CSF is reabsorbed primarily at the arachnoid villi. Which pressure gradient drives this reabsorption?

• A CSF osmotic pressure exceeding venous osmotic pressure
• B Active transport of Na+ by arachnoid granulation epithelium
• C Negative intrathoracic pressure during inspiration creating a CSF flow gradient
• D CSF hydrostatic pressure exceeding venous sinus hydrostatic pressure ✓

Explanation

CSF is produced by choroid plexus (~500 mL/day) and maintained at normal ICP (~10 mmHg in horizontal position). Reabsorption at arachnoid granulations (villi projecting into dural venous sinuses) is driven by the hydrostatic pressure gradient: normal CSF pressure (~10–15 mmHg) > venous sinus pressure (~5 mmHg), net ~5–10 mmHg favouring absorption. The arachnoid villi act as one-way valves. When venous sinus pressure rises above CSF pressure (e.g., sinus thrombosis), reabsorption fails and ICP rises — the mechanism of idiopathic intracranial hypertension.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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