Our brain is connected to the rest of our body through countless blood vessels, however unlike other organs and regions of the body not everything that is in the blood, can actually get into the brain, and visa versa. This is called the blood brain barrier. Often people mistake it with the meninges, although the meninges are chemically connected to the rest of the body and medication can freely pass it.
The blood brain barrier exists because many of the molecules used in our body for signalling processes between tissues are the same molecules used in the brain for signalling. If there would not be a blood brain barrier, then molecules in our blood could freely permeate the brain and essentially “mess up” the messages being relayed within neural pathways.
The blood brain barrier has proven useful not only to keep our “body messages” separated from “brain messages”, but also to keep toxins and drugs out of our brain. This does pose a problem though for medicating people who have neurological disorders, such as Parkinson’s, Alzheimer’s or depression, where you do want drugs to permeate the blood brain barrier and diffuse into different areas of the brain itself.
This means that medication being designed to reach into the brain has to be made such that is actually can pass this barrier.
There are a few exceptions to the “coverage” of the blood brain barrier: certain areas of the brain are not separated from the blood stream. These are generally areas that either secrete into the bloodstream, or “sample” from it to determine salt concentrations and the presence of potential toxins.
Areas and glands not protected by the blood brain barrier are the pineal gland, the pinituary gland, the median eminence of the hypothalamus and the area postrema. The latter one is a chemical trigger zone that will make you sick if a toxin or poison is detected in your blood stream.