The BBB "Blood Brain Barrier” results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restricts the passage of solutes.
At the interface between blood and the brain, endothelial cells are stitched together, by these tight junctions, which are composed of smaller subunits, frequently biochemical dimers, that are transmembrane proteins such as occludin, claudins, junctional adhesion molecule (JAM), or ESAM, for example.
The blood-brain barrier is composed of high-density cells restricting passage of substances from the bloodstream much more than endothelial cells in capillaries elsewhere in the body.
Several areas of the human brain are not "behind” the BBB. These include the circumventricular organs.
One example of this is the pineal gland, which secretes the hormone melatonin "directly into the systemic circulation” as this hormone can pass through the blood-brain barrier.
Recover-Me ”piggy backs” on the substances Palatinose (Isomaltulose) through the BBB and thus in to the brain.