Blood–Brain Barrier

Abstract The blood–brain barrier consists of endothelial cells lining brain capillaries. It serves to restrict and control the movement of substances between the general circulation and brain extracellular fluid. It participates in regulating the volume and composition of fluid surrounding the brain through specific transport processes, and thus contributes to homoeostasis of the central nervous system. Some of these processes may be regulated hormonally, or modulated by adjacent cells including astrocytes. The barrier function of the blood–brain barrier is due to: (1) tight junctions that restrict movement of substances between the endothelial cells, (2) specific transport proteins that determine which substances can cross the barrier transcellularly and (3) enzymes that may degrade or alter substances prior to passage. Systemically administered drugs intended to treat neurological disorders must be designed to bypass the restrictive elements of the blood–brain barrier. Pathological conditions associated with the central nervous system may alter blood–brain barrier function. Key Concepts: The blood–brain barrier regulates brain extracellular fluid. Brain capillaries form a tight barrier except in specialised areas. Tight junctions restrict paracellular movement of substances across the blood–brain barrier. Astrocytes contribute to differentiation of the blood–brain barrier. Transport across the blood–brain barrier may be passive or active. Enzymes contribute a metabolic barrier to the blood–brain barrier. A strategy must be developed to deliver drugs to the brain. Altered blood–brain barrier function in disease. Structural and functional properties of the blood–brain barrier.