Cerebrospinal fluid · Lobeworks/17

Cerebrospinal fluid (CSF) is the clear, nearly protein-free liquid that fills the ventricles of the brain and the space around the brain and spinal cord, and that cushions the central nervous system, floats it, and carries away part of its waste.

Cerebrospinal fluid. Cerebrospinal fluid (CSF) is the clear, nearly protein-free liquid that fills the ventricles of the brain and the space around the brain and spinal cord, and that cushions the central nervous system, floats it, and carries away part of its waste.

Most of it is secreted by the choroid plexus, tufts of capillaries covered by a special epithelium inside the ventricles; the rest seeps from the brain itself. An adult holds about 125 to 150 mL at any moment and makes about 500 mL a day, so the whole volume is replaced three or four times daily. It flows from the lateral ventricles through the third and fourth into the space under the arachnoid membrane, around the brain and down the cord, and drains back into the venous blood. Floating in it, a brain of about 1.4 kg weighs only a few tens of grams, which keeps its own weight from crushing its lower surface.

It sits behind its own barrier. The choroid plexus epithelium is sealed by tight junctions, the blood-CSF barrier, a cousin of the blood-brain barrier in the capillary walls: both keep the brain's fluid steadier and cleaner than blood.

It is the window clinicians sample. A lumbar puncture draws CSF from below the end of the cord, and its cells, proteins and pressure report infection, bleeding or inflammation inside the skull without opening it.

Whether sleep washes the brain is disputed. The glymphatic hypothesis holds that CSF flows in along arteries, through the tissue with the help of astrocyte water channels, and out along veins, faster during sleep (Xie et al., 2013, in mice); a 2024 mouse study measured the opposite, clearance reduced in sleep and anaesthesia, and the methods behind both are argued over.

The brain floats in a liquid it makes for itself.

CSF turns a soft organ of 1.4 kg into one that weighs almost nothing in its skull, and gives clinicians the one fluid that tells them what happens inside.

Questions: Does sleep wash the brain? Maybe, and the evidence points both ways. The glymphatic hypothesis holds that cerebrospinal fluid flows into the tissue along arteries and out along veins, carrying away waste such as amyloid, and a 2013 mouse study measured the space between cells growing by about 60 % in sleep with clearance rising to match. A 2024 mouse study using a different method found clearance reduced in sleep and anaesthesia instead. The two disagree on measurement more than on anatomy, and neither has been shown directly in the human brain, so sleep cleans the brain is a claim to hold loosely. What does cerebrospinal fluid do for the brain that blood cannot? It floats the brain and gives it a calm bath. Suspended in about 150 mL of CSF, a brain of about 1.4 kg weighs only a few tens of grams, so its own weight does not press its underside against the skull, and the fluid cushions it against knocks. Blood cannot do that job, and its chemistry swings too much for neurons: the blood-brain barrier and the blood-CSF barrier keep both the brain's own fluid and the CSF steadier and cleaner, and CSF carries part of the brain's waste away as it drains.