Describe the physiology of cerebrospinal fluid
- 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.
Supply of O2 and simple sugars and amino acids, and removal of CO2 occurs occurs in CSF.
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:
* 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.
- [Na+] is unchanged
- [Mg2+] and [Cl-] are increased
- Concentrations of everything else is less
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 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
- Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
- Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.