The blood-brain barrier is a selective transport system, not a wall

What the barrier is physically made of

The barrier is formed by the cells lining brain capillaries, which are joined by protein complexes that seal the spaces between them. In most of the body those spaces are comparatively open, allowing dissolved substances to pass between cells without crossing any membrane. Sealing that route means anything entering brain tissue must pass through the lining cells themselves rather than around them.

Supporting cells including specialised glia and pericytes wrap the vessels and contribute signals that maintain the sealed state. The barrier is therefore a property of a whole cellular assembly rather than a discrete structure existing independently of the cells that form and maintain it.

Transporters do the actual admitting

Because the passive route is closed, the brain depends on transport proteins that move specific molecules across the lining cells deliberately. Glucose and particular amino acids each have dedicated transporters, and the capacity of those transporters sets the supply available to brain tissue. Small lipid-soluble molecules can diffuse through membranes directly, which is why some drugs enter readily while chemically similar ones do not.

Larger molecules including many proteins require routes involving vesicles that carry cargo across the cell and release it on the far side. Designing anything to reach brain tissue therefore means designing for a transporter or for a vesicle route rather than for permeability generally.

Pumps that remove what gets in

The lining cells carry efflux pumps that recognise a broad range of molecules and return them to the bloodstream after entry. These pumps explain why some compounds that should cross easily on chemical grounds nonetheless reach only low concentrations in brain tissue. The same pumps are a major obstacle in developing treatments for conditions of the nervous system, since they remove candidate drugs efficiently.

Their broad specificity means they also handle many dietary and environmental compounds, which is presumably the function they evolved for. Any claim that a substance crosses the barrier needs to account for efflux as well as for entry, and most such claims do not.

Regional variation and regulated openness

Certain small brain regions lack the sealed arrangement deliberately, because they need to sample the composition of circulating blood. These regions participate in functions including hormonal feedback and the detection of substances that trigger nausea. Barrier properties also change under specific physiological conditions, and that regulated variation is part of normal function rather than failure.

Increased permeability is documented in several disease states, and whether it initiates the process or results from it varies by condition. Reporting barrier changes as uniformly harmful therefore misrepresents a system that adjusts its permeability as part of ordinary operation.

Measurement and the limits of claims

Barrier integrity in people is assessed indirectly, through imaging with contrast agents or through markers found in cerebrospinal fluid. Both methods detect substantial disruption reliably and are much less sensitive to the subtle changes often discussed in popular writing. There is no consumer test for barrier function, which makes any personal claim about having a leaky or an unusually tight barrier unverifiable in practice.

Marketing frequently asserts that an ingredient crosses the barrier, and such assertions are usually inferred from chemistry rather than measured. Symptoms affecting cognition or the nervous system need clinical assessment, since the conditions that cause them are diagnosed by examination and testing.