Apparent Volume of Distribution

Relates total amount of drug in body to the concentration of drug in PLASMA.

qT = Vd(Cp)

Units of Vd = ml, liter, liter/kg body weight

Vd = Dose/Co = Dose/[AUC(ke)]

where AUC is area under the curve of the plasma concentration versus time curve

Vd - does not correspond to any anatomical space

High Vd (e.g 20 liters/kg) - present in high concentration in tissue relative to plasma.

Low Vd (e.g. 0.1 liters/kg) - present in high concentration in plasma relative to other tissues.

Volume of plasma in a 70 kg male is approximately 2 liters.

Drug Type Vd
Binds to plasma proteins Plasma volume (0.05 liters/kg)
Not highly bound to plasma

but don't enter cell

Total extracellular water volume (0.2 liters/kg)
Enter cells freely Total body water volume (0.55 liters/kg)
Binds to tissues Extremely high Vd's >10 liters/kg