[Andrew K Fletcher (OP)]

Retrograde (reversed) blood flow in healthy astronauts during 50 days space flight. 
JAMA Network
Exposure to a weightless environment during spaceflight results in a chronic headward blood and tissue fluid shift compared with upright posture on Earth. To investigate the association of the change with cerebral venous outflow, researchers obtained jugular vein ultrasonography of healthy crew members participating in long-duration spaceflight missions to the International Space Station. Stagnant and/or reverse internal jugular vein flow was observed in 6 of 11 crew members on flight day 50. This ultrasonography video illustrates an example of retrograde (grade 4) flow; blood in the internal jugular vein is flowing in the reverse direction (toward the head, left). Click

to view a video example of stagnant internal jugular venous flow and https://ja.ma/2NtZlGX for full details of the investigation.

[chris]

Very interesting - and nice to see you, Andrew!

PS - I altered you thread title to ensure that it complies with forum rules (that topics titles should be formatted as a question); please don't alter it back.

[Andrew K Fletcher (OP)]

Clearly there is a problem for circulation in astronauts exposed to microgravity. This poses serious problems for the current assumptions in circulation.

1. if our circulation is struggling against gravity, why does it reverse direction when gravity is reduced?
2. Indeed, why does the blood-flow reverse against the pressure from the heart?
3. Why does Professor Michel Cabanac's experiment show blood flow reversal, against the normal flow from the heart, where higher evaporation rates, due to exercise are achieved?

This paper deserves a great deal of attention. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/10.1001/jamanetworkopen.2019.15011

[alancalverd]

It also poses a serious question of possible blood flow disturbance with a long-term prone or supine patient in normal gravity.

[evan_au]

Maybe the heart doesn't need to pump so hard, so it slacks off a bit, and doesn't pump as efficiently?
- And maybe there is a bit more backwash through the valve when the atrium contracts?
- This is despite the couple of hours of intensive exercise done every day by astronauts/cosmonauts...

[Bored chemist]

This poses serious problems for the current assumptions in circulation.

Not really.
Measuring blood-flow is fairly common in medicine.
https://en.wikipedia.org/wiki/Hemodynamics
They will have made one of two observations:
(1) Blood sometimes flows "backwards"- in which case the space observation is nothing new.
(2) Blood flow doesn't reverse in Earth bound patients- in which case the space research is irrelevant.