Pharmacology Teaching and Research

Online Lecture: Pharmacokinetics.

                      Volume of Distribution and Binding of Drugs to Plasma Proteins

The distribution of a drug in body fluids is often measured as a volume of distribution (Vd), and is a measure of the fluid volume that would be required to contain the amount of drug present in the body at the same concentration as that measured in the plasma.

It is important to note that the measured volumes are not actual physical volumes but are apparent volumes based on the dilution of drug in plasma.

However the liquid volumes in various compartments can put the observed volumes of distribution into context.

If the majority of the drug is bound to plasma protein then only the unbound fraction can have a biological effect or be metabolised/excreted.

Therefore, the degree of binding to plasma proteins significantly influences the pharmacokinetic and pharmacodynamic properties of a drug.

The efficacy of the drug will be related to the exposure to the amount of unbound drug in plasma, i.e. the proportion free to penetrate into surrounding tissues.

The bound drug in plasma can also serve as a reservoir for free drug removed by various elimination processes thus prolonging the duration of action.
The proportion of binding can also be affected if two drugs are used at the same time because each drug may displace a proportion of the other from being bound to the plasma protein, resulting in a greater proportion of unbound drug.
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