Chapter 24 - Absorption, Distribution, and Pharmacokinetics

Abstract

This chapter includes a general discussion of absorption, distribution, and pharmacokinetics of chemicals, especially pesticides. It also discusses the factors that influence the transfer of chemicals across biological membranes. Human exposure to pesticides is typically by oral, dermal, and inhalation routes. Whereas, occupational exposure to pesticides is likely to occur by inhalation or dermal contact, exposure of small children is more likely to be by oral and/or dermal routes. Absorption generally occurs from gastrointestinal tract and respiratory tract. Once in the bloodstream, the chemical is available for distribution throughout and elimination from the body. Factors that influence the rate and extent of distribution of a chemical to a particular tissue include blood flow to the tissue the mass of the tissue, the ability of the chemical to cross membranes, and the affinity of the chemical for the tissue relative to blood. The rate of distribution of a chemical from blood to tissues can be perfusion- or diffusion-rate- limited. Pharmacokinetics is the modeling and mathematical description of the time course of chemical disposition. Pharmacokinetic models typically describe the change in blood concentration of the chemical with time. There are two basic approaches to characterizing the pharmacokinetics of a chemical in the body: compartmental and noncompartmental.