Which term describes the movement of a drug from systemic circulation into target tissues?

• A. Absorption
• B. Active Transport
• C. Distribution
• D. Elimination

Answer: (C)

Movement from the bloodstream into tissues is distribution. After a drug enters systemic circulation, it moves from the blood into various tissues where it can exert effects or be stored, transformed, or eventually eliminated. This step depends on how well blood reaches a tissue (tissue perfusion), how easily the drug crosses capillary walls and cell membranes (permeability), and the drug’s chemical properties, especially lipophilicity and molecular size. Highly perfused organs like the brain, liver, and kidneys tend to receive the drug quickly, while fat or bone may take longer. The fraction of drug bound to plasma proteins is pharmacologically inactive until it dissociates, so only the unbound (free) portion can distribute into tissues. The concept is often summarized by the volume of distribution, which describes how extensively a drug disperses into body tissues relative to the blood. Absorption describes entry of the drug into circulation, elimination describes removal from the body, and active transport is a mechanism a drug might use to cross membranes—not the overall movement from blood into tissues.