Telano & Baker, (2022, p 1) state about CSF:

"In the adult population, its secretion varies between individuals, usually ranging from 400 to 600 ml per day. The constant secretion of CSF contributes to complete CSF renewal four to five times per 24-hour period in the average young adult. The reduction of CSF turnover may contribute to the accumulation of metabolites seen in aging and neurodegenerative diseases".

CSF production is via a network of modified ependymal cells known as the choroid plexus (CP). The CP is a highly specialized, simple, cuboidal epithelium continuous with ependymal cells lining the ventricles of the brain. This simple cuboidal epithelium surrounds clusters of fenestrated capillaries allowing for the filtration of plasma. CP cells have dense microvilli present on their apical surface. They are interconnected via tight junctions, creating a blood-CSF barrier that helps control the composition of CSF.

"CSF assists the brain by providing protection, nourishment, and waste removal. CSF provides hydromechanical protection of the neuroaxis through two mechanisms. First, CSF acts as a shock absorber, cushioning the brain against the skull. Second, CSF allows the brain and spinal cord to become buoyant, reducing the effective weight of the brain from its normal 1,500 grams to a much lesser 50 grams."

Lee (2022) argues that the aging brain tries to compensate for its loss of buoyancy by production of amyloid plaques. This strategy ultimately fails as the it leads to impaired CSF flow dynamics.

On the website of the American Association of Neurological Surgeons (AANS), hydrocephalus in adult patients is described as a condition that does not cause an increase in pressure in the CSF spaces as in children but a shrinkage in the brain. In ex-vacuo hydrocephalus, as seen in Alzheimer patients, the shrinking brain cause a vacuum that makes the ventricles enlarge. The extra CSF takes the place of the diminishing brain mass. Other reasons for adult onset hydrocephalus include head injuries, cranial surgery, hemorrhage, meningitis or a tumor. It is linked to a "gradual blockage of the CSF-draining pathways in the brain. The ventricles enlarge to handle the increased volume of CSF, thus compressing the brain from within and eventually damaging or destroying the brain tissue"(Adult-Onset Hydrocephalus – Symptoms, Diagnosis and Treatments, n.d.). To normalise the flow of the CSF, an extra opening can be made in the third ventricle so that the CSF can flow into the subarachnoidal space, shunts can be used to drain the fluid back into the blood or the blockage (like a tumor or blood clot) can be surgically removed.

True or false

According to this article the brain is floating in the CSF. [[BLANK:0]]
The waste removal function of the CSF may become less efficient as we age because the CSF replacement happens slower. [[BLANK:1]]
Choroid plexuses are modified cuboidal ependymal cells. [[BLANK:2]]
The CSF is filtered from blood in sinusoidal capillaries that are surrounded by ependymal cells. [[BLANK:3]]
The ependymal cells form an important part of the blood brain barrier. They prevent cells, glucose and proteins from filtering through. [[BLANK:4]]
CSF is produced outside the brain and reabsorbed inside the brain. [[BLANK:5]]
CSF is the same as lymph. [[BLANK:6]]
CSF acts as a shock absorber and provides nutrition to the Peripheral Nervous System. [[BLANK:7]]
Because the brain is buoyant in the CSF, gravitation affects it much less than it would have if it was not. [[BLANK:8]]
There are three apertures in the fourth ventricle through which CSF leaves the inner space - two lateral and one medial. [[BLANK:9]]

References

Adult-onset Hydrocephalus – Symptoms, Diagnosis and Treatments. (n.d.). Retrieved September 12, 2022, from

Lee, S. J. (2022). Alzheimer’s disease is a result of loss of full brain buoyancy. Medical Hypotheses, 164, 110857.

Telano, L. N., & Baker, S. (2022). Physiology, Cerebral Spinal Fluid. In StatPearls. StatPearls Publishing.