Cerebrospinal fluid (CSF) is a clear, watery fluid that surrounds the brain and the spinal cord. It is an ultrafiltrate of blood plasma and is contained within the subarachnoid space and the central canal of the spinal cord. In this article, we will explore the production and drainage of CSF as well as its numerous functions. Further information on the anatomy of the CSF production and drainage can be found here.
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 covers two lateral ventricles, and the roof of the third and fourth ventricles. Around 500 ml of CSF is produced each day, with around 150 ml being present in the body at any given time.
The choroid plexus is composed of fenestrated capillary loops, covered by a layer of specialised ependymal cells. The blood plasma freely permeates through the capillary loops. However, a barrier exists at the ependymal cells as they are connected by tight junctions. Hence, the ependymal cells form the blood–cerebral spinal fluid barrier, regulating the composition of CSF. Several different mechanisms play a role in the transport of ions and micronutrients via ependymal cells, including active and passive transport. Additionally, as water follows the osmotic gradient created by the active transport of ions, blood plasma and CSF have approximately the same osmolarity.
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, small protrusions of arachnoid matter into the venous sinus. Physiologically, the pressure of CSF within the subarachnoid space is greater than that within the venous sinus. Hence, the CSF will drain into the venous sinuses. Interestingly, this is achieved via the creation of giant CSF-containing vacuoles in the arachnoid cells.
Further information on the anatomy of the CSF production and drainage 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 caused by the brain hitting the cranium.
- Homeostasis – regulates the distribution of metabolites surrounding the brain, keeping the external environment stable.
- Clearing waste – waste products produced by the brain cells are excreted into the CSF, which then drains into the bloodstream.
Clinical Relevance – Hydrocephalus
Hydrocephalus is an abnormal increase in the volume of CSF within the ventricular system. This increase in volume results in elevated pressure within the cranium, which can cause irreversible damage to the brain tissue. There are two types of hydrocephalus: communicating (non-obstructive) and non-communicating (obstructive).
Communicating (non-obstructive) hydrocephalus is caused by the imbalance in CSF production and absorption. There is either the failure of absorption or increased production of CSF. The enhanced production of CSF may be caused by tumours of the choroid plexus but it is a very rare occurrence. More commonly, communicating hydrocephalus results from CSF not being reabsorbed into the dural venous sinuses as the function of arachnoid villi is impaired. This can be attributed to the subarachnoid haemorrhage or meningitis, causing scarring and fibrosis of the subarachnoid space.
Non-communicating (obstructive) hydrocephalus is caused by an obstruction of CSF outflow. These obstructions are brought by multiple different pathophysiologies such as congenital malformations, tumours, or vascular malformations, and are most likely to occur at the narrow points such as the interventricular foramen, cerebral aqueduct, medial foramen of Magendie, or the lateral apertures of the fourth ventricle. The obstruction leads to the dilatation of the ventricular system superior to the obstruction.
Management of hydrocephalus involves placing a shunt from the ventricles to the abdominal cavity (ventriculoperitoneal shunt). It is a surgical procedure and therefore carries the risk of infection. However, it is the only long-term treatment available.