Cerebrospinal Fluid

Describe the physiology of cerebrospinal fluid

CSF is a transcellular fluid in the ventricles and subarachnoid space. ~150ml (2ml/kg) of CSF exist in a normal individual, divided evenly between the head and spinal column.

Functions

• Mechanical Protection
  Due to its low specific gravity, CSF reduces the effective weight of the brain (by a factor of 30) and therefore reduces trauma caused by the acceleration and deceleration of the brain.
• Buffering of ICP
  CSF can be displaced to the spinal subarachnoid and have its rate of reabsorption increased in order to offset an increase in ICP by another space-occupying lesion.
• Stable Extracellular Environment
  Neurons are sensitive to ionic changes in the extracellular environment. Ionic concentrations in CSF are tightly controlled, which ensures stable neuronal activity. Additionally, toxins are actively removed from CSF.
• pH Regulation
  pH of extracellular fluid is important in the control of respiration, and is also tightly regulated.
• Nutrition
  Supply of O₂ and simple sugars and amino acids, and removal of CO₂ occurs occurs in CSF.

Formation

CSF is produced in the choroid plexus (70%) and brain capillary endothelial cells (30%) at a rate of 0.4 ml.min^-1 (500ml.day^-1). It is produced by a combination of ultrafiltration and secretion from plasma:

• Na⁺ is actively transported
  Drives flow of Cl^- ions and water.
• Glucose is transported via facilitated diffusion down its concentration gradient

Factors Affecting Formation

Formation is relatively constant within normal parameters (altering the rate of absorption is the predominant means to control pressure), though it is reduced by:

• Decreased Choroidal Blood Flow
  CPP <70mmhg reduces="" CSF formation.

Contents

* CSF [protein] is variable:

• Highest in the lumbar sac
• Lowest in the ventricles
• Always lower than plasma [protein]
  This means CSF is a poor buffer solution, which increases its sensitivity to derangements in respiratory acid-base status.

In summary:

• [Na⁺] is unchanged
• [Mg²⁺] and [Cl^-] are increased
• Concentrations of everything else is less

Circulation

CSF flow is driven by respiratory oscillations, arterial pulsations, and ongoing production in the choroidal plexus.

• Production in the choroidal plexus in the lateral ventricles
• To the third ventricle via the Foramen of Munro
• To the fourth ventricle via the Aqueduct of Sylvius
• To the cisterna magna via the two lateral Foramina of Luschka and the midline Foramen of Magendie
• It may now pass either:
  • Cranially, to the basilar cisterns and via the Sylvian fissure to the cortical regions
  • Caudally, to the spinal subarachnoid space via the central canal

Reabsorption

Reabsorption of CSF:

• Occurs in the arachnoid villi, which are located in the dural walls of the sagittal and sigmoid sinuses
  • 85% of reabsorption occurs in intracranial arachnoid villi
  • Remainder by spinal arachnoid villi
• Is predominantly via pinocytosis and opening of extracellular fluid spaces
• Is pressure-dependent
  • Reabsorption occurs when the CSF pressure is 1.5mmHg greater than venous pressure
    Typically an ICP < 7mmHg results in minimal CSF reabsorption. Above this, CSF absorption increases in a linear fashion up to 22.5mmHg.

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.