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Body Compartment Model

Volume of Distribution is considered in relation to body “compartments”.

Some large molecules stay mainly in the plasma and have a very small Vd. Hydrophilic or highly polar small molecule drugs, like penicillins, distribute into the extracellular fluid (ECF) and have a relatively small Vd. Highly lipid soluble drugs, such as tricyclic antidepressants, distribute far more widely into tissues and have a large VD.

Inject drug, and progressively add compartments to see what happens as the drug distributes more widely in the body.

You will see that Ab (Amount in body), Vd and Cp are clearly related.




Step 1

Inject Monoclonal Antibody

dose (Ab) = 100mg
conc (Cp) = 20mg/L
vol(Vd) = 5L

Step 2

Inject Hydrophilic Drug

dose (Ab) = 100mg
conc (Cp) = 6.7mg/L
vol(Vd) = 15L

Step 3

Inject Lipophilic Drug

dose (Ab) = 100mg
conc (Cp) = 0.02mg/L
vol(Vd) = 5000L