The flexibility of the arteries shouldn't change the circumstances from a hydraulic system, since all that happens is the arteries flex until their increasing tension equalizes with the pressure exerted upon them by the blood. That is what is demonstrated by the ballon example.
The gravity effect on blood could be significant especially since we hang out in that position for a long while. Although that would suggest somebody who does a feet down free immersion (ie. Herbert) should not be getting that effect and brings up the question of whether he has a blood shift and if so, then gravity could not be the only cause.
As well, I previously discovered that on 50m inhale dives I did not seem to have any significant blood shift, yet on shorter 30-40m half inhale dives, I was eventually getting what seemed like significant blood shift. This I was speculating based upon the sensation of burn in the legs shortly after turning and kicking from the bottom. It should be noted that the sensation of oxygen depletion at the end of the dives was relatively equal and therefore for the burn, during the half inhale dive, to be caused by a difference in O2 uptake by the muscles, from the blood alone, without a difference in blood shift, I would have anticipated a significant increase in O2 craving at the completion of the half inhale dive.
Maybe, to see if there is an immediately significant gravitational effect, I will go do a comparison of:
1. 1:30 static, followed by a set of squats for 1:30.
vs.
2. 1:30 static standing on my head, followed by a set of squats for 1:30.
Be back soon...