Blood-brain barrier

The blood-brain barrier is the filter that separates the blood from the brain. It works like a highly selective membrane: the brain's blood vessels use it to regulate which molecules and ions pass between the blood and the brain.

This barrier is located in the brain's capillaries, very thin blood vessels. On the inside, their wall is made of endothelial cells, and those cells are joined to one another by tight junctions. Around those junctions are supporting structures: pericytes, cells that partly cover the outside of the capillary; astrocytes, which are cells of the nervous system itself; and the basal lamina, a supporting layer. Picture a tile wall with sealed grout lines: less seeps through between the tiles than through an ordinary wall. In the same way, the tight junctions and what surrounds them help make passage through these capillaries more restricted than in any other capillary in the body.

Even so, some substances cross easily. Gases such as oxygen and small lipid-soluble molecules, the ones that dissolve in fat, pass through freely. By contrast, glucose, which is a sugar, and amino acids, the building blocks of proteins, need transporter proteins to carry them from one side to the other.