The cerebrospinal fluid (CSF) is a clear, colorless fluid that bathes the entire surface of the central nervous system and cushions the brain and spinal cord against concussion or violent changes of position. It is contained within a system of fluid-filled cavities called ventricles. An adult human has a total volume of 125–150 milliliters (less than a cupful) of CSF and produces 400–500 milliliters of it every day. Since it's replaced several times a day, CSF flushes the central nervous system.
The CSF is produced mainly by a structure known as the choroid plexus in the lateral, third and fourth ventricles. It flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro). The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the aqueduct of Sylvius). CSF then flows into the subarachnoid space through the foramina of Luschka, of which there are two, and the foramen of Magendie.
Absorption of the CSF into the bloodstream takes place in the superior sagittal sinus through structures called arachnoid villi. When the CSF pressure is greater than the venous pressure, CSF will flow into the bloodstream. However, the arachnoid villi act as one-way valves: if the CSF pressure is less than the venous pressure, the arachnoid villi will not let blood pass into the ventricular system.
Functions of the CSF include:
The CSF protects the brain from damage by "buffering" the brain. In other words, the CSF acts to cushion a blow to the head and lessen the impact.
Because the brain is immersed in fluid, the net weight of the brain is reduced from about 1,400 gm to about 50 gm. Therefore, pressure at the base of the brain is reduced.
The one-way flow from the CSF to the blood takes potentially harmful metabolites, drugs and other substances away from the brain.
The CSF serves to transport hormones to other areas of the brain. Hormones released into the CSF can be carried to remote sites of the brain where they may act.
Disorders affecting the cerebrospinal fluid
The CSF is normally maintained at a constant pressure which is related to the blood pressure in the great veins and is raised when venous tension is increased by coughing or straining. Any disease process or tumor within the cranium increases the CSF pressure as it expands, causing headache and nausea. Any block to the free flow of CSF results in damming-up, which balloons the brain to produce the condition known as hydrocephalus, or "water on the brain".
The composition of the fluid and its contained cells may be affected by disease, so examination of the fluid is often of value in diagnosis. While the cord ends at the first lumbar vertebra, its membranes continue as far as the sacrum. The lumbar spine thus encloses a space, or theca, filled with cerebrospinal fluid, and this may be entered by a long needle inserted between the spinous processes – a procedure known as lumbar puncture.