Cerebrospinal fluid, also called CSF,is a clear fluid that surrounds the spinal cord and brain. The flow of CSF is essential to the cushioning, protection and support of the brain. It is also essential for the removal of waste products from the brain.
Cerebrospinal fluid flow is sometimes called the third circulation of the brain and is discussed at length throughout my book,The Downside of Upright Posture. CSF pressure is normally very low and only slightly higher than pressure in the veins of the brain. The slight difference in pressure, however, is necessary for CSF to flow from its point of highest pressure, where it is produced, to its point of lowest pressure where it empties into the the veins of the brain and exits the cranial vault.
Cerebrospinal Fluid Production
CSF is produced in the chambers of the brain called ventricles. The two highest ventricles in the brain are the lateral ventricles. The third ventricle lies below them and the fourth ventricle lies below the third. A dense plexus of blood vessels, called the telea choroidea, lies close to the ventricles. Support tissues of the brain, called glial cells that are not nerve cells but connective tissue fibers, surround the blood vessels and form an extra-fine sieve called the blood brain barrier. A layer of salt secreting skin-like cells similar to sweat glands line the inside of the ventricles.
CSF is produced by both active and passive processes. Active production of CSF requires energy to cause the cells lining the ventricles to secrete salt into the ventricles. The salt solution then creates a concentration gradient that draws blood out of the blood vessels of the telea choroidea, across the blood brain barrier and into the ventricles. About two cups of CSF is produced on a daily basis by active production.
CSF is also produced by passive means. The passive production of CSF occurs as a result of upright posture. Upright posture causes blood and CSF to flow out of the brain faster. The faster flow of fluids out of the brain causes a pull on the blood vessels that lie close to the ventricles. In this way upright posture and gravity pull blood into the ventricles. The increase in passive production that comes from upright posture is necessary to compensate for the increase in outflow of CSF from the brain during upright posture.
Whether by active or passive means, as the blood from the blood vessels that lie close to the ventricles crosses through the tight mesh of glial support tissues it gets filtered down to water, for the most part, with a few compounds like sugars and some proteins mixed in. The concentration of proteins in particular, however, is much lower than found elsewhere in the body. In fact, the lack of proteins in CSF may make the brain more susceptible to injury due to decreases in blood flow and oxygen, such as occurs following a heart attack or stroke. The impact of CSF protein concentrations on ischemic (decreased blood flow) injury in the brain will be discussed further as this website continues to develop.
Cerebrospinal Fluid, Hydrocephalus and Edema
CSF also mixes with the fluids that surround the cells of the brain called interstitial fluids. CSF is thus connected to waste removal from the brain. A back-up in venous outflow causes interstitial edema or swelling. A back-up in CSF causes hydrocephalus. CSF mixes with and leaves the brain with venous blood, therefore, a back-up in the venous drainage system of the brain can lead to a back-up in CSF. This has led some scientists to suggest that edema and hydrocephalus in the brain are essentially one and the same.
Under extreme conditions an increase in CSF volume can cause the ventricles to enlarge as in the picture below of a child with hydrocephalus. The ventricles are the large black shadows in the middle of the brain. In this case they are too large, and compressing the brain.
Cerebrospinal Fluid and Cranial Hydrodynamics
Cerebrospinal fluid flow is determined by changes in cranial hydrodynamcis, which is affected by cardiovascular changes in blood flow and pressure, as well as respiratory changes in pressure. CSF flow is also affected by posture. Upright posture increases CSF flow.
Cardiovascular waves from the heart and arteries cause rhythmical rises and falls in blood volume and pressure in the arteries of the brain. The changes in arterial pressure affect surrounding cerebrospinal fluid pressure. As pressure goes up CSF is driven out of the skull and into the spine. As arterial pressure drops, CSF is pulled from the spinal cord into the brain.
Respiratory waves similarly affect cranial hydrodynamics for two reasons. The first reason is it changes pressure acting on the heart and blood vessels inside the ribcage. As pressure inside the ribcage goes up during exhalation it increases pressure in the arteries in the brain. As pressure goes down during inhalation pressure in the arteries of the brain go down as well. Thus, respiration increases the height or amplitude of the wave like wind on water.
The second reason, similar to respiration's affect on the arteries, respiration also affects the vertebral veins of the spine. This is important because most veins have valves as in the picture on the left. The vertebral veins, however, have no valves to check or prevent reverse flows. This allows venous blood to flow in either direction. Therefore, respiratory waves are transmitted to the brain via the vertebral veins as well.
In brief, during inhalation the diaphram muscle moves downward and the ribcage expands similar to opening a bellows to prepare to blow on a fire. The increase in space creates a void which draws air in. Pressure in the brain likewise drops during inhalation and CSF is drawn into the brain just like the bellows. During exhalation the diaphram moves upwards and the ribcage shrinks in size. The decrease in space blows air out of the lungs similar to closing the bellows and blowing on the fire. CSF and venous pressure drop in the brain in synchrony with respiration. Consequently, CSF gets pulled into the cranial vault during inspiration as pressure drops and pushed out on exhalation as pressure increases.
Radiologists refer to the combined impact of cardiovascular and respiratory waves on CSF flow in the brain as B waves or volume waves. The also refer to the rise and fall of CSF pressure as compliance or pulsatility. CSF compliance or pulsatility is considered to be a good thing in the brain. It helps move things along. A lack of compliance or pulsatility causes CSF and interstitial fluids to become sluggish and stagnant. Sluggish, stagnant fluids, in turn, cause waste and toxic substances to accumulate. When they do it is called cytotoxic edema. Cytotoxic edema is a suspect in the cause of neurodegenerative processes such as demyelination seen in multiple sclerosis.
Cerebrospinal Fluid Pathways
CSF flows out of the ventricles of the brain where it is produced and into the subarachnoid space of the brain and cord. Some CSF also flows down through a channel in the center of the cord called the central canal. The central canal, however, appears to close with age. In several key areas, the subarachnoid spaces surrounding the brainstem area are enlarged into cisterns. The cisterns cushion and protect the brain. They also keep the brainstem from sinking into the foramen magnum, which is the large hole in the base of skull for the passage of the cord. Sinking into the foramen magnum ,a condition known as Chiari malformations, compresses the brainstem, as well as blood vessels and CSF pathways. I suspect this may play a role in certain cases of Alzheimer's, Parkinson's and multiple sclerosis.
About sixty percent of CSF ends up in the cord and forty percent stays in the brain. The CSF in the cord and the brain need to continuously flow so that they can be removed from the system in order to prevent back-ups. From what we know, most of the CSF in the brain and in the cord empty into the venous drainage system of the brain. It appears, however, that some may escape through alternative outlets as well, such as along nerve roots and lymphatic pathways. In fact, in certain condtions it leaks out of the nose and ears.
To keep it simple, for now, it is presumed that most of the cerebrospinal fluid produced by the brain is eventually absorbed by arachnoid granulations (Pacchionian bodies) in the brain. This includes cerebrospinal fluid that ends up in the cord. Most CSF makes its way back to the brain. Interestingly, most of the arachnoid granulations appear to be located in and around the superior sagittal sinus at the top of the brain as depicted below. The venous lacunae are to the sides of the superior sagittal sinus. Lacunae means lake. These little lakes contain dense networks of veins with arachnoid granulations. Some scientists suspect that most of the absorption of CSF occurs in these lake-like venous lacunae.
The production and the flow of cerebrospinal fluid is thus essential to the health of the brain. Maintaining its correct volume is equally important. An insufficient volume of CSF can cause the brainstem to sink in the cranial vault into a pressure conus or Chiari malformation, as mentioned above. An excess in CSF volume on the other hand can cause hydrocephalus.
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