Biomedical Engineering Reference
In-Depth Information
7.5.3 Pharmacokinetic Models
A pharmacokinetic model describes the movement of a drug or anesthetic agent to sites
throughout the body, as shown in Figure 7.10. Much of the model focuses on drug
absorption, distribution, metabolism, and excretion. Pharmacokinetic models usually have
three or fewer compartments. Metabolic models have many more compartments than phar-
macokinetic models, typically six or more compartments. Pharmacokinetic models usually
are linear, whereas metabolic models are nonlinear in order to capture an adequate repre-
sentation of the system. Pharmaceutical drugs do not appear naturally in the body and
can be traced easily. Since metabolic solutes occur naturally in the body, following the
movement of the solute is possible only with the use of isotopes.
Assume that the drug absorption site is the GI tract, where the drug passes to the blood
via the liver. Since drugs are relatively small, they easily move through the capillary walls
and into the interstitial fluid. From there, the drug then moves into the targeted organs
and other tissues. As shown, the drug is removed via the kidneys, GI tract, and biotrans-
formation in the liver and other sites. Transfer from compartment to compartment is
through diffusion. The rate of movement from each compartment depends on the transfer
rate. Typically, the removal of the drug from the body occurs more slowly than movement
among the plasma, body fluids, and tissues. For example, tissues and organs that are
highly perfused, such as the thyroid gland, liver, and kidney, have a large transfer rate,
and tissues with low perfusion have a small transfer rate. If the removal transfer rates
Hepatic Duct
GI Tract Luman
Fe
ces
Gut Absorption
Biliary Secretion
Portal
Vein
Spleen
Liver
GI Tract
Hepatic Artery
Urine
Kidneys
Blood
Other
Body
Tissues
Thyroid
Gland
Body
Fluids
Muscle
FIGURE 7.10
A pharmacokinetic model describing the movement of a solute in the body.
