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Cerebrospinal fluid (CSF) is a clear liquid that is around and within the organs of the central nervous system. Estimates are that there is approximately 125 mL to 150 mL of CSF in the body at any given time. However, it is important to note that there is continuous generation and reabsorption of CSF. Depending on the rate of production and absorption (which varies individually), the supply of CSF can be replaced about every 7.5 hours. Most of this fluid is produced in the ventricles of the brain by the choroid plexus—however, the ependymal cells, which line the ventricles, produce a smaller portion. After production, the fluid travels through the ventricles and then around the brain and spinal cord. It is then reabsorbed directly into the blood through structures in the arachnoid mater called arachnoid villi (arachnoid granulations). Importantly, this fluid can be examined clinically through a lumbar puncture. With a lumbar puncture, physicians can look for abnormalities in the CSF, which can be helpful when creating a differential diagnosis.