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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 selective tissue access and distinguish the blood-brain barrier from an absolute shield.
Selective access to brain tissue
The lecture emphasizes that brain capillaries restrict passage more than many other capillaries. Membrane properties and transport mechanisms help determine access. Small, lipid-soluble, uncharged molecules may cross by passive diffusion; other substances depend on specific transport systems. Transport can also move substances away from brain tissue. The barrier is not a guarantee that a drug cannot enter.
Reading the diagrams carefully
Page 26 depicts endothelial cells, tight junctions, transport proteins and an astrocyte. The source's description of the BBB as a CSF barrier produced by astrocytes is an oversimplification: the figure itself places the blood-to-brain interface at the capillary endothelium. The placental discussion similarly introduces transfer rather than an absolute shield. Maternal and fetal concentrations should not be assumed identical for every drug. Pregnancy-specific treatment decisions are not inferred from this diagram.
- Blood-brain barrier
- A selective interface regulating movement between blood and brain tissue.
- Placental transfer
- Movement of substances across the placental interface between maternal and fetal compartments.
Professor deck 04, PDF pages 24, 25, 26, 27. 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.