CSF Fluid

Question 1

CSF fluid

Answer 1

Cerebrospinal fluid (CSF) is formed by ependymal cells in the choroid plexus of the lateral, third and fourth ventricles (approx 500ml/day). This fluid is constantly reabsorbed, so that only 100-160ml is present at any one time. It occupies the space between the arachnoid and pia matter.

CSF passes from the lateral ventricles to the third ventricle via the foramen of Munro. From there it passes to the fourth ventricle via the aqueduct of Sylvius (cerebral aqueduct). Then it passes through the foramen of Magendie and foramen of Lushka into the subarachnoid space and spinal cord.

It then returns to the vascular system by being reabsorbed by the arachnoid villi and entering the dural venous sinuses.

The normal intracerebral pressure (ICP) is 5 to 15 mmHg. The rate of formation of CSF is constant and is not affected by ICP. Absorption of CSF increases linearly as pressure rises above about 7 cmsH2O pressure. At a pressure of about 11cmsH2O, the rate of secretion & absorption are equal.

The CSF has a composition identical to that of the brain ECF but this is different from plasma. The major differences from plasma are:

The pCO2 is higher (50 mmHg) resulting in a lower CSF pH (7.33)
The protein content is normally very low (0.2g/l) resulting in a low buffering capacity
The glucose concentration is lower
The chloride and magnesium concentration is higher
The cholesterol content is very low
The pH is lower (about 7.33 compared to 7.41 for plasma)
The concentration of calcium and potassium is lower.
The concentration of sodium is unchanged

The following table summarises these differences:                      ReducedProtein content
Glucose
Cholesterol
pH
Calcium
Potassium
UnchangedSodium
IncreasedChloride
Magnesium

There are no lymphatic channels in the brain and CSF fulfills the role of returning interstitial fluid and protein to the circulation.

The CSF is separated from blood by the blood-brain barrier. Only lipid soluble substances can easily cross this barrier and this is important in maintaining the compositional differences.