Brain Physiology Relevant to Hydrocephalus

Hydrocephalus is characterized by an abnormal accumulation of cerebrospinal fluid (CSF) within cavities inside the brain called ventricles.

Brain Ventricles

The brain and spinal cord form the central nervous system (CNS) and 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 called the meninges, which are special membranes that completely surround the brain and spinal cord. CSF flows between these membranes in an area called the subarachnoid space and cushions the brain and spinal cord against forceful blows, distributes important substances, and carries away waste products.

Cerebrospinal fluid 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: two lateral ventricles, the third ventricle, and the fourth ventricle. The ventricles are connected by narrow passageways.

Cerebrospinal Fluid Pathways

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 known as the aqueduct of Sylvius into the fourth ventricle. From the fourth ventricle, it passes through three small openings called 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 called arachnoid villi near the top and midline of the brain. The CSF then passes through the arachnoid villi into the superior sagittal sinus, a large vein, 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 one 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.

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