Cranial hydrodynamics is a term used by radiologists to describe fluid mechanics in the brain, which includes blood and cerebrospinal fluid called CSF. Intracranial compliance is term used to describe pressure changes in the brain inside the skull caused by changes is CSF pressure as it pulsates on its course through the brain being driven by waves arising from the heart and arterial pulse waves along with respiratory waves, which amplify them. Techically they are called cardiorespiratory or B waves. Some scientists also call them volume waves in the brain.
In brief, the brain pulsates just like the beat of the heart. Things that disturb intracranial compliance and the rhyhmical movement of fluids can have profound affects on the health of the brain because it lies within a somewhat restricted closed container. The other two elements within the container are blood and cerebrospinal fluid mentioned above. An increase in the volume of any one element within the cranial vault must result in a decrease of one or both of the other elements. Two of the elements, blood and CSF are similar to water and therefore not very compressible. The brain, on the other hand is made of billions of nerves and over fifty percent fat, so it is much more compressible than blood or CSF.
CSF produced in the chambers of the brain called ventricles. A dense network of blood vessels surround the ventricles. CSF is made from blood that is drawn from the network of blood vessels through an extra-fine fine filter, called the blood brain barrier. The blood is drawn by a concentration created by salt, as well as a pressure gradient created by upright posture.
CSF is used to support the brain, which floats inside the skull as well as for cushioning and protection. Moreover, the brain has no lymph system. Instead, waste products are removed through the CSF system, which empties into the main vein at the top of the head called the superior sagittal sinus.
CSF production, flow and drainage from the brain is inexorably linked to venous drainage from the brain. CSF pressure in the ventricles is normally slightly higher than venous pressure so that it’s flow is toward the veins.
Upright posture creates a slightly negative pressure in the superior sagittal sinus, the main vein at the top of the head. This draws venous blood and CSF toward it. From here blood flows downhill along a steep slope like a waterfall to the base of the skull. In the bottom of the skull drain into either the jugular or vertebral vein routes.
The CSF pressure gradiant is very small so that venous drainage issues, such as back pressure in the veins in the basement of the skull, can affect intracranial compliance. It can also affect cranial hydrodynamics resulting in either too much CSF in the brain, called normal pressure hydrocephalus in adults or too little. Moreover, a decrease in the volume of CSF decreases brain support and raises the risk of pressure conus and Chiari like conditions. I cover cranial hydrodynamics thoroughly in my book, as well as Chiari and pressure conus conditions.