The blood–brain barrier is formed by brain endothelial cells, which are connected by tight junctions with an extremely high electrical resistivity of at least 0.1 ω⋅m.he BBB is distinct from the quite similar blood–cerebrospinal-fluid barrier, which is a function of the choroidal cells of the choroid plexus, and from the blood–retinal barrier, which can be considered a part of the whole realm of such barriers. Several areas of the human brain are not on the brain side of the BBB.