems. These transport systems have a limited capacity. When several substrates are present simultaneously, competition for the carrier may occur (see p. 268). 

The same considerations hold for acidic molecules, with the important difference that alkalinization of the urine (increased pH) will promote the deprotonization of -COOH groups and thus impede reabsorption. Intentional alteration in urinary pH can be used in intoxications with proton-acceptor substances in order to hasten elimination of the toxin (alkalinization phenobarbi-tal acidification amphetamine). 

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