What determines whether a substance can diffuse across the blood-brain-barrier?

Question

What determines whether a chemical substance is able to cross the blood-brain-barrier via passive, transmembrane diffusion? What structurally differentiates these chemicals?

Answer 1

Only small, uncharged substances can pass through the membrane. Because the membrane has a hydrophobic region, charged and polar molecules can't pass through. Non-polar molecules or uncharged molecules can pass through the membrane without help from a membrane protein. They must also be small though. Good examples are CO₂ and O₂.