Cerebrospinal fluid
Also known as: CSF, spinal fluid, ventricular fluid
The clear fluid that surrounds and cushions the brain and spinal cord, circulates through the ventricles, and plays a key role in waste clearance from the brain.
Cerebrospinal fluid (CSF) is a clear, colourless liquid produced mainly by specialised structures called the choroid plexuses, located in the brain's ventricles. Roughly 500 ml is produced each day; the total volume in the adult brain and spinal column at any moment is around 150 ml, meaning the entire CSF volume turns over several times daily.
CSF flows through the ventricular system, around the surface of the brain and spinal cord (in the subarachnoid space), and is reabsorbed into the venous circulation — primarily at structures called arachnoid granulations.
Functions
- Mechanical cushioning — the brain essentially floats in CSF, reducing the effective weight it bears and buffering it against impact
- Pressure regulation — CSF volume adjusts to compensate for changes in blood volume within the skull (the Monro–Kellie doctrine)
- Metabolic waste clearance — CSF carries metabolic by-products away from brain tissue; this function is closely linked to the glymphatic system
- Nutrient and signalling molecule transport — CSF distributes glucose, electrolytes, and signalling molecules throughout the CNS
CSF as a biomarker
CSF collected by lumbar puncture (spinal tap) is a window into the brain's biochemistry. CSF biomarkers now form part of the diagnostic criteria for several neurodegenerative conditions:
- Amyloid-β 42 / Aβ40 ratio — reduced in Alzheimer's pathology, reflecting amyloid deposition in brain tissue
- Phosphorylated tau (p-tau) — elevated in Alzheimer's disease, reflecting neurofibrillary tangle formation
- Neurofilament light chain (NfL) — a general marker of neuroaxonal injury; elevated in many neurodegenerative and vascular conditions
- α-synuclein — relevant to Parkinson's disease and related disorders
CSF on MRI
On a T1-weighted MRI, CSF appears very dark (low signal). As the brain loses tissue through atrophy, the spaces normally occupied by tissue — ventricles and cortical sulci — expand and fill with CSF. This is why enlarged ventricles and prominent sulci are standard MRI signs of brain atrophy and ageing.
Brain age models implicitly capture this: a brain with more CSF-filled space relative to tissue volume than expected for its age will have a larger brain age gap.
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Related terms
Atrophy
The loss of brain tissue volume or cortical thickness over time, reflecting a reduction in the size or number of neurons and their supporting cells.
Glymphatic system
A brain-wide network of fluid channels, driven largely during sleep, that clears metabolic waste — including proteins linked to neurodegeneration — from brain tissue.
MRI
A non-invasive imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the brain's soft tissue structure.
Dementia / Alzheimer's disease
Dementia is a syndrome of progressive cognitive decline severe enough to impair daily life; Alzheimer's disease is its most common cause, characterised by amyloid plaques, tau tangles, and gradual neurodegeneration.