Why is it bad to have blood in cerebrospinal fluid?
There is something called the blood brain barrier. CSF is produced in the brain by modified ependymal cells in the choroid plexus. It circulates from the choroid plexus through the interventricular foramina (foramen of Monro) into the third ventricle, and then through the cerebral aqueduct (aqeduct of Sylvius) into the fourth ventricle, where it exits through two lateral apertures (foramina of Luschka) and one median aperture (foramen of Magendie). It then flows through the cerebromedullary cistern down the spinal cord and over the cerebral hemispheres. There is no connection between this system and the blood stream (at least not directly),so blood in the CSF is indicative of traumatic brain injury, or intracerebral hemorrhage (especially subarachnoid hemorrhages). It can also be present from spinal injuries.