Which of the following is NOT a factor that lowers blood-brain barrier permeability?

• A. Lack of pores between capillaries
• B. Endothelial cells contain ATP-dependent transport
• C. Capillaries are surrounded by astrocytes
• D. High protein concentration in interstitial fluid

Answer: (D)

Permeability of the blood-brain barrier is kept low by several protective features: tight endothelial junctions that eliminate paracellular routes, ATP-dependent transport systems in the endothelial cells that actively regulate and often export substances, and astrocyte end-feet encasing the capillaries to maintain and reinforce the barrier. The lack of pores between capillary endothelial cells directly reduces leakage between cells, while ATP-driven transporters curb entry of many compounds by actively moving them back into the blood. Astrocytes provide signaling and support that help sustain the tight barrier properties, further lowering permeability.

High protein concentration in the interstitial fluid, however, is not a mechanism that lowers BBB permeability. In fact, elevated interstitial protein often indicates barrier disruption and increased permeability, not protection. So that factor does not contribute to reducing permeability.