ABSTRACT
We have seen that a drug can produce an effect only when it is in the immediate vicinity of its site of action. With the obvious exceptions of chemicals that act like the cathartic magnesium sulfate or the plasma substitutes, drugs do not make their initial contact with the body at, or even near, their locus of action. In almost all cases, drugs must move from where they are administered to the tissues or cells where they will act. For example, when aspirin is swallowed for the relief of a headache or a hypnotic is taken to produce sleep, these agents must go from the gastrointestinal tract to their respective sites of action in the brain to exert their characteristic effects. (One would hardly consider rubbing an aspirin on the forehead to relieve the headache!) To do this, they must pass through various cells and tissues which act as barriers to their movement. Just as receptors show specificity with regard to the drugs with which they combine, so too do barriers show a certain degree of selectivity in the ease with which they permit drugs to pass through them. Thus the anatomic structures which act as barriers to the migration of materials are called semi-permeable, allowing certain chemicals to pass freely, others to pass with difficulty and still others to be almost entirely excluded from passage. We have seen that the specificity of a drug-receptor combination is the consequence of the physicochemical properties and structural configuration of the receptor at the site of action as well as of the physicochemical properties and structure of the drug molecule. In an analogous way, the selectivity of migration through the anatomic barrier is the consequence of the physicochemical properties and structural configuration of the barrier as well as of the migrating molecule. We have also noted that the forces responsible for a particular drugreceptor combination are not unique to that combination but underlie all the reactions between drugs and tissue elements of a living system. In a parallel fashion, the mechanisms that serve to move a drug across a particular barrier are those which move any substance across any biologic barrier. This movement is called biotransport, and the mechanisms underlying the transfer of chemicals across biologic barriers are called transport processes or transport mechanisms.
