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Blood pressure depends on the total volume of fluid in the organism. As total fluid volume drops (as in dehydration), blood pressure also decreases.
The actual mechanics are a little bit subtler. There are two "fluid compartments" inside each organism: a blood compartment which consists of all the fluid in circulation (but not the blood cells or dissolved molecules also in the blood) and a lymphatic compartment which is pretty much everything else. (O.K., it's not everything else because a third compartment would be all the individual water molecules inside cells, but because cells don't change their volumes much it's almost invariably ignored).
The total volume of both fluid compartments depends on Starling's law - I leave it to you to Wiki that - which relates volume to pressure, which in turn depends on ion concentrations and osmotic gradients between the compartments.
In dehydration, fluid volume loss is typically from the lymphatic/tissue compartment. (A counter example would be fluid loss through bleeding in which case the loss is plainly from the blood compartment, but that's something of a special case so we'll leave it alone). As the fluid pressure in the tissue declines, fluid leaves the blood compartment to compensate for the loss. So total blood volume decreases.
And as blood volume decreases - losing fluid but not cells or dissolved molecules - the concentration of those cells and dissolved molecules becomes higher. The way clinicians diagnose dehydration is by taking a blood sample and checking the relative concentrations of those formed elements and ions.
Thank you blues!! Your answer is always very good!!
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blues, i have one questio
When dehydrating , means low water intake = high osmotic pressure -> releases ADH -> reabsorbs water
and also low water intake = blood pressure decrease = release aldosterone -> Nacl and water are reabsorbed
so...does it mean that both NaCl and water are being reabsorbed??
then what will be in the urine then..
Look at the details, not just the overall picture.
Neither are totally reabsorbed. The overall rate of water re-absorption is higher than the rate of the other ions and chemicals. So what you get are a relatively high concentration of dissolved materials in a relatively low volume of fluid.