Dose is the amount of chemical that actually penetrates into the body, and the effective dose ultimately determines toxicity. Distribution refers to the dynamic movement and temporary storage of a chemical within the body. Virtually every chemical has a defined target organ, of which the most common are the brain, heart, lungs liver, kidney, and colon. Different kinds of tissues and organs take up and release chemicals at different rates. Sometimes distribution may have an effect on elimination.
All chemicals move in and out of various bodily tissues at varying rates.
Some are dissipated within hours, while others, especially those that bind with lipid tissue, may accumulate and remain for years. While not all chemicals are metabolized in the body, metabolites are helpful in assessing toxicity. While some chemicals are not toxic in certain animals, the metabolites in humans are found to be extremely toxins. A common class of metabolites are conjugates , which are combinations of the parent compound and other naturally-occurring chemicals in the body. Conjugates can be excreted more rapidly than the parent chemical.