CSF is not static. Anatomy and physiology are inexorably intertwined. CSF is constantly produced (as an ultrafiltrate of blood at the choroid plexus) and resorbed. Resorption ultimately occurs at the arachnoid granulations, lymphatics of the skull base and dura, the 'paravascular cast' of the glymphatic system via interstitial fluid, and the spinal canal.

With every heart beat, there is CSF systole. The influx of arterial blood can be conceptualized as a net pulsation, squeezing the brain and propelling CSF through the ventricular system to the spinal CSF spaces and cisterns ('intracranial systole'). There is instantaneous to-and-fro flow, which can manifest as flow voids on certain MRI sequences, and net bulk flow. There is instantaneous

After sutures are closed, the intracranial compartment has a fixed volume. To a first approximation, we can consider three internal components: brain, blood, and CSF.

If CSF volume decreases, the blood volume will commensurately increase as the brain volume is relatively fixed. The change in blood volume is more marked in venous structures compared to arteries.

Spontaneous intracranial hypotension (SIH) arises from spinal CSF leaks. This results in CSF hypovolemia and potentially alterations in craniospinal CSF space compliance. There is resultant increased intracranial venous volume and 'brain sag' that account for findings seen on MRI exams of the brain. In distinction, leaks from the skull base typically present with CSF rhinorrhea, CSF otorrhea and/or ascending meningitis, but not SIH.