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Deep inside your head are four linked spaces.
They are full of a clear liquid called cerebrospinal fluid — CSF for short.
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The fluid flows through them in one direction.
Made deep in the middle, it travels down through a narrow channel and out around the brain.
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It is made all the time, and soaked up all the time.
At the top it drains back into the blood. Made and drained stay in balance.
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Sometimes the narrowest channel blocks.
Now the fluid arriving above it has nowhere to go.
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So the upper spaces swell.
The build-up presses outward. Doctors call this hydrocephalus.
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A shunt gives the fluid a new way out.
A soft tube runs from the swollen space, under the skin, all the way down to the belly.
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The pressure comes down.
The spaces shrink back toward normal, and the fluid keeps moving. You are still completely you.