Equations
The volume of distribution is given by the following equation:
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 physiologic value; 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 units for Volume of Distribution are typically reported in (ml or liter)/kg body weight. The fact that VD is a ratio of a theoretical volume to a fixed unit of body weight explains why the VD for children is typically higher than that for adults, even though children are smaller and weigh less. As body composition changes with age, VD decreases.
The VD may also be used to determine how readily a drug will displace into the body tissue compartments relative to the blood:
Where:
- VP = plasma volume
- VT = apparent tissue volume
- fu = fraction unbound in plasma
- fuT = fraction unbound in tissue
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