For example, does the blood of the freediver accumulates in his/her head while being upside down, as it would on land, or not, as it would in microgravity environment ?
What is the difference between an astronaut in the ISS and a freediver in perfect neutral buoyancy?
A freediver could not be in perfect neutral buoyancy. The air in his lungs causes his chest to be more buoyant than his legs. So he would be turned chest up, legs down. Been there, done that. If you let air out until you sink, the mean density of your chest is still lower than the density of your legs. When you exhale completely some air remains in your lungs.
If there is some water left in your mouth or in your diver's mask, it will flow to the lowest point of the cavity just like it would do when you are not floating in water. Just like within a submarine mentioned by Organic Marble.
But blood does not accumulate in the head while being upside down in the water. Rising blood pressure is compensated by rising water pressure when going down from feet to head.
There is a similar effect of fluid redestribution within the human body for a freediver, an astronaut and a participant to a bedrest study. Under the effects of the earth's gravity, blood and other body fluids are pulled towards the lower body. In zero gravity, in bedrest or in the water, the blood is shifted from the lower to the upper body by the elasticity of the blood vessels of the legs. The kidneys are activated to remove the excess water from the upper body.
The hydrostatic gradient counteracts rising blood pressure from feet to head. It does compensate it exactly only if the density of the blood is the same as the salt water around the diver.
$\begingroup$ So let's say you put on the freediver a tight suit which is weighted in such a way that the specific gravity of the freediver is 1 with full lungs. The bloodflow of this diver would then be the same than the blood flow of an astronaut in weightlessness ? $\endgroup$ Sep 20, 2019 at 14:59
$\begingroup$ You may try to compensate buoyancy of the body with weights, but you will get neutral buoyancy only for a certain depth of water and a certain air content of the lungs. Going down or up some meters and the neutral buoyancy is gone. $\endgroup$ Sep 20, 2019 at 15:04
$\begingroup$ Yes it's fundamentally an unstable equilibrium, and the amplitude of this instability is affected by many different factors. But let's focus on the question putting this aside with the thought experiment of a neutral buoyancy. So the hydrostatic gradient counteracts rising blood pressure from feet to head. Does it compensate it exactly ? I.e. is the bloodflow the same than in microgravity ? $\endgroup$ Sep 20, 2019 at 15:12
$\begingroup$ I learned this trick to let out the air underwater until I don't rotate chest up. It unnerved my girlfriend. $\endgroup$– JoshuaSep 22, 2019 at 4:22
An astronaut practicing an EVA in the Neutral Buoyancy Laboratory (a large swimming-pool like facility) is still affected by gravity. They are pulled down relative to the suit - which is buoyed up by its internal air volume and attached flotation devices. If they are upside down, the blood would tend to accumulate in their head.
Buoyant forces do not remove the effect of gravity on the internals of a floating object. Crewmembers do not fly/float about within submarines and an object dropped in a submarine falls as normal.
An astronaut doing an EVA in space is not affected relative to the suit. There is no buoyancy force and the same inertial forces affect the astronaut and suit.
$\begingroup$ I agree with you about EVA simulations but could you elaborate on "the same is true for a freediver" ? Would blood in a closed plastic bag accumulate at the bottom of the bag if it is neutrally buoyant ? $\endgroup$ Sep 20, 2019 at 14:53
1$\begingroup$ Sure. See my added comments about dropping objects in a submarine. $\endgroup$ Sep 20, 2019 at 14:53
$\begingroup$ It seems different to me, as in a submarine there is a rigid barriere protecting you inside from the water hydrostatic pressure. I agree submarine analogy is relevant according to EVA simulations, but not convinced yet about a freediver $\endgroup$ Sep 20, 2019 at 14:56
$\begingroup$ @Uwe's answer mentions some considerations about hydrostatic pressure, so maybe it's more what you are looking for. If your "bag of blood" had a volume of air inside it above the blood, it would have to be at or above the pressure of the surrounding water (or it would collapse) so I don't see much difference from the submarine case. $\endgroup$ Sep 20, 2019 at 14:58
$\begingroup$ It's actually the crux of the question and maybe I did not make myself clear, I'm sorry. I picture gravity as an intensive force field acting on every single particle of the body, whereas buoyancy acts "in total" if you take the submarine case, but for a freediver or a bag with flexible interfaces, the hydrostatic pressure can be "transmitted" throughout body layers. $\endgroup$ Sep 20, 2019 at 15:03
The viscosity of the surrounding medium has a lot of impact concerning your ability to move. If, for some reason, your body starts rotating, you'll come to a rest quickly in water, but it'll take a very long time on the ISS (unless you can get a hold of a wall) and you'll rotate forever in free space.
In water, you can move around easily by swimming; swimming movements on the ISS or in free space won't have any effect except possibly make you wobble a bit.
$\begingroup$ A diver is also hindered by a bunch of bulky equipment and a tight body suit, which can also restrict movement compared to someone on the ISS just wearing normal clothing. A closer comparison might be someone on an EVA in a space suit, since that is also bulky and restrictive (hence why astronauts receive underwater training). But that's still different for the reasons you mentioned. $\endgroup$ Sep 22, 2019 at 15:28
1$\begingroup$ @DarrelHoffman a freediver does not wear a bunch of bulky equipment and prefers a flexible suit or no suit at all. $\endgroup$ Sep 22, 2019 at 16:40
One big difference is that a diver in neutral buoyancy still has his sense of equilibrium working correctly, as gravity still acts on the inner ear.
$\begingroup$ The semicircular canal system of the vestibular organ detects rotational movements. Feeling rotations should work in zero and nonzero gravity. The otolithic organs sense linear accelerations as well as nonzero gravity. $\endgroup$ Sep 22, 2019 at 13:46
The main difference is that an astronaut in the ISS is in constant freefall at orbital speed, whereas the diver in equilibrium is not falling at all. That's why fluids in the diver will behave just as in a human standing around on land, if we only consider the gravitational effects.
The astronaut would have more similarity to a parachutist in that regard.
$\begingroup$ As an experienced diver I can assure you fluids in the body behave different while standing around on land and being under water in a horizontal position. A diver is not exposed to gravitational effects only, hydrostatic ambient pressure does matter. By the way, a diver with too much weight may be in danger when falling. $\endgroup$ Sep 23, 2019 at 15:06
$\begingroup$ @Uwe That's why I said
if we only consider the gravitational effects. That's the main difference. A diver will feel the weight, just as someone on land, whereas the astronaut won't, just like a parachutist (to a certain extent where he's still accelerating). I'm not denying the other factors, but they are less profound regarding the question in the title. $\endgroup$– GabrielSep 23, 2019 at 15:23