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Brain Tour : Our Brain : Cerebrospinal FluidThe brain, as well as the rest of the CNS, is further protected against injury by cerebrospinal fluid (CSF). CSF circulates through the subarachnoid space, around the brain and spinal cord, and through the ventricles of the brain.
The ventricles are cavities in the brain that are connected to each other, with the central canal of the spinal cord, and with the subarachnoid space. There are four ventricles, two lateral ventricles, one third ventricle and one forth ventricle.
 The entire CNS contains between 80 and 150 ml of cerebrospinal fluid. That is around half a can of soft drink. It is clear and colourless. It contains proteins, glucose and urea and salts. CSF has two principal functions related to homeostasis: protection and circulation. It serves as a shock-absorbing medium to protect the brain and spinal cord from jolts what would otherwise be traumatic. The fluid buoys the brain so that is "floats" in the cranial cavity. In its circulatory function, CSF delivers nutritive substances filtered from blood and removes wastes and toxic substances produced by the brain and spinal cord cells. Cerebrospinal fluid is formed by filtration and secretion from the choroid plexuses, specialised capillaries in the ventricle. CSF circulates continually.
From the ventricle, it flows into the subarachnoid space around the back of the brain and downward around the posterior surface of the spinal cord, up the anterior surface of the spinal cord, and around the anterior part of the brain. From there it is gradually reabsorbed into the veins, mostly into a vein called the superior sagital sinus. The absorption actually occurs through arachnoid villi, which are finger like projections of the arachnoid that push into the superior sagital sinus. Normally, cerebrospinal fluid is absorbed as rapidly as it is formed. If an obstruction or inflammation causes the CSF to accumulate in the ventricles or the subarachnoid space, the condition is called hydrocephalus. This accumulation of fluid can place pressure on the brain and cause brain damage. Hydrocephalus is treated by inserting a shunt in the ventricles to drain off the excess fluid into a vein in the neck.
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