Cerebrospinal fluid is a clear, watery fluid that surrounds the brain and the spinal cord. It is an ultra-filtrate of blood plasma and is contained within the subarachnoid space and the central canal of the spinal cord.
Contents of CSF
|Osmolarity||295 mOsm/L||295 mOsm/L|
|Glucose (fasting)||2.5 – 4.5 mmol/L||3.0 – 5.0 mmol/L|
|Protein||200 – 400 mg/L||60 – 80 g/L|
|Sodium||144 – 152 mmol/L||135 – 145 mmol/L|
|Potassium||2.0 – 3.0 mmol/L||3.8 – 5.0 mmol/L|
|Chloride||123 -128 mmol/L||95 – 105 mmol/L|
|Calcium||1.1 – 1.3 mmol/L||2.2 – 2.6 mmol/L|
|Urea||2.0 – 7.0 mmol/L||2.5 – 6.5 mmol/L|
Flow of CSF
The CSF is produced by the choroid plexus which can be found in the two lateral ventricles, and in the roof of the third and fourth ventricles. Around 500 ml is produced each day, with around 150-250 ml being present in the body at any one time.
The choroid plexus is composed of a fenestrated endothelium, a pial layer and a layer of specialised ependymal cells. The blood plasma is filtered through the fenestrated endothelial layer, only allowing passage for certain substances. This is followed by active transport of substances through the ependymal cells. Some substances get transferred by passive transport and diffusion of water. This allows osmolarity to be maintained.
CSF is produced continuously which keeps the fluid in circulation around the central nervous system. The fluid will move from the lateral ventricle to the third and then to the fourth ventricle. From the fourth ventricle, the fluid moves out into the subarachnoid space and/or the central canal of the spinal cord through the two lateral foramina of Luschka and the medial foramen of Magendie.
CSF gets drained into the superior sagittal venous sinus through the arachnoid villi. The pressure gradient between the subarachnoid space and the venous sinus results in the fluid moving through the arachnoid villi.
Further information on the anatomy of the ventricles and drainage of CSF can be found here.
Functions of CSF
The CSF has many functions:
- Buoyancy – the brain weighs ~1400g, but due to the presence of CSF creating a bath, it only has a net weight of 50g. The brain otherwise is only supported within the arachnoid space by blood vessels and nerve roots which are fragile structures.
- Protection – CSF acts as a shock absorber preventing damage from occurring to the brain when the cranium is jolted/hit.
- Homeostasis – regulates the distribution of metabolites surrounding the brain keeping the environment ideal to prevent any damage to the nervous system.
- Clearing waste – waste products produced by the brain move into the CSF which then clears out through the arachnoid granulations into the venous sinus so it can be absorbed into the bloodstream.
Clinical Relevance – Hydrocephalus
Hydrocephalus is an abnormal increase in volume of CSF within the ventricular system. This increase in volume results in increased pressure within the cranium, which can cause irreversible damage to brain tissue. There are two types of hydrocephalus: communicating (non-obstructive) and non-communicating (obstructive).
Communicating hydrocephalus is caused due to CSF not being reabsorbed into the dural venous sinuses due to functional impairment of the arachnoid villi. This can be due to a subarachnoid haemorrhage, meningitis or congenital absence of arachnoid villi resulting in scarring and fibrosis of the subarachnoid space.
Non-communicating hydrocephalus is caused when there is an obstruction of CSF outflow. These obstructions are most likely to occur at narrow points such as the interventricular foramen, cerebral aqueduct, median aperture or the lateral apertures of the fourth ventricle. The outflow obstruction can lead to dilatation of the ventricular system.
Management for hydrocephalus involves placing a shunt from the ventricles to the atria of the heart or the abdominal cavity. It is a surgical procedure and therefore carries the risk of infection but is the only treatment available.