Examples of in vivo control volumes. The generality of control volume analysis shown through example of four in vivo control volumes enclosing CSF or blood. Control surfaces coincide with the surrounding fluid/tissue interfaces. To completely enclose the specified volumes control surfaces must intersect fluid pathways; it is at these locations where velocity measurements are needed. Individual ventricles, such as the third ventricle a), can be used to determine parameters relevant at the scale of a single ventricle. Similarly, the entire ventricle system b) may be used to estimate CSF production by the choroid plexus. The largest CSF control volume contains all the CSF in a given individual c), in both the ventricular system and the subarachnoid spaces. Blood too must be accounted for, here by a control volume colored red d) which contains all the intracranial arterial, capillary and venous blood. In a) and b) arrows represent the nominal direction of bulk flow and dotted lines in c) represent the control surface.