Explore the system
What can cross a membrane?
Compare an ordinary capillary with the blood-brain barrier. Change molecular properties and predict the route.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 1 · What can cross a membrane? · PDF 13, 14 · 1.5, 1.6, 1.7
Course notes and existing practice
Learning objective
Explain why circulation, membrane passage and protein binding influence tissue exposure.
Blood is a transport compartment
After systemic entry, drug can move between blood and tissues while absorption and elimination continue. Distribution is not instantaneous uniform mixing throughout the body. Tissue blood flow, membrane properties and binding help determine where drug is found and how rapidly concentrations change.
Bound and unbound drug
The deck contrasts movement across lipid membranes with passage through capillary structures and emphasizes plasma protein binding. A drug bound to a large plasma protein does not simply pass out as if it were a free small molecule. Binding is reversible, so bound and unbound fractions can exchange over time. Brain capillaries require separate treatment rather than extending a general capillary-pore description to every tissue.
- Distribution
- Movement of drug between blood and body tissues.
- Plasma protein binding
- Reversible association of a drug with proteins in plasma, distinguishing bound from unbound drug.
Professor deck 04, PDF pages 21, 22, 23. Course material is paraphrased. Qualifications and excluded clinical generalizations are documented in the Pharmacokinetics editorial record. Source review does not establish clinical validation.
Examples and practice explanations are original study aids, not professor quotations, exam requirements or medication instructions.