Cerebrospinal fluid (CSF) is the fluid that coats the spinal cord and brain. CSF is contained by two meningeal membranes called the pia mater and the arachnoid mater. This fluid protects the central nervous system from infection and provides a shock-absorbent buffer for impacts that could damage the brain. It is secreted by the choroid plexus, which is the structure enclosing the brain ventricles. CSF samples are taken during the lumbar puncture procedure.