Some science behind the scenes
The choroid plexus is a structure in the ventricles of the brain where cerebrospinal fluid (CSF) is produced. There are four choroid plexi in the brain, one in each of the ventricles. The diagram below shows the choroid plexus in the fourth ventricle, in the section closest to the bottom half of the cerebellum.
The choroid plexus (CP) consists of many capillaries, separated from the ventricles by choroid epithelial cells. Liquid filters through these cells from blood to become cerebrospinal fluid. There is also much active transport of substances into, and out of, the CSF as it is made.
The craniosacral system's fluid intake is via the choroid plexus, which allows passage of fluid from the vascular system into the ventricular system of the brain.
The choroid plexus is selective in its passage of solutes from blood into the craniosacral system. The fluid which passes through the choroid plexus is known as cerebrospinal fluid. Cerebrospinal fluid is returned into the venous system by the arachnoid villae [see skull] . These villae are most concentrated in the sagittal venous sinus within the cranial vault, but are found in significant numbers throughout the intracranial venous drainage system.
So in summary, CSF goes into the craniosacral system from the blood via the choroid plexus and goes out again via the veins in the skull. Generally it is the blood that takes away any unwanted impurities that have entered the system.
The CP epithelial layer has tight junctions in between the cells on the side facing the ventricle. These tight junctions prevent the majority of substances from crossing the cell layer into the CSF; thus the CP acts as a blood-CSF barrier.
The so called ‘Blood brain barrier’ is distinct from the similar blood–cerebrospinal-fluid barrier and from the blood–retinal barrier, which can be considered a part of the whole realm of such barriers.
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