The brain and spinal cord float in cerebro-spinal fluid (CSF), a salty fluid within the meninges. CSF is also present in the central canal of the spinal cord, in the space between the brainstem and cerebellum (fourth ventricle), in the lateral ventricles within each cerebral hemisphere and in channels that connect them (the aqueduct and third ventricle).
The nervous system develops from a tubular structure in the embryo called the neural tube. The central cavity of this tube remains in adults as the central canal of the spinal cord.
Cerebrospinal fluid (CSF) is contained in the ventricles and the subarachnoid space and bathes the brain and spinal cord. The brain effectively floats in CSF and the fluid acts as a cushion, protecting the brain during sudden movements.
The total volume of CSF is 150 mls, and is produced at a rate ogf about 550 mls/day. Consequently it's turnover rate is 3-4 times a day.
CSF is formed in roughly equal amounts by the choroid plexus and the walls of the cerebral ventricles. CSF flows through the ventricular system and is absorbed by the arachnoid villi and to a lesser extent by the cerebral venous system.
The normal intracerebral pressure (ICP) is 5 to 15 mmHg, which depends on the relative rates of secretion and re-absorption.
CSF has a composition different from plasma. Significantly for clinical purposes, the glucose concentration is lower and the chloride concentration is higher than plasma. CSF contains only a little protein; its pH is lower than blood, due to the higher concentrtion of carbon dioxide.
A system of vessels similar to lymphatics has recently been discovered within the brain and these carry fluid out of the neural tissue. They also allow immune cells to enter the brain.
The epithelial cells of the choroid plexus are polarised and carry fluid and ions from the blood stream into the ventricles. The cells provide a barrier because of the tight junctions that unite their cell membranes, and separate the apical and baso-lateral memebranes.
The sodium-potassium pump, K+ channels and the Na+/K+/2Cl cotransporter are expressed in the apical membrane.
The basolateral membrane contains Cl-/HCO3- exchangers, and a variety of co-transporters.
These ion channels and the active sodium pump, together with the tight junctions, allow differences in composition between the interstitial fluid and the CSF.
Many textbooks state that the brain does not have a lymphatic system.
However recent studies have identified vessels that express all the molecular hallmarks of lymphatics: they are present in the dural sinuses and appear to be able to carry fluid and immune cells from the CSF to the cervical lymph nodes.