The Blood-brain Barrier
In the human brain, there are approximately 400-425 miles of capillaries. Because the brain is basically a small neurochemistry factory, which makes our behavior a function of its interior chemical balances, the brain must protect its own chemical integrity by carefully controlling the levels of chemical substances in the brain's blood supply. That safeguard comes by way of a blood-brain barrier (BBB), where entry into the brain is restricted to the familiar chemicals for which the brain has initiated or orchestrated production. This barrier is present in all vertebrate brains.
The blood-brain barrier, which is composed of the lumenal and the ablumenal membranes, develops during the first trimester of human fetal life. However, if a foreign chemical has a molecular structure similar to one of the neurotransmitters used by the brain, the intruding chemical will often enter the brain undetected, which can wreak behavioral havoc. This is how hallucinations and other sensory perception distortions occur.
