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Drug Absorption and Distribution

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Timothy S. Tracy

Unless a drug acts topically (i.e., at its site of application), it first must enter the bloodstream and then be distributed to its site of action. The mere presence of a drug in the blood, however, does not lead to a pharmacological response. To be effective, the drug must leave the vascular space and enter the intercellular or intra-cellular spaces or both. The rate at which a drug reaches its site of action depends on two rates: absorption and distribution. Absorption is the passage of the drug from its site of administration into the blood; distribution is the delivery of the drug to the tissues. To reach its site of action, a drug must cross a number of biological barriers and membranes, predominantly lipid. Competing processes, such as binding to plasma proteins, tissue storage, metabolism, and excretion (Fig. 3.1), determine the amount of drug finally available for interaction with specific receptors.

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