Intravenous dosage(a)P 1EliminationkelEliminationTwo compartment model
One compartment model
EliminationAbsorptionAbsorptionkabskabsOral dosageP 1 P 2kelk 12k 21P 1EliminationkelP 1 P 2kelk 12k 21(b)Figure 8.3 Compartmental models in which a drug is administered (a) by intravenous injection
and (b) orally. The rate constant for the appropriate movement of the drug isksubscript. P 1 is the
plasma and highly perfused tissue compartment, which is the central compartment in all com-
partmental models. It is the first destination of a drugThe simplest model is the one compartmental model, in which the compartment
represents the circulatory system and all the tissues perfused by the drug (Figure
8.3). Other types of model are in use but unless stated otherwise all discussions
in this text will be based on a one compartment model.
The information obtained using pharmacokinetic models has a wide variety
of uses such as correlating drug doses with pharmacological and toxic responses
and determining an optimum dose level for an individual. However, as these
models are based on a simplification of what is a very complex system it is
necessary to treat the results of these analyses with some degree of caution until
the model has been rigorously tested.
8.4 Intravascular administration
The main methods of intravascular administration are intravenous (IV) injec-
tion and infusion. When a single dose of a drug is administered to a patient by
intravenous injection, the dose is usually referred to as anIV bolus. Intravas-
cular administration places the drug directly in the patient’s circulatory system,
which bypasses the body’s natural barriers to drug absorption. Once it enters the
circulatory system the drug is rapidly distributed to most tissues since a dynamic
equilibrium is speedily reached between the drug in the blood and the tissue.
This means that a fast IV bolus injection will almost immediately give a high
initial concentration of the drug in the circulatory system but this will immedi-
162 PHARMACOKINETICS