Upon entering the systemic circulation, a drug is distributed within the body by the flow of blood and lymph within the arteriovenous system. Usually the biophase—the place of drug–receptor interaction—is not the bloodstream but the tissue. To exert its pharmacologic effect, a drug has to cross not one but many membranes. The vehicle bringing the drug molecule to the membrane and carrying it away is the biological fluid. Although the circulating bloodstream and lymph