CSF is secreted by the choroid plexus (~500 mL/day, ~0.35 mL/min at rest), flows from the lateral ventricles through the interventricular foramina into the third ventricle, down the narrow aqueduct of Sylvius into the fourth ventricle, then out into the subarachnoid space, where it is reabsorbed by the arachnoid granulations into the venous sinuses. Superimposed on this net one-way circulation is a to-and-fro pulsatile flow driven by the cardiac cycle: each systolic choroidal-artery pulse pushes a bolus of CSF caudally through the aqueduct, which then rebounds cranially in diastole.
Bulk volume obeys a simple mass-balance ODE integrated every frame:
dV/dt = Q_in − Q_out
Q_out = (P_icp − P_venous) / R_out (Davson's equation)
where Q_in is choroid-plexus production, R_out is outflow resistance (dominated by the aqueduct and arachnoid granulations), and P_venous ≈ sagittal-sinus pressure (~5 mmHg). Intracranial pressure is derived from excess ventricular volume via the Monro-Kellie doctrine — skull, brain, blood and CSF share a fixed total volume, so once compensatory reserve (buffering by venous blood displacement) is exhausted, pressure rises steeply with volume (an exponential pressure–volume curve, modeled here as ICP = ICP₀ + k·(V − V₀)² for V > V₀).
- Choroid plexus production — sets Q_in; higher production (e.g. choroid plexus papilloma) raises steady-state ICP.
- Aqueduct / outflow resistance — models narrowing of the aqueduct of Sylvius or reduced arachnoid granulation absorption; the classic driver of obstructive vs. communicating hydrocephalus.
- Heart rate — sets the frequency of the pulsatile back-and-forth flow bolus visible in the aqueduct, matching phase-contrast MRI CSF flow studies used clinically to diagnose aqueductal stenosis.
- Simulate Aqueductal Stenosis — snaps resistance to a pathological value and lets you watch ventricles dilate and ICP climb in real time, the mechanism behind obstructive hydrocephalus.
Real-world relevance: this is the same pressure-volume-compliance model (Marmarou's model) used in clinical intracranial-pressure monitoring and in reasoning about shunt-valve sizing for hydrocephalus treatment.