Cerebrospinal fluid

make CSF and send it into the subarachnoid space.  CSF (shown in blue) surrounds the brain and spinal cord.]]
.  CSF flows through the subarachnoid space - the space between the dura mater and the arachnoid layer]]
**Cerebrospinal fluid** (**CSF**) bathes and protects the central nervous system (the brain and the spinal cord). "Cerebro" means "brain"; "spinal" is a short version of "spinal cord"; and fluid is a liquid.

CSF is made by networks of blood vessels called choriod plexuses in each of the brain's four ventricles.

CSF flows through the subarachnoid space – the space between the two layers of meninges that are closest to the brain (the arachnoid layer and the pia mater). CSF also fills the brain's ventricles, and flows down the middle of the spinal cord (the *central canal*).  CSF is 99% water.  It carries:  If the brain did not have CSF to float in, it would sit on the bottom of the skull.  The brain's weight would push the bottom of the brain against the skull.  Blood would not be able to get to the bottom of the brain because the blood vessels would be getting crushed by the weight of the brain on top of them.  Without getting blood (and the oxygen it carries), the neurons in the bottom of the brain would die.

### CSF is the brain's cushion
CSF protects the brain by acting like a cushion.  Without CSF, every time a person moved their head, their brain would hit the inside of their skull.  This could injure the brain.  

When a person hits their head, CSF acts like the airbag in a car and can sometimes keep the brain from slamming into the inside of the skull.  However, when a person hits their head very hard – in a car accident, for example – the CSF cannot protect the brain from hitting the skull.  This can cause concussions, bleeding in the brain, brain damage, or even death.  Sometimes, after a chemical reaction, chemicals that the brain does not need are left over.  These chemicals are called "waste products."  For example, when the brain's cells use oxygen and glucose (sugar) to create energy, carbon dioxide (CO2) is left over.

To keep waste products from building up in the CSF, the choroid plexuses make new CSF about four times a day.  The bloodstream can then carry these chemicals to organs that can get rid of them, like the kidneys and the lungs.  For example, the bloodstream carries carbon dioxide to the lungs, where it can be breathed out.

## Testing CSF
Doctors can use  of CSF to find out if a person has a brain infection, like meningitis, encephalitis, or syphilis.  CSF samples can also show bleeding from certain parts of the brain.  Swelling in the brain, caused by some inflammatory diseases like multiple sclerosis, can show in a CSF sample as well.

Usually, doctors take samples of CSF by doing a lumbar puncture (spinal tap).

Cancer cells in the CSF are a sign that a person has brain cancer, or has cancer that started somewhere else and spread to the brain.

## Problems with CSF
### Too much
Head injuries and some diseases can make too much CSF build up in the brain.  This is called hydrocephalus.  The extra fluid puts pressure on (squeezes) the brain.  This is called increased intracranial pressure.  If the pressure gets high enough, the brain's blood vessels get crushed, and blood cannot get to the brain.  If a person's brain does not get enough blood and oxygen, they will become  and their brain cells will die.  Eventually, the person will die.  The rest will have physical disabilities, intellectual disabilities, and/or other brain problems.

Most people with CSF leaks get better on their own, after the hole in the dura mater heals.
* Bacteria, viruses, or other pathogens can get in through the same hole CSF is leaking out of.  This can cause brain infections like meningitis.
* Without enough CSF to float in, the brain sits on the bottom of the skull.  The brain's weight can: 
** Crush the blood vessels in the bottom of the brain
** Press on the brainstem, causing very low blood pressure
** Push the brain down through the hole in the bottom of the skull, where the brainstem turns into the spinal cord.  This can cause paralysis, brain damage, coma, and death.

## Related pages
* Central nervous system
** Brain
** Spinal cord
* Meninges

## References