Cerebrospinal Fluid (CSF) Circulation, Composition, Functions
What is cerebrospinal fluid?
Cerebrospinal fluid (CSF) is a clear fluid that surrounds the brain and spinal cord. There is about 150 milliliters of CSF within the cerebral cavity that encloses the brain and spinal cord which allows the brain to “float” in the fluid.
Formation of Cerebrospinal Fluid (CSF)
Most of the CSF is secreted by the choroid plexus of the four ventricles. This accounts for about two-thirds of the 500 to 700 milliliters of CSF that are produced in a day. The remaining quantities of CSF are secreted by the ependymal surfaces of the ventricles and the arachnoid mater. A small amount of CSF also comes from the blood flow in the brain.
CSF is formed by an active process where sodium ions are transported across the epithelial cells and pushed outside of the choroid plexus. The positive sodium ions then attract negative chloride ions. This changes the osmotic gradient and the CSF with the higher ion concentration draws water across the choroid plexus membrane (osmosis). Glucose, bicarbonate ions and sodium are then transported out of the blood capillaries by other processes. This brings the composition of CSF similar to that of plasma, although the quantities of chloride ions, potassium ions and glucose are lower in the CSF.
The quantity of protein in the CSF may vary between 15mg/dL to 40mg/dL and glucose concentration is approximately 50 to 80mg/dL. (1)
Flow of Cerebrospinal Fluid
Fluid secreted from the choroid plexus of the lateral passes through the first and third ventricles and into the fourth ventricle. Minute amounts of CSF are added to the bulk from the lateral ventricles in the third and fourth ventricle. By exiting the fourth ventricle through the two lateral foramina (of Luschka) and the midline foramen (of Magendie), the cerebrospinal fluid enters the cisterna magna. This then drains into the subarachnoid space which surrounds the entire brain and spinal cord. Eventually CSF flows through the arachnoidal villi and is emptied into the several venous sinuses of the cerebrum. It is then returned into the venous circulation.
Functions of the Cerebrospinal Fluid
The main function of the cerebrospinal fluid is to act as a shock absorber thereby cushioning the brain. There is a very slight difference in the specific gravity of the brain and CSF which allows the brain to be “suspended” in the CSF. With a moderate blow or sudden jerking of the neck and head, the brain will move with the skull and not collide with it. With more severe blows, however, the brain can collide with the skull on the side opposite to the impact (contrecoup phenomenon) although the impact is usually reduced significantly due to the presence of CSF.
Brain tissue is delicate and the brain is a heavy organ. By being suspended in CSF, it is not weighed down against the bony skull which could damage the lower parts of the brain tissue.
Alterations in the volume of CSF is a compensatory mechanism to deal with raised intracranial pressure associated with a hemorrhage (bleeding in the cranial cavity), hematoma (accumulation of blood) or cerebral edema (swelling of the brain).
Since the brain lacks a true lymphatic system, excess protein in the brain tissue spaces (which cannot enter into the veins of the brain) are carried through the perivascular spaces and into the subarachoid spaces by the cerebrospinal fluid. By passing through the arachnoid villi, the CSF carries the protein back into the venous blood stream. This route via the perivascular spaces may also be utilized to flush out cellular debris in the brain following an infection and other metabolic wastes.
References
- Cerebrospinal Fluid. Neuropathology – Northeastern Ohio Universities College of Medicine
