question: What is the role of the blood-brain barrier?
answer: The blood–brain barrier (BBB) is a highly selective semipermeable membrane barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). The blood–brain barrier is formed by brain endothelial cells and it allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function.