Drugs are not distributed homogeneously (equally) to all tissues. This is because different drugs have to different tissues.
Suppose the beaker in the previous example is coated with a porous material that can adsorb the dye and concentrate it. The resulting dye concentration in the solvent will be lower than that in the coating material.
Based on this equation, the beaker behaves as if it has volume larger than its actual volume.
This is the apparent
volume of the beaker
The volume of distribution of a drug in a particular patient is constant.
The can be estimated if the amount of the drug in the body and the resulting plasma concentration are known.
Volume of distribution =
Amount of drug in body
Drug concentration in plasma
It is practically impossible to measure the amount of the drug in the body. The only time the amount of the drug in the body can be estimated is right after iv bolus administration. The amount of the drug in the body at that time, is equal to the administered dose.
