Absorption
Absorption is the transfer of chemicals from the site of exposure into the systemic circulation. The rate of absorption depends on the amount of chemical in the environment, rate of exposure, properties of the chemical, area of exposed surface and characteristics of the local epithelium.
Toxicants in the systemic circulation enter the extracellular fluid. Lipid-soluble chemicals move readily into cells by diffusion, while hydrophilic chemicals need some kind of transport.
Reabsorption of excreted toxicants may occur across tubular cells in the kindneys or after metabolic transformations in the intestine.
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Distribution
Some mechanisms that facilitate distribution of chemicals to a target include endothelial cell porosity, special membrane transport, acumulation in organelles and intracellular binding. Mechanisms opossing distribution include association with non-target proteins (mostly binding to plasma proteins), special barriers, preferential distribution to storage sites, and export from cells.
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Excretion
Excretion is the removal of xenobiotics from blood and their return to the exterior environment. Non-volatile chemicals are excreted mainly by filtration at the glomeruli (if < 60 kDa) or by active transport at the proxumal tubular cells and hepatocytes. Volatile chemicals are mostly excreted by the lungs.
Some chemicals may be eliminated before reacing the systemic circulation by the first-pass effect. Especially chemicals absorbed through the GI track, must pass through the liver and lungs before distribution to the rest of the body. The liver may eliminate a significant amount via detoxication or bile secretion..
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