The second stage of metabolism refers to the way in which a drug is transported from the site of introduction to the site of action. When the body absorbs a drug, a portion of the drug binds to plasma protein and may compete with other drugs for this storage site.
Another portion is transported in “free” form through the circulation to all parts of the body. It is the “free” drug that is pharmacologically active. As the free drug moves from the circulatory system, it crosses cell membranes to reach its site of action. As the drug is metabolized and excreted, protein bound drug is freed for action. Lipid-soluble drugs are distributed to and stored in fat and then released slowly in to the bloodstream when drug administration is discontinued. The amount of the drug that is distributed to body tissues depends on the permeability of the membranes and blood supply to the absorption area.
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