Volume of distribution

In pharmacology, the volume of distribution (V_D, also known as apparent volume of distribution) is the theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma. It is defined as the distribution of a medication between plasma and the rest of the body after oral or parenteral dosing.

The V_D of a drug represents the degree to which a drug is distributed in body tissue rather than the plasma. V_D is directly correlated with the amount of drug distributed into tissue; a higher V_D indicates a greater amount of tissue distribution. A V_D greater than the total volume of body water (approximately 42 liters in humans) is possible, and would indicate that the drug is highly distributed into tissue.

In rough terms, drugs with a high lipid solubility (non-polar drugs), 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. Volume of distribution may be increased by renal failure (due to fluid retention) and liver failure (due to altered body fluid and plasma protein binding). Conversely it may be decreased in dehydration.

The initial volume of distribution describes blood concentrations prior to attaining the apparent volume of distribution and uses the same formula.

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 V_D for that drug is known. The V_D 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.

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