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Definition of Volume of distribution

Volume of distribution Definition from Medicine Dictionaries & Glossaries
Terms and symbols used in pharmacology
The volume, in an organism, throughout which a drug appears to have been distributed; the volume into which a drug appears to have been dissolved after administration to an organism. Symbolized by Vd.
Suppose a drug has been completely absorbed from its site of application, has reached an equilibrium in its distribution among the several tissues of the body, and that no biotransformation or excretion of the drug has occurred. If one knew the mass (dose) of drug administered and the average concentration of the drug in the body, the apparent volume into which the drug had been dissolved could be determined from the relationship or definition: concentration = mass/volume. Since these idealized conditions are unobtainable in practice, the volume of distribution of a drug can only be approximated using experimental data.
With the assumption that the concentration of the drug in the plasma (or serum) reflects the average drug concentration in its whole volume of distribution, plasma concentration can be plotted against time after drug administration, and the resulting line can be extrapolated to yield a fictive concentration (C0) "predicted" to have existed at the instant the drug was administered - further assuming instantaneous and complete administration, absorption, and distribution of the drug. Obviously, C0, is the value expected to have occurred at a time when mechanisms of biotransformation and excretion had no significant effect on the amount of drug in the body. Needless to say, it is assumed for proper interpretation of C0, that the drug as measured in the plasma is identical to the agent that was administered, and that the drug underwent no chemical alteration in the course of administration, absorption, or distribution.
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Volume of distribution Definition from Encyclopedia Dictionaries & Glossaries
Wikipedia English - The Free Encyclopedia
The volume of distribution (VD), also known as apparent volume of distribution, is a pharmacological, theoretical volume that the total amount of administered drug would have to occupy (if it were uniformly distributed), to provide the same concentration as it currently is in blood plasma. Therefore, if VD is greater, it shows that the drug is more diluted than it should be (in the blood plasma), meaning more of it is distributed in tissue (i.e. not in plasma). It is defined as the distribution of a medication between plasma and the rest of the body after oral or parenteral dosing. It is defined as the theoretical volume in which the total amount of drug would need to be uniformly distributed to produce the desired blood concentration of a drug. In rough terms, drugs with high lipid solubility (non-polar), low rates of ionization or low plasma binding capabilities have higher volumes of distribution than drugs which are more polar, more highly ionized or exhibit high plasma binding in the body's environment.

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