The brain and spinal cord are bathed in and cushioned by a crystal clear fluid, spinal fluid, which is produced in large quantities by a structure called the choroid plexus of the brain. In the normal course of events, the fluid circulates around and is absorbed into the venous side of the bloodstream through the arachnoid villi that lie over the surface of the brain. Hydrocephalus can have many causes. There, the fluid is absorbed into the bloodstream from the abdominal cavity.
Shunts are fickle things, and are sometimes prone to blockage.
Shunt blockage can create something of a medical emergency for replacement.