Cerebrospinal fluid (CSF) is a clear liquid, which is very similar to the liquid portion of blood (plasma). It contains various salts and products such as sodium, calcium, bicarbonate, chloride, magnesium and glucose. CSF is a medium for the transport of chemicals to and from the brain, as well as providing buoyancy and protection for the brain.
CSF is produced by the choroid plexus, the parenchyma, and the ventricular wall. These structures are capable of producing 400-600 cc's of fluid a day, which is enough to completely fill the fluid spaces of the nervous system 4 times over.
This fluid then circulates through the ventricles exiting the fourth ventricle through the Foramin of Magendie and the Foramina of Luschka. These empty into the cisterna magnum and the brain-stem cisterns. From here, the fluid flows into the subarachoid fluid space (around the brain and spinal cord). Eventually it is reabsorbed back into the blood over the surface of the brain. The CSF then reaches structures (arachoidal villi) along the superior, mid-line surface of the brain where it is reabsorbed back into the blood vessels (the sagital sinus).
This orderly cycle of spinal fluid production, flow, and absorption maintains a protective environment for the nervous system. In addition, there is no loss of fluids or of the salts contained in the spinal fluid.
There are several key passageways through which the CSF must pass in order to exit the ventricular spaces and to reach the subarachnoid spaces. First, each of the two lateral ventricles has an outlet into the third ventricle called the foramen of Monroe. The third ventricle in turn has an outlet (the aqueduct of Sylvius or aqueduct) to the fourth ventricle. Finally, the fourth ventricle has three outlets, the foramen of Magendie and the paired foramena of Luschka. Additionally, the subarachnoid space has a potential blockage point for the flow of CSF to the arachnoidal villi, at an opening in the tent-like structure that divides the upper and lower parts of the brain (the tentorial notch). Distortion or enlargement of the brain in the region of this opening can compress the subarachnoid space, preventing fluid from flowing up to the arachnoidal villi.