Which statement is true about the blood-brain barrier?

• A. It prevents all substances from crossing.
• B. It restricts passage of many hydrophilic substances while allowing lipid-soluble molecules.
• C. It does not affect pharmacology.
• D. It is identical to systemic capillaries.

Answer: (B)

The blood-brain barrier is a highly selective gate formed by tight junctions between brain capillary endothelial cells that regulate what can enter the brain from the blood. Because of these tight connections, many hydrophilic (polar) substances cannot cross readily, while lipid-soluble (lipophilic) molecules can diffuse through the cell membranes of the endothelium. In practice, small nonpolar molecules and gases diffuse easily, whereas most polar or large molecules require specific transporters. This selective permeability is why the barrier has a major impact on pharmacology: many drugs that are polar or large cannot reach therapeutic concentrations in the brain unless they use dedicated transport systems or are designed to be lipid-soluble. The barrier is not identical to systemic capillaries, which are generally more leaky and lack such tight junctions, underscoring its unique role in protecting neural tissue while shaping how substances access the brain.