Cerebrospinal fluid

Definition The serum like fluid that circulates through the ventricles of the brain, the cavity of the spinal cord, and the subarachnoid space Since the brain floats in CSF, the fluid acts in effect to reduce the weight of the brain from some 1000 g to about 50 g, and also protects the brain from knocks on the head

The subarachnoid space on the outside of the brain is in continuity with a similar space around the spinal cord and also with the series of interconnected cerebral ventricles within the brain (see figure). Each of the paired lateral ventricles, in the cerebral hemispheres, contains a leaf-like, highly vascular choroid plexus. It is from these structures that the bulk of the CSF is secreted.

From the lateral ventricles CSF drains into the central third ventricle, and thence through the aqueduct in the midbrain into the fourth ventricle. From the fourth ventricle, the CSF exits into the subarachnoid space through several openings, and fills the ‘basal cisterns’ beneath the brain. hence the flow of CSF is mainly up and over the whole brain surface, whilst some flows down around the spinal cord. Completing the circuit back to the bloodstream

Secretion and volume: The secretion of CSF is an active transport process that moves fluid and solutes from the blood plasma into the ventricles, the choroid plexuses being a specialized part of the blood-brain barrier. CSF secretion involves the pumping of ions and specialized ion channels, with the energy coming from glucose and oxygen in the blood. In the adult human CSF is formed at a rate of about 0.5 ml/min; the total volume is about 200 ml, of which 30 ml is in the ventricles and the remainder in the subarachnoid space. The circulation of CSF leads to the fluid being completely replaced about every 4 hours.

Composition and appearance: CSF is a weak salt solution with similar inorganic ion concentrations to plasma (Ca2+, Na+, Cl , K+, and Mg2+), but with small and significant differences whereas the protein content is about 100 times less than that of plasma (0.5 g/liter compared to 50-70 g/liter).

Glucose concentration is also lower than that of blood 50-85 mg/dl, it’s the major source of energy for the brain. CSF is normally a clear, amazingly ‘bright’ fluid, and if it is cloudy or contains a raised level of protein or traces of blood this is usually an indication of brain infection, some types of brain or spinal cord tumour, or trauma.

Function 1. The major function of the CSF is to cushion the brain within its solid vault. 2. It carries nutrients to the brain and the spinal cord. 3. The CSF also acts as a drainage route for waste products of brain metabolism.

Diagnosis and therapy: Abnormalities of the CSF can be important in diagnosis of some medical conditions; the fluid can be sampled by lumbar puncture from the extension of the subarachnoid space (the lumbar sac) below the lower end of the spinal cord. usually between the third and fourth lumbar vertebrae Detecting the levels of protein and glucose in CSF may be extremely beneficial in the diagnosis of subarachnoid hemorrhage and central nervous system infections (such as meningitis). A cerebrospinal fluid culture examination may yield the microorganism that has caused the infection.  

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