Volume of distribution - Definition and Overview

The volume of distribution (V_D) 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:

<math>{V_{D}} = \frac{total \ amount \ of \ drug \ in \ the \ body}{drug \ blood \ concentration}<math>

The V_D 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 V_D may also be used to determine how readily a drug will displace into the body tissue compartments relative to the blood:

<math>{V_{D}} = {V_{P}} + {V_{T}} \left(\frac{fu}{fu_{t}}\right)<math>

Where:

V_P = plasma volume

V_T = apparent tissue volume

fu = fraction unbound in plasma

fu_T = fraction unbound in tissue Some examples:

Warfarin V_D = 8L Reflects a high degree of plasma protein binding

Theophylline V_D = 30L Represents distribution in total body water

Chloroquine V_D = 15000L Shows highly lipophilic molecules which sequester into total body fat