The brain is shielded from potentially toxic substances by the presence of a barrier.  The blood brain barrier (BBB) is an anatomical interface that separates the brain from the circulatory system.  The endothelial cells of the blood vessels, perivascular cells (pericytes), astrocytes and neurons form the “neurovascular unit” and regulate effective functioning of the CNS.

Blood Brain Barrier: Authorized Molecules Only

Source: Knowing Neurons