ABSTRACT
In order for a drug to exert its characteristic effects, it must reach its site of action. This usually entails movement, since most drugs make initial contact with the body some distance away from where they act. Although the transport processes described in Chapter 4 can account adequately for passage of drugs across any biologic membranes that impede their progress, they can hardly explain movement over a great distance. The forces that drive passive or facilitated diffusion, or active transport and endocytosis, are sufficient only to move solutes across the very short span of cellular membranes themselves. How, then, is the movement of drugs over greater distances accomplished? Just as oxygen from the lungs or food substances from the intestine gain access to every cell of the organism by way of the bloodstream, so too does the circulatory system serve as the common pathway for carrying drugs from the inner side of a biologic barrier to any tissue or organ. Hence, unless a drug is administered purposely to produce its effect locally or is injected directly into the bloodstream, access to its site of action involves two separate processes. The first of these is absorption, the movement of the solute into the bloodstream from the site of administration. The second process is distribution, the movement of solute from the blood into the tissue (Fig. 5-1).
