Between capillaries and interstitial fluid, what structure regulates which substances reach brain tissue?

• A. Blood-brain barrier
• B. Blood-CSF barrier
• C. Dural folds
• D. Choroid Plexus Structures

Answer: (A)

The structure that regulates what moves from brain capillaries into the brain tissue is the blood-brain barrier. It’s built from tight junctions between the endothelial cells lining brain capillaries, reinforced by support from astrocyte end-feet and pericytes. Those tight junctions dramatically limit paracellular diffusion, so most substances must cross endothelial cells through highly selective transport mechanisms. This creates a controlled environment, allowing essential nutrients (like glucose via specific transporters) to enter while keeping out many toxins and fluctuations in ions and other molecules that could disrupt neural function. Lipid-soluble substances can diffuse more readily, but many polar or large molecules require specific transporters or are actively effluxed.

In contrast, the blood-CSF barrier at the choroid plexus regulates exchange between blood and cerebrospinal fluid, not directly with brain interstitial fluid. Dural folds are merely folds of protective dura mater and don’t govern molecular traffic to brain tissue. Choroid plexus structures mainly produce CSF and participate in CSF composition, rather than forming the primary barrier between capillaries and brain interstitial fluid.