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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(
The apparent volume of distribution (Vd)
• Concentrations (mass per unit volume or amount per unit volume), not masses (mg
or mg), are usually measured in plasma or serum. Therefore, a term is needed to
relate the measured concentration (Cp) at a time to the mass of drug (X) at that time.
This term is defined as the apparent volume of distribution (V).
• It is not a physiological volume.
The concept of the apparent volume of distribution
The following figure illustrates the concept of apparent volume of distribution.
1. Beakers A and B contain equal but unknown volumes of water
.
2. Only beaker B contains a small quantity of charcoal (an adsorbing agent).
3. Let us assume that we add 1 gm of potassium iodide (KI), which is soluble in
water, to each beaker.
4. A suitable analytical procedure determines the concentration (mg/mL) of KI in
each beaker.
5. Let us assume that the KI concentration in beakers A and B is determined to
be 100 and 50 µg/mL, respectively.
Drug concentration Drug concentration in
in Beaker Beaker with charcoal
The concentration of KI is different although the volume of water in both
beakers is the same
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