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Figure 10.4 (a) A highly simplified diagram of a kidney tubule to illustrate the filtration and secretion of drugs from the blood into the tubular filtrate, and their subsequent reabsorption or loss in the urine. (b) Schematic representation of the influence of urinary pH on the passive reabsorption of a weak acid and a weak base from the urine in the renal tubules; at a high pH the passive reabsorption of the weak base and the excretion of the weak acid are enhanced, while at a low pH values the reabsorption of the weak acid and the excretion of the weak base are enhanced.

efficacy of both agents is reduced by precipitation and the danger of renal blockade is, of course, increased.5

Precipitation of drugs in vivo

Pain on injection may be the result of precipitation of a drug at the site of injection brought about either by solvent dilution or by alteration in pH. Precipitation of drugs from formulations used intravenously can, of course, lead to thromboembolism. The kinetics of precipitation under realistic conditions must be appreciated, since a sufficiently slow rate of infusion may obviate problems from this source as the drug precipitates and then redissolves. A simple equation6 yields the flow rate (Q) of blood or normal saline required to maintain a drug in solution during its addition to an i.v. fluid:

excretion

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