The arachnoid villi absorb excess cerebrospinal fluid, or CSF, that collects in the venous sinuses surrounding the brain. Acting as one-way valves, the arachnoid villi, or arachnoid granulations, help to ensure that the pressure and volume of CSF surrounding the brain does not reach dangerous levels. Although a cushion of CSF is required to protect the brain from injury, excessive amounts and a buildup of pressure can cause the potentially disabling or fatal condition called hydrocephalus, which is sometimes referred to colloquially as "water on the brain."
The arachnoid villi protrude from the thin second layer that covers the brain, which is called the arachnoid. These small protrusions allow the excess CSF to exit the sinus area and enter the bloodstream. The larger and calcified protrusions are called pacchionian bodies.
The majority of the CSF is absorbed by the arachnoid villi that are located in the superior saggital sinus, which is the large central area inside the skull that runs from the front of the head to the back. Under normal conditions, the absorption rate of CSF by the arachnoid villi is similar to the rate of CSF production in the choroid plexus.