The fundamental feature that distinguishes the CSVS from the systemic (caval) venous system is the lack of venous valves. In 1940, Batson demonstrated that the VVS was angiographically linked to the cranial venous system, and that retrograde flow from the VVS into the brain was possible because of the lack of venous valves. Anderson's experiments in living humans similarly demonstrated that contrast material injected into the VVS reached the intracranial venous sinuses and the internal cerebral veins in retrograde fashion. Groen and colleagues confirmed the lack of valves and the connection of the vertebral venous plexus with the cranial sinuses, the subcutaneous cranial veins, the intercostal veins, and the sacral venous plexus, thus demonstrating "the existence of a wide communication of the vertebral venous system with the intracranial, intrathoracic and intra-abdominal veins." Later, angiographic studies by Lasjaunias and Berenstein verified retrograde flow into the radicular veins, and showed that retrograde flow into the nerve roots, the spine, and the vertebral bodies was possible. According to Gisolf's mathematical model, one could also anticipate that increased central venous pressure would result in caudal-to-cranial venous blood flow via the VVS, which is exactly what Batson[2,13] postulated more than a half century before. Thus, retrograde and bidirectional flow are inherent characteristics of all elements of the CSVS, made possible by the lack of venous valves.
This ability to enable retrograde and bidirectional flow is a critical distinction between the CSVS and the systemic venous system, one so important that Herlihy divided the venous systems of the human body into two major parts, with one of these being the structures that I am naming the CSVS. This distinction highlights the fact that the CSVS is a venous system capable of bidirectional flow, which does not function to return blood to the heart, a characteristic that is essential to its physiologic functions in both health and disease.
This bidirectionality is a fundamental concept that runs counter to the prevailing notion that the circulatory system is circular and unidirectional. This formulation began with Andrea Cesalpino (1519-1603) who preceded Harvey in describing the circular motion of the blood in the caval-heart-aortic pathway, and then continued with Harvey's seminal de Motu Cordis in 1628. In this work, Harvey described the significance of the venous valves in allowing the flow of venous blood in the caval system only in the direction of the heart, and stated that "the blood in the animal body moves around in a circle continuously." This last statement, so accurate in describing the flow of blood in the systemic circulation, is fundamentally inaccurate with regard to the CSVS, which rather than being a part of the circular, unidirectional caval-aortic system, is, as Herlihy discussed, an "ebb-and-flow" system that is predominantly linear and bidirectional.
This ebb-and-flow formulation explains much: the unusually large capacity of this venous system (200-1000 mL;[1,2,37] by contrast, the total volume of the CSF is 150 mL); the lack of valves; the variable flow through this system with changes in posture;[19-21,26,37,38,41-46] and the ability of tumor cells, emboli, and infection to travel in both directions along this system, both to the brain and from it. This valveless, ebb-and-flow system enables the CSVS to provide pressure homeostasis for the cerebral circulation, because the CSVS functions, as Batson[9,12,14] and Epstein and colleagues have described, as a large-capacity "venous lake," out of which blood may flow into the brain and into which blood may flow from the brain, depending on variations in posture and intrathoracic or intra-abdominal pressure.
just food for thought....