The cerebrospinal fluid is a transparent liquid material that fills the brain cavities. There are in the brain four cavities which are referred to as ventricles. There are in the brain four ventricles, two lateral ventricles and two others which are referred to as the third and fourth ventricles. These ventricles communicate with each other through small canals that are called foramina.
The lateral ventricle communicates with the third ventricle through the foramen of monroe. On the other hand, the third ventricle communicates with the fourth ventricle through the a canal which is called the cerebral aqueduct. There are two passages in the fourth ventricle and which communicate with the subarachnoid space.
These passageways are called the foramen of luchka and the foramen of magendie. The cerebrospinal fluid has an important function that is related to protecting the brain tissue against trauma. It does this by absorbing the shock that the brain can suffer in traumatic events such as car accidents. Also the cerebrospinal fluid has glucose and electrolytes which can function as nutrients for the brain cells.
The cerebrospinal fluid is produced mainly in the lateral ventricles in area of the ventricles which is called the choroid plexus. The cells of the choroid plexus secrete cerebrospinal fluid by way of an osmotic effect that sodium and chloride ions exert on the fluid in the extracellular space.
Sodium is continuously actively pumped to the ventricular space by way of specialized pumps that use energy and ATPase enzymes for this purpose. After its transport to the ventricular space sodium ions attract also chloride ions by way of electrostatic interactions of the positively charged sodium ions with the negatively charged chloride ions.
These sodium chloride ions then exert an osmotic effect on fluids in the extracellular space leading to its flow inside the ventricles. This is the main mechanism by which the choroid plexus produces fluid in the lateral ventricles.
The balance between the amount of cerebrospinal fluid that is produced by the choroid plexus is exactly controlled under normal nonpathological conditions by the reabsorption of the cerebrospinal fluid in the arachnoid villi in the subarachnoid space. This regulation of cerebrospinal fluid pressure inside the ventricles is due to the reabsorption function of the arachnoid villi which permits the flow of cerebrospinal fluid from the ventricles on demand as the pressure inside the ventricles increases.
As the pressure of the cerebrospinal fluid inside the ventricles increases due to the production of the cerebrospinal fluid by the choroid plexus the arachnoid villi allows more and more of the cerebrospinal fluid to be reabsorbed into the venous system of the brain. It is the arachnoid villi that controls the cerebrospinal fluid pressure inside the ventricles by adjusting the amount of fluid that is permitted out of the ventricles.
The rate at which the cerebrospinal fluid is formed by the chroid plexus does not seem to have a significant effect in controlling the pressure of the cerebrospinal fluid inside the ventricles.
Under pathological conditions there can be elevation of the cerebrospinal fluid pressure inside the ventricles. This can happen either due to congenital obstruction of the canals which communicate between the ventricles in which case it is called non-communicating obstruction. The other cause to elevated pressure in the ventricles is a defect in the arachnoid villi in which its function to reabsorb cerebrospinal fluid into the venous system is impaired.
Tumours in the brain can obstruct the flow of the cerebrospinal fluid through the arachnoid villi. Also brain hemorrhage or brain infection can cause obstruction to the flow of fluid outside the ventricles through the arachnoid villi. The mechanism to this obstruction in hemorrhagic brain conditions is that red blood cells in the brain can leak to the ventricular space due to the hemorrhage.
Thus, they can physically obstruct the arachnoid villi preventing it from making its normal function of reabsorption of the cerebrospinal fluid to the venous system. Also in the case of brain infection, white blood cells can leak to the ventricular system and appear in the cerebrospinal fluid. This can also cause obstruction to the flow of cerebrospinal fluid through the arachnoid villi by obstructing it. The mechanism in both cases is similar to each other.
In addition the increased cerebrospinal fluid pressure inside the ventricles can affect the optic nerve of the eye. Thus causing edema in the optic disc. This condition is sometimes celled papilledema which can impair the vision in the affected individuals.