The clear fluid that cushions the brain and spinal cord: where it's made, how it circulates through four ventricles, what a lumbar puncture tells doctors, and what happens when it backs up.
Spinal Cord & Body Wiring
Intermediate
Cerebrospinal Fluid and the Ventricles
The Editors
May 28, 2026
3 min read
In plain English
Your brain floats in a clear liquid called cerebrospinal fluid. It's made inside hollow spaces in the brain called ventricles, flows around the brain and spinal cord, cushions them from bumps, and carries away waste. If it can't drain properly, pressure builds up — a condition called hydrocephalus.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
Go deeper next
Cerebrospinal fluid (CSF) is a clear, colourless liquid that surrounds the brain and spinal cord and fills a system of cavities inside the brain called the ventricles. Ancient anatomists thought the ventricles were the seat of the mind; they are actually part of the brain's plumbing.
Made, moved and recycled
CSF is produced mainly by the choroid plexus, a frilly tissue in the ventricles where blood capillaries meet specialised ependymal cells. An adult makes about half a litre a day, but only about 125–150 millilitres exist at any one time — the whole volume is replaced three or four times a day.
The fluid flows from the two large lateral ventricles (one in each hemisphere) to the narrow third ventricle between the thalami, down a slim channel (the cerebral aqueduct) to the fourth ventricle in front of the cerebellum, and out into the subarachnoid space around the brain and spinal cord. It is reabsorbed into the blood through arachnoid granulations and other routes, including lymphatic vessels.
What it does
Buoyancy: a 1.4-kg brain effectively weighs only about 25–50 grams floating in CSF, preventing it from crushing its own lower parts. Cushioning against knocks. Chemical stability and the removal of waste.
The lumbar puncture
Because the spinal cord ends high in the back, doctors can safely insert a needle lower down to draw CSF — a lumbar puncture or spinal tap. CSF tests help diagnose meningitis, bleeding around the brain, multiple sclerosis and, using amyloid and tau proteins, Alzheimer's disease.
Hydrocephalus
If CSF flow is blocked or reabsorption fails, fluid builds up and the ventricles enlarge — hydrocephalus ("water on the brain"). In babies, whose skull bones haven't fused, the head enlarges. It is commonly treated with a shunt, a tube that drains fluid to the abdomen. Normal pressure hydrocephalus in older adults causes walking problems, bladder problems and memory difficulty, and can be partly reversible — a rare treatable cause of dementia-like symptoms.
Everyday examples & things to try
Analogy
An egg in a jar of water
Put a raw egg in a jar full of water and shake it gently — it survives far better than in an empty jar. CSF does the same for the brain, turning a heavy, soft organ into a nearly weightless, cushioned one.
Key takeaways
- CSF is made by the choroid plexus (~500 mL/day) and fills the four ventricles and subarachnoid space.
- It provides buoyancy, cushioning and chemical stability and helps clear waste.
- Lumbar puncture samples CSF; blocked flow causes hydrocephalus.
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