A semipermeable membrane that serves as a selective barrier separating the circulating blood and the extracellular fluid in the central nervous system
The blood-brain barrier, as the name implies, is the barrier in the brain that separates the blood from the extracellular fluid in the central nervous system. The three major components of this barrier are endothelial cells, tight junctions, and the astrocytic endfeet. Unlike the endothelial cells lining the blood vessels in other parts of the body, the endothelial cells of the blood vessels in the brain do not have spaces in between them. There are tight junctions that stitch these cells together, thereby preventing the easy passage of substances. The tight junctions are transmembrane proteins (e.g. occluding, claudins, junctional adhesion molecule). The astrocytic endfeet also referred to as glia limitans are astrocyte projections. They surround the endothelial cells. Essentially, the blood-brain barrier allows the passage of water, oxygen, carbon dioxide, glucose, lipid-soluble molecules, and other non-ionic solutes across by passive diffusion. Glucose and amino acids important to neurons may pass through the barrier through other transport mechanisms.