What is the significance of arachnoid villi in CSF absorption?

• A. They produce CSF
• B. They act as one-way valves that drain CSF into dural sinuses, maintaining normal ICP
• C. They filter blood to provide nutrients
• D. They form the blood-brain barrier

Answer: (B)

Maintaining normal intracranial pressure depends on CSF being absorbed back into the bloodstream through arachnoid villi. These villi are small protrusions of the arachnoid mater that extend into the dural venous sinuses, especially the superior sagittal sinus. They act as one-way outlets, allowing CSF to flow from the subarachnoid space into venous blood when CSF pressure exceeds venous pressure. This drainage balances CSF production by the choroid plexus and prevents buildup of CSF in the brain.

They don’t produce CSF—the choroid plexus does that. They aren’t responsible for filtering blood to provide nutrients, which is more about general capillary exchange and CSF composition. And they don’t form the blood-brain barrier, which is created by tight junctions between endothelial cells and supporting glial structures. When absorption via these villi is impaired, CSF can accumulate, leading to hydrocephalus and elevated intracranial pressure.