What protective filtering system limits which substances can pass from the blood into the human brain?

The blood–brain barrier is the protective filtering system that limits substances moving from blood into brain tissue.

It is formed mainly by specialized endothelial cells lining brain capillaries. These cells are joined by unusually tight junctions, reducing the movement of many molecules between them. Supporting structures, including pericytes, astrocyte end-feet, and the vessel basement membrane, help maintain the barrier’s properties.

The barrier protects neurons from toxins, pathogens, and sudden chemical changes in the bloodstream. It also creates a major challenge for medicine: many potentially useful drugs cannot easily reach the brain. Small lipid-soluble molecules and certain gases cross relatively readily, while glucose and some amino acids require specific transport systems.

The blood–brain barrier is not equally tight everywhere. Regions called circumventricular organs have more permeable capillaries because they monitor blood chemistry or release hormones. Inflammation, infection, tumors, trauma, and some neurological diseases can disrupt the barrier.