Distribution: membranes, protein binding and barriers
Explains which drugs reach the brain (and a fetus), and why.
Where this fitsConnects to 2 · Unlocks 1
- Connects to
- Glia and the blood-brain barrier — Glia (astrocytes) build the blood-brain barrier.
- Making neurotransmitters: precursors and rate-limiting steps — Transmitters can't cross, which is why the brain makes its own.
- Unlocks
- Half-life and redistribution — Redistribution into tissues shapes how quickly effects fade.
Orient
Distribution: psychoactive drugs are quickly and evenly diluted and distributed by diffusion through bloodstream once absorbed. Barriers along the way: cell membranes (lipid bilayer, only permeable to small and lipid-soluble molecules), capillary walls, the blood-brain barrier (BBB) and the placental barrier.
Course terms
Your professor's wording, word for word from the slides. Learn the term and this definition — the explanation underneath is only there to help it make sense.
- Cell membrane
“Only permeable to small and lipid-soluble molecules”Slides 04, p. 22
Made of a phospholipid layer (lipid bilayer); the first barrier to absorption and distribution.
- Capillaries
“Tiny, cylindrical blood vessels branching from arteries and veins”Slides 04, p. 23
Pores let most drugs through, but drugs bound to plasma proteins cannot pass.
- Blood-brain barrier (BBB)
“Capillaries in brain do not allow chemicals (including drugs) to pass as easily as capillaries in rest of body”Slides 04, p. 24
Produced and maintained by glial cells (astrocytes).
- Passive transport
“Small, uncharged/nonpolar, lipid-soluble molecules passively diffuse through barrier”Slides 04, p. 25
Charge/polarity is why neurotransmitters can't cross the BBB.
- Active transport
“Channels and proteins that allow for chemicals that don’t passively transport to enter the brain”Slides 04, p. 25
Needed for glucose and some amino acids.
- Placental barrier
“This membrane system is unique in its role of separating two distinct human beings”Slides 04, p. 27
Easier to cross than the BBB; fetal blood level ≈ mother's.
- Teratogens
“Some drugs can cause birth defects - teratogens”Slides 04, p. 27
e.g., fetal alcohol syndrome.
From your course: Course slides 04, pp. 21–27
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Full lesson: Distribution, membranes and protein binding · Blood-brain and placental barriers