Which is more susceptible to dehydration — extracellular and intracellular fluid?

Question

I want to ask in human body, there is intracellular and extracellular fluid that makes up the total body water of our body, which is drained first when we do activities? extra or intra? if I want to do a bioimpedance measurement, does measuring only the extracellular fluid good? Thankyou

Answer 1

You should understand that the body is an open system which is held at a steady state. Water can diffuse in and out of the cell such that the osmolarity is balanced on both sides of the membranes. If the water potential changes in either of these compartments (intra- and extra- cellular) because of addition or removal of water molecules, both the compartments would be affected such that finally the osmolarity is same in both the compartments. It may, however, take some time to achieve the steady state.

If you refer to sweating when you say "activity" then you should note that it is a complex process with extensive neurological control. The water that is secreted by the eccrine sweat gland primarily comes from the blood. However, profuse sweating will lead to water loss from both the intra- and extra- cellular compartments.

Prolonged exposure to hyperthermic conditions and/or prolonged exercise in the heat can induce water deficits due to profuse sweating, resulting in hypohydration. This water deficit lowers both intracellular and extracellular volumes and results in plasma hyperosmolality and hypovolemia; both of which impair sweating.

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_{Shibasaki, Manabu, and Craig G. Crandall. "Mechanisms and controllers of eccrine sweating in humans." Frontiers in bioscience (Scholar edition) 2 (2010): 685-696.}

So, for your experiment it is better to measure the osmolarity of the extracellular fluid or the blood plasma because:

• It is easier to measure
• It would not be sensitive to local and temporary effects