ventricular system and cerebrospinal fluid

Deep inside the brain there is not solid tissue all the way through. Instead, the brain is hollowed out by a connected set of fluid-filled spaces called the ventricular system, four chambers linked by narrow channels. They are filled with a clear, watery liquid called cerebrospinal fluid, or CSF. The fluid is made on tufts of tiny blood vessels called the choroid plexus that line these chambers, slowly leaking and filtering a fresh batch out of the bloodstream all day long.

Once made, the fluid does not just sit there. It flows from chamber to chamber, then out through small openings into a thin gap that wraps around the whole brain and spinal cord, before being soaked back up into the blood. Because the brain floats in this fluid, it weighs far less than it would in air, so it does not crush itself under its own weight, and the liquid acts as a shock absorber that softens the jolts of everyday bumps and movement. The same flow also rinses the brain, carrying away waste products and helping deliver some nutrients, a kind of slow internal plumbing and cleaning service.

When this plumbing is blocked, fluid backs up and presses outward, a dangerous condition called hydrocephalus that can swell the chambers and damage surrounding tissue. Doctors also tap a little of this fluid with a needle in the lower back, a lumbar puncture, to look for signs of infection, bleeding, or disease, because the CSF carries clues about what is happening throughout the nervous system.