Cerebrospinal Fluid (CSF): Defination, Structure and Function

Cerebrospinal fluid (CSF) is a clear, plasma-like fluid (an ultrafiltrate of plasma) that bathes the central nervous system (CNS). It occupies the central spinal canal, the ventricular system, and the subarachnoid space.

Cerebrospinal Fluid (CSF):

Cerebrospinal fluid (CSF)

• CSF performs vital functions including: Support; Shock absorber; Homeostasis; Nutrition; Immune function.
• Adult CSF volume is estimated to be 150 ml with a distribution of 125 ml within the subarachnoid spaces and 25 ml within the ventricles.
• Ependymal cells, which create cerebral spinal fluid (CSF), line the ventricles of the brain and central canal of the spinal cord
• CSF is predominantly secreted by the choroid plexus with other sources playing a more poorly defined role, an adult produces between 400 to 600 ml per day.
• The constant secretion of CSF contributes to complete CSF renewal four to five times per 24-hour period in the average young adult.
• Reduction of CSF turnover may contribute to the accumulation of metabolites seen in aging and neurodegenerative diseases. The composition of CSF is closely regulated, and any variation can be used for diagnostic purposes
• Out-pouching of the arachnoid mater (arachnoid granulations) are responsible for the resorption of  CSF into the dural venous sinuses. 
• Disequilibrium in synthesis and resorption or obstruction of circulation results in  CSF accumulation and raised intracranial pressure called hydrocephalus.
• CSF be examined clinically through a lumbar puncture. With a lumbar puncture, physicians can look for abnormalities in the CSF, which can be helpful when creating a differential diagnosis

Structure and Function

1. Support - The CSF supports the weight of the brain estimated at 1500 gm and suspends it in neutral buoyancy to a net weight of about 25 gm. Hence, the entire brain density is cushioned, protecting it from crushing into the bony cranium.
2. Shock absorber - It protects the brain from damage during head trauma. Otherwise, even minor head bopping will result in severe brain injury.
3. Homeostasis - The biochemical constituents and volume of the CSF play vital cerebral homeostatic roles:

• Maintains stable intrinsic CNS temperature
• Biochemical constituents and electrolytes maintain the osmotic pressure responsible for normal CSF pressure which is essential to maintaining normal cerebral perfusion
• Biochemical waste products diffuse into the CSF and are removed as CSF is resorbed through arachnoid granulations into venous circulation, a small percentage of CSF also drains into lymphatic circulation.

4. Nutrition - The CSF contains glucose, proteins, lipids, and electrolytes, providing essential CNS nutrition.

5. Immune function - The CSF contains immunoglobulins and mononuclear cells.

The 5 minute video below is a great summary of the CSF

Ventricles and CSF

• CSF is produced mainly by a structure called the choroid plexus in the lateral, third and fourth ventricles.
• CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro).
• The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the Aqueduct of Sylvius).
• CSF then flows into the subarachnoid space through the foramina of Luschka (there are two of these) and the foramen of Magendie (only one of these).
• Absorption of the CSF into the blood stream takes place in the superior sagittal sinus through structures called arachnoid villi.
  

When the CSF pressure is greater than the venous pressure, CSF will flow into the blood stream. However, the arachnoid villi act as "one way valves"...if the CSF pressure is less than the venous pressure, the arachnoid villi will NOT let blood pass into the ventricular system.

Blood Supply and Lymphatics

The choroid plexus is a group of fenestrated blood capillaries located in the ventricular system. The choroid plexus mostly synthesizes CSF.

Arachnoid granulations are responsible for CSF resorption; they drain CSF into the dural venous sinuses.

CSF drains into the lymphatic circulation, via lymph ducts contiguous to the olfactory duct, as it passes through the cribriform plate.

Clinical Significance

Hydrocephalus is a pathological condition of abnormal accumulation of CSF caused by increased CSF production, blockage of flow, or decreased absorption. The ventricles distend in order to accommodate elevated CSF volumes, potentially causing damage to the brain by pressing its tissue against the boney skull. Hydrocephalus may be congenital or acquired.

CSF Leak is a condition in which CSF is able escape from the subarachnoid space through a hole in the surrounding dura. The volume of CSF lost in a leak is very variable, ranging from insignificant to very substantial amounts.

Syringomyelia due to blockage of CSF circulation.

Meningitis is a condition in which the coverings of the brain become inflamed. There are two classifications of meningitis: aseptic and bacterial. Aseptic meningitis can be caused by agents such as fungi, medications, and cancer metastasis, but the majority of aseptic meningitis cases are caused by viruses. Fever, nuchal rigidity, and photophobia are classic presenting symptoms. Diagnosis is made via an analysis of CSF obtained through LP.

Subarachnoid Hemorrhage (SAH) is the leakage of blood into the subarachnoid space where it mixes with the CSF. SAH is most commonly caused by trauma with 80% of nontraumatic SAHs being caused by aneurysm rupture. Other nontraumatic causes of SAH include arteriovenous malformations and vasculitis.

Lumber Puncture and CSF Analysis - Lumbar puncture is a sterile procedure, done to obtain CSF samples for diagnostic purposes. It involves passing a needle into the subarachnoid space at the levels between L2 and L5 vertebrae. However, most commonly lumbar puncture is performed between L4 and L5. Biochemical, microbiologic and cytologic studies are then carried out on the sample.

Conclusion

Cerebrospinal fluid (CSF) plays an essential role in maintaining the homeostasis of the central nervous system.

The functions of CSF include: (1) buoyancy of the brain, spinal cord, and nerves; (2) volume adjustment in the cranial cavity; (3) nutrient transport; (4) protein or peptide transport; (5) brain volume regulation through osmoregulation; (6) buffering effect against external forces; (7) signal transduction; (8) drug transport; (9) immune system control; (10) elimination of metabolites and unnecessary substances; and finally cooling of heat generated by neural activity.

For CSF to fulfil these functions, fluid-like movement in the ventricles and subarachnoid space is necessary.

Abnormalities in the CSF flow or introduction of infection and or irritants can seriously affect an individuals functioning.