The blood–brain barrier (BBB) is a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS).
What It’s Made of. Certain molecules are able to get past the blood-brain barrier, such as the glucose and oxygen that is so important for the brain’s survival. Certain cells such as macrophages, which are important in fighting infections, can also pass through the blood-brain barrier.
Technically, the BBB is a shorthand for the Blood-CNS barrier, which has two parts: the Blood-Brain portion and the Blood-Spinal Cord Barrier portion.