This is the most important means by which a drug enters the tissues and is distributed through them. It refers simply to the natural tendency of any substance to move passively from an area of high concentration to one of low concentration. In the context of an individual cell, the drug moves at a rate proportional to the concentration difference across the cell membrane, i.e. it shows first-order kinetics (see p. 99); cellular energy is not required, which means that the process does not become saturated and is not inhibited by other substances.
The extent to which drugs are soluble in water or lipid is central to their capacity to cross cell membranes. Water or lipid solubility is influenced by environmental pH and the structural properties of the molecule.
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