I'm no physiologist, but...
Quote: (Originally Posted by
dteubner)
When you are vertical in air then there is a big pressure difference between your head and your feet becasue the heart is pumping blood up to your head and down to your feet. So if your blood pressure is 120/80 (millimetres of mercury) then the pressure in your scalp is 45cm of water less than this (90/50) and your feet 135cm more (220/180). A similar effect happens in the veins.
Is there really such a pressure difference? I would have thought that in a closed system with a pump and a small variation in height above the pump the pressure would be more equalised. If it wasn't then the blood from the brain would have difficulty on the return journey as it is linked to the higher pressure blood vessels that are returning from the legs.
Quote: (Originally Posted by
dteubner)
If you are vertical in the water then there is an additional pressure gradient. Say you are standing in a pool with your head just under the water. The top of your head is experienceing 1ATA and your feet are at 1.18 ATA. Because blood has essentially the same density as water this gradient abolishes the blood pressure differences described above. So immersion casues a very significant effect on blood flow and blood distribution, as well as eliminating the effect of orientation (head up vs down or whatever).
How does immersion cause this? Blood may be the same density as water, but it is enclosed within the body and not mixing with the water [sharks permiting
]. The circulatory system is a liquid so there should me no [OK, minimal because it does contain dissolved gas] compression of the blood vessels and reduction in blood volume. Blood is also still being pumped around, so there will be the pressure generated by the heart to circulate the blood which will still be the same pressure as the surface, surely?
Having said all that, I deco on the flat so there is a minimum pressure difference across my body.