Learning objective
Explain why drug in blood is not the same as drug at a receptor, and why a falling blood level need not mean excretion.
Moving among compartments
Distribution is the exchange of drug between blood and tissues. Blood flow, membrane permeability, lipid solubility and reversible binding affect where drug goes. A plasma sample measures one compartment, not every tissue. A rapid early decline in plasma concentration can reflect redistribution into tissues even before much drug has left the body.
Free and bound drug
Drug can bind reversibly to plasma proteins. The unbound fraction is generally available for membrane passage and filtration; a bound molecule can become available after dissociating. Do not treat binding as chemical destruction or assume that a high total concentration guarantees a high free concentration at the target.
Barriers are selective
Cell membranes have a lipid interior and embedded proteins. At the blood-brain barrier, tightly joined endothelial cells limit passage between cells. Supporting glia help maintain this environment. Passive permeability, carrier-mediated transport and efflux influence entry; lipid solubility alone does not guarantee unrestricted brain access. The placenta permits transfer of many drugs and is not a universal shield. Transfer does not by itself establish a particular fetal effect.
Check your understanding
Supplemental knowledge check
Original PolyDiscipline practice based on the explanation and cited sources. Unscored; answers are temporary to this page visit and do not award mastery or XP.
Supplemental knowledge check
Original PolyDiscipline practice based on the explanation and cited sources. Unscored; answers are temporary to this page visit and do not award mastery or XP.
Explain it without notes
Why is the blood-brain barrier more than a wall of fat?
Compare your explanation 1
Tight endothelial junctions, membrane properties, transport systems and efflux all matter. Glial support is part of the environment, not a replacement for endothelial junctions.
Distinguish protein binding, redistribution and metabolism.
Compare your explanation 2
Binding is reversible association, redistribution is a change of compartment, and metabolism chemically transforms the molecule.
Revisit the connected course lessons and visuals
Julien’s Primer of Drug Action, supplied consolidated PDF pages 12, 13, 14, 15, 18 (Chapter 1). Locators refer to PDF captures, not printed pages. Original paraphrase and practice; source conformance reviewed, not independently clinically certified.