Certain dyes, when injected intravenously, fail to stain the parenchyma of brain and spinal cord, although they pass easily into the non-nervous tissues. However, the same dyes, when injected into the ventricles, enter the brain substances easily. This indicates that a barrier exists at the capillary level between the blood and nerve cells. In simple terms, the blood brain brain barrier can be defined as the separation of the circulating blood and the brain extracellular fluid in the Central Nervous System (CNS).
This means that substances present in the circulating blood cannot all pass through the blood vessels into the brain extracellular fluid. The possible structures constituting the blood-brain barrier are as follows.
(a) Capillary endothelium without fenestrations,
(b) Basement membrane of the endothelium.
(c) The end feet of astrocytes covering the capillary walls.
The barrier permits a selective passage of blood contents to the nervous tissue, and thus the toxic and harmful substances are ordinarily prevented from reaching the brain. Thus this barrier is very important for protection of brain against any harmful agents in the blood.