Cerebrospinal fluid and its function

Cerebrospinal fluid (CSF) is the clear, colorless and transparent fluid that circulates through ventricles of

brain, subarachnoid space and central canal of spinal cord. It is a part of extracellular fluid (ECF).

PROPERTIES AND COMPOSITION OF CEREBROSPINAL FLUID

Properties

Volume : 150 mL (100 mL to 200 mL)

Rate of formation : 0.3 mL per minute

Specific gravity : 1.005

Reaction : Alkaline.

Composition

Composition of CSF is given in Figure 163.1. Since CSF is a part of ECF, it contains more amount of sodium than potassium. CSF also contains some lymphocytes. CSF secreted by ventricle does not contain any cell. Lymphocytes are added when CSF flows in the spinal cord.

FORMATION OF CEREBROSPINAL FLUID

SITE OF FORMATION

CSF is formed by choroid plexuses, situated within the ventricles. Choroid plexuses are tuft of capillary

projections present inside the ventricles and are covered by pia mater and ependymal covering. A large

amount of CSF is formed in the lateral ventricles.

MECHANISM OF FORMATION

CSF is formed by the process of secretion that involves active transport mechanism. Formation of

CSF does not involve ultrafiltration or dialysis.

SUBSTANCES AFFECTING THE FORMATION OF CSF

1. Pilocarpine, ether and extracts of pituitary gland stimulate the secretion of CSF by stimulating

choroid plexus

2. Injection of isotonic saline also stimulates CSF formation

3. Injection of hypotonic saline causes greater rise in capillary pressure and intracranial pressure and

fall in osmotic pressure, leading to increase in CSF formation

4. Hypertonic saline decreases CSF formation and decreases the CSF pressure. The increased

intracranial pressure is reduced by injection of 30% to 35% of sodium chloride or 50% sucrose.

CIRCULATION OF CEREBROSPINAL FLUID

Major quantity of CSF is formed in lateral ventricles and enters third ventricle by passing through foramen of Monro. From here, it passes to fourth ventricle through aqueductus Sylvius. From

fourth ventricle, CSF enters the cisterna magna and cisterna lateralis through foramen of Magendie (central opening) and foramen of Luschka (lateral opening). From cisterna magna and cisterna lateralis, CSF circulates through subarachnoid space over spinal cord and cerebral hemispheres. It also flows into

central canal of spinal cord.

ABSORPTION OF CEREBROSPINAL FLUID

CSF is mostly absorbed by the arachnoid villi into dural sinuses and spinal veins. Small amount is absorbed along the perineural spaces into cervical lymphatics and into the perivascular spaces.

The mechanism of absorption is by filtration due to pressure gradient between hydrostatic pressure in the

subarachnoid space fluid and the pressure that exists in the dural sinus blood. Colloidal substances pass

slowly and crystalloids are absorbed rapidly. Normally, about 500 mL of CSF is formed everyday

and an equal amount is absorbed.

PRESSURE EXERTED BY CEREBROSPINAL FLUID

Pressure exerted by CSF in man varies in different position, viz.

Lateral recumbent position : 10 to 18 cm of H2O

Lying position : 13 cm of H2O

Sitting position : 30 cm of H2O

Certain events like coughing and crying increase the pressure by decreasing absorption. Compression of

internal jugular vein also raises the CSF pressure.

FUNCTIONS OF CEREBROSPINAL FLUID

1. Protective Function

CSF acts as fluid buffer and protects the brain from shock. Since, the specific gravity of brain and CSF is

more or less same, brain floats in CSF. When head receives a blow, CSF acts like a cushion and prevents the movement of brain against the skull bone and thereby, prevents the damage of brain.

However, if the head receives a severe blow, the brain moves forcefully and hits against the skull bone,

leading to the damage of brain tissues. Brain strikes against the skull bone at a point opposite to the point

where the blow was applied. So, this type of damage to the brain is known as countercoup injury.

2. Regulation of Cranial Content Volume

Regulation of cranial content volume is essential because, brain may be affected if the volume of cranial

content increases. It happens in cerebral hemorrhage and brain tumors. Increase in cranial content volume is prevented by greater absorption of CSF to give space for the increasing cranial contents.

3. Medium of Exchange

CSF is the medium through which many substances, particularly nutritive substances and waste materials

are exchanged between blood and brain tissues.

COLLECTION OF CEREBROSPINAL FLUID

CSF is collected either by cisternal puncture or lumbar puncture. In cisternal puncture, the CSF is collected by passing a needle between the occipital bone and atlas, so that it enters cisterna magna. In lumbar puncture, the lumbar puncture needle is introduced into subarachnoid space in lumbar region, between the third and fourth lumbar spines.

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