The blood-brain barrier (BBB) results from the selectivity of tight junctions between endothelial cells in CNS vessels that restrict the passage of solutes.
At the blood-brain interface, endothelial cells are sewn together by these tight junctions, which are composed of smaller subunits, often biochemical dimers, which are transmembrane proteins such as occludin, claudins, junctional adhesion molecule (JAM) or ESAM.
The blood-brain barrier is composed of high-density cells that restrict the 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.
An example is the pineal gland, which secretes the hormone melatonin "directly into the systemic circulation" because this hormone can cross the blood-brain barrier.
Recover-Me "piggybacks" on the substances Palatinose (isomaltulose) across the BBB and into the brain.