Quote: (Originally Posted by
Freef)
No more than being horizontal in bed. If you were vertical in the water the blood would be in your legs to the same extent as if you were on dry land.
Sorry David but this isn't true.
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.
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).