The ability of a toxin to produce an effect depends on factors other than its ability to react with a receptor. These include factors that influence how much of the toxin actually arrives at the receptor. Physicochemical parameters describe the equilibrium distribution of substances among phases in the environment and within organisms. However, the environment and organisms are often far from equilibrium. Therefore, we need to take a kinetic approach, one that describes the rates at which substances move within organisms. In the next few sections we focus on the fate and transport of substances within organisms, or pharmacokinetics. In later sections we discuss transport among compartments of the environment and various parts of the ecosystem. Environmental transport is described using many of the same principles and methods as used here.
The way that a chemical is distributed in the environment and within an organism varies with physicochemical properties that are independent of its toxicity. Toxins may be absorbed by different mechanisms and by different routes, such as by inhalation or ingestion, each with its own efficiency at delivering the toxin. Once absorbed, the toxin finds its way to various parts of an organism, including storage sites. The organism can eliminate the toxin by biochemical reactions or excretion, or may convert it to a more toxic substance. These processes can be described mathematically, giving models that can be used for understanding and predicting the phenomena.
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