Abstract
T he brain is choosy about what it allows to enter its tissues. Because it can function only in a chemically stable environment, the brain shuts out most water-soluble substances in the blood-even some potentially helpful medications. Three structures stand as gatekeepers to the brain's interior: the network of cere bral capillaries, the arachnoid mem brane that covers the brain's surface and the diffuse, highly vascularized tissue called the choroid plexus. The choroid plexus and the arach noid membrane act together as barri ers between the blood and the cere brospinal fluid (CSF), a watery broth of nutrients, ions and other essential molecules. The CSF bathes the exterior of the central nervous system and fills the ventricles, the four large cavities inside the brain. Because the CSF ex changes substances freely with the interstitial fluid that surrounds the brain's neurons and supportive glial cells, the blood-CSF barrier is vital for keeping dangerous compounds out of the brain. The arachnoid membrane is generally impermeable to water-solu ble substances, and its role in forming a blood-CSF barrier is largely passive.
Published Version
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