Which features characterize CNS capillaries that reduce drug passage?

• A. Fenestrated capillaries
• B. Non-fenestrated capillaries with astrocyte support
• C. Abundant pores
• D. No tight junctions

Answer: (B)

The blood-brain barrier is defined by continuous, non-fenestrated capillaries whose endothelial cells are joined by tight junctions, plus astrocyte end-feet that envelop the vessels and reinforce barrier properties. This setup minimizes paracellular diffusion and regulates transcellular transport, making it harder for many drugs to enter the CNS. Non-fenestrated endothelium means there are no openings between cells for substances to pass, and the astrocyte support helps maintain the tight junctions and restrict vesicular transport.

If capillaries were fenestrated or had abundant pores, or if there were no tight junctions, many substances could leak through, increasing CNS exposure.