Cerebrospinal fluid (CSF) is a clear fluid that surrounds the brain and spinal cord. It is produced by choroid plexuses in the ventricles and absorbed through arachnoid villi. The blood-brain barrier (BBB) protects the brain by preventing many substances in blood from entering brain tissue while allowing important nutrients to pass through. It is formed by tight junctions between endothelial cells in brain capillaries and surrounding astrocyte foot processes. Together, CSF and the BBB help maintain a stable environment for the brain.