Brain Physiology Relevant to Hydrocephalus
The brain and spinal cord form the central nervous system (CNS). They’re surrounded and protected by the bones of the skull and the vertebral column. Between the brain and skull are three other protective coverings. These are the meninges — special membranes that completely surround the brain and spinal cord. CSF flows between these membranes in an area called the subarachnoid space. CSF cushions the brain and spinal cord against forceful blows, distributes important substances and carries away some waste products.
CSF is believed to be primarily produced within the ventricles by delicate tufts of specialized tissue called the choroid plexus. The ventricles can be thought of as chambers filled with fluid. There are four ventricles in all: the two lateral ventricles, the third ventricle, and the fourth ventricle. The ventricles are connected by narrow passageways.
Produced mainly in the lateral and third ventricles, CSF flows from the lateral ventricles through two narrow passageways into the third ventricle. From the third ventricle, it flows down another long passageway (the aqueduct of Sylvius) into the fourth ventricle. From the fourth ventricle, it passes through three small openings (foramina) into the subarachnoid space surrounding the brain and the spinal cord.
It has traditionally been thought that CSF is primarily absorbed through tiny, specialized cell clusters (arachnoid villi) near the top and midline of the brain. CSF then passes through the arachnoid villi into a large vein (the superior sagittal sinus) and is absorbed into the bloodstream. Once in the bloodstream, it’s carried away and filtered by the kidneys and liver in the same way as other bodily fluids. Small amounts of CSF are also absorbed into lymphatic channels.
Our bodies produce approximately a pint of CSF every day, continuously replacing it as it’s absorbed. Under normal conditions, a delicate balance exists between the amount of CSF produced and the rate at which it is absorbed. Recent research raises the possibility that the rhythm of cerebral blood flow and CSF pulsations may have bearing on this delicate balance.