Blood flow in the internal jugular veins during the spaceflight - Is it actually bidirectional?

Recently intriguing results of the research performed on astronauts of the International Space Station have been published. Unexpectedly, in some crew members a stagnant and bidirectional flow in the internal jugular vein was found, and in one of the astronauts this vein seemed to be totally thrombosed. If it actually were the case that in the settings of weightlessness there is a substantial risk of jugular vein thrombosis, any long-term human space missions would be extremely dangerous. Yet, we interpret these findings differently. In our opinion, what has been explained as bidirectional flow, actually represented the flow separation, and what has been described as occluded vein was rather the vein with recirculating and low-velocity flow. In this paper, basing on physical laws governing the flow of fluids, we describe how the flow separation and recirculation can develop in a dilated internal jugular vein, how it can affect cerebral outflow through this vein, why such a phenomenon is more likely in the settings of microgravity and how future research on this topic should be directed.