Page 26 - Pharmacology Mnemonics and Short Notes

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Apparent volume of distribution (Vd)
 It is the hypothetical volume of the fluid into which a drug

distributed

Amount of drug in body

V d =
Plasma concentration of drug

V d is not a real volume, small volume indicates extensive plasma protein
binding, but large volume indicates extensive tissue binding.

V d is increased by increased tissue binding, decreased plasma binding and

increased lipid solubility.

N.B. in average 70 kg adult, the total body water is 42 liter,
extracellular volume is 14 liter and plasma volume is 4 liter.

A-Patterns of Drug Distribution
The drug is transported in the blood in either 2 forms bound form OR
free form

Plasma protein bound Drug Free Drug
Inactive Active
Non-Diffusible Diffusible

Cannot be metabolized Can be metabolized
Cannot be excreted by kidneys Can be excreted by kidneys

•The protein bound drug acts as a store from which a small amount of
free drug is released.

•The protein responsible for binding of most drugs is albumin.
B-Competition for Plasma Protein Binding Sites
One drug may displace another from its binding sites on plasma

proteins. The displaced drug will show higher free blood level with
enhanced activity & possibly toxicity e.g. Aspirin can displace Warfarin

(anti-coagulant)  Hemorrhage.
Contents C-Passage of drug to CNS via BBB and to fetus via placental

membrane

Pharmacology Mnemonics and Short Notes By Muhammad Ramzan Ul Rehman