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The volume of distribution (VD) , also known as apparent volume of distribution, is a pharmacological term used to quantify the distribution of a drug throughout the body after oral or intravenous dosing. It is defined as the volume in which the amount of drug would need to be uniformly distributed to produce the observed blood concentration. It is given by the equation:
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Therefore the dose required to give a certain plasma concentration can be determined if the VD for that drug is known. The VD is not a real volume; it is more a reflection of how a drug will distribute throughout the body depending on several physicochemical properties, e.g. solubility, charge, size, etc. The VD may also be used to determine how readily a drug will displace into the body tissue compartments relative to the blood:
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Where:
VP = plasma volume
VT = apparent tissue volume
fu = fraction unbound in plasma
fuT = fraction unbound in tissue
Some examples:
Warfarin VD = 8L Reflects a high degree of plasma protein binding.
Theophylline, Ethanol, VD = 30L Represents distribution in total body water.
Chloroquine VD = 15000L Shows highly lipophilic molecules which sequester into total body fat
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