CSF, Ventricles & Brain Anatomy
Arachnoid granulations (villi)
Arachnoid granulations are small projections that absorb cerebrospinal fluid back into the bloodstream.
In short: Arachnoid granulations (villi)
- They are the main sites of CSF absorption.
- They return CSF to the venous blood, mainly at the superior sagittal sinus.
- Impaired absorption here causes communicating hydrocephalus.
- They are also called arachnoid villi (smaller form) or Pacchionian granulations.
What it is
Arachnoid granulations (and their smaller form, arachnoid villi) are small, finger-like projections of the arachnoid membrane that protrude into the brain's venous channels. They are the main sites where CSF is absorbed back into the bloodstream.
Understanding Arachnoid granulations (villi)
After circulating through the subarachnoid space, CSF is absorbed through the arachnoid granulations into the large venous channels of the brain, particularly the superior sagittal sinus. This absorption balances the continuous production of CSF, keeping the total volume and pressure stable. When absorption through these structures is impaired — for example, after bleeding or infection that scars or clogs them — CSF accumulates, causing communicating hydrocephalus. (Other absorption pathways, including lymphatic routes, are increasingly recognized as well.)
Why it matters
Understanding arachnoid granulations helps families see how CSF is normally absorbed and why impaired absorption — a common result of hemorrhage or infection — leads to the communicating type of hydrocephalus.
Common questions
What do arachnoid granulations do?
They absorb CSF back into the bloodstream.
Where do they return CSF?
Mainly into the superior sagittal sinus.
What happens if absorption is impaired?
CSF accumulates, causing communicating hydrocephalus.
Bottom line
Arachnoid granulations absorb CSF into the bloodstream; impaired absorption here causes communicating hydrocephalus.
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