Lumbar CSF pressure is normally 70-180 mm H2O. Up to pressures well above this range, the rate of CSF formation is independent of intraventricular pressure. However, absorption is propoional to the pressure. At a pressure of 112 mm H2O, which is the average normal CSF pressure, filtration and absorption are equal. Below a pressure of approximately 68 mm H2O, absorption stops. Also know: Large amounts of fluid accumulate when the capacity for CSF reabsorption is decreased (external hydrocephalus, communicating hydrocephalus). Fluid also accumulates proximal to the block and distends the ventricles when the foramens of Luschka and Magendie are blocked or there is obstruction within the ventricular system (internal hydrocephalus, noncommunicating hydrocephalus). Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 33. Circulation through Special Regions. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.