Quote: (Originally Posted by
Freef)
I'm no physiologist, but...
Is there really such a pressure difference?
Yes, of course there is. There is a essentially a column of blood from the head to the feet and so that blood exerts a pressure in the feet equal to the height of the column. The actual pressure at any point in the system is the sum of the pressure generated by the system (the heart in the ateries and the venous tone in the veins) and the hydrostatic prssure of the column.
Quote:
How does immersion cause this?
By getting rid of the pressure column effect because the external pressure at any point in the system is essentially equal to the column of blood. So the actual pressure in the circulation is simply the generated pressure alone.
Quote:
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 obviously incompressible but there is an important concept called venous capacitance. A vein will expand when the pressure of the blood in it rises. Immersion changes the distribution of venous blood by decreasing the pressure in the veins below the heart. This actually increases the venous pressure at the heart, increasing the venous return and the cardiac output. Increasing the cardiac output increases the blood fow to the kidneys which increases the urine output so the blood volume does fall (although indirectly).
The point is that, as Simon pointed out in his original post, effects of posture on blood flow and distribution in a dry environment will not happen when immersed.