A cortical microvessel stained for blood–brain barrier protein ZO-1
The blood–brain barrier (BBB) is a highly selective permeability barrier. It controls what gets from the bloodstream into the brain, and what does not.
For example, things that the brain needs to survive (water, glucose, and amino acids) can get through the barrier. However, the barrier stops many harmful things, like bacteria and viruses, from getting into the brain. This helps protect the brain from getting an infection.
The blood-brain barrier is formed by capillary endothelial cells. It allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion. It also allows the selective transport of molecules such as glucose and amino acids which are crucial to nerve function. On the other hand, the blood-brain barrier may prevent the entry of neurotoxins by means of an active transport mechanism.
A few small areas in the brain do not have a blood-brain barrier.