Introduction
The intensity of toxic action of foreign chemicals is normally directly related to the concentration of their active chemical entities at target sites. Thus, delivery to and presence of a toxicant at the target site is a determining factor for toxic outcomes. Toxicokinetics is a collective (mathematical) term which describes the disposition of foreign chemicals in the body by determining the types, rates and extent of absorption, distribution, metabolism and excretion of the chemical and is important for toxicological risk assessment. The term elimination includes both metabolic and excretory processes. The main routes of absorption, distribution and excretion of toxicants in the body are illustrated in Fig. 4a and 4b. Foreign chemicals generally enter the body by ingestion, inhalation or through dermal contact. They are absorbed into the blood circulation and distributed to the various organs and tissues. In general, foreign chemicals must be metabolically transformed, for a major part in the liver in order to be able to be eliminated from the body. The parent compound and/or its metabolites will be excreted either through the kidneys into the urine, via the bile and the gastrointestinal tract into the faeces, or if they are volatile liquids or gases through the lungs into the expired air. Other excretory pathways also exist, such as through saliva, sweat, and milk, which may be quantitatively important in certain instances (e.g. chloroorganic compounds in milk). Some poorly excreted chemicals may accumulate and be stored in body tissues such as fat and bone.
Toxicokinetic studies are performed to aid in the evaluation and interpretation of toxicological effects data (toxicological risk assessment). Mathematical models may be applied to describe parts or the whole process of disposition. In addition, toxicokinetic studies may provide data useful for dose level selection and timing in toxicodynamic studies. Furthermore, since the toxicokinetic behaviour of a chemical may vary between higher and lower doses, and between animals and humans, toxicokinetic information is very important when extrapolating effects data between these situations. Physiologically based toxicokinetic (PBTK) models which consist of several components where organs are grouped together according to blood flow are especially useful.