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Water balance
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We are mostly water. In an average 70 kilogram man, 60% of body weight is water: about 42 liters. And that water is not free-floating: it is split across two compartments, the intracellular one, inside the cells, and the extracellular one, outside them.
The split is not down the middle. Two thirds of the water sits inside the cells and only one third outside. And that outside third divides again: three quarters is interstitial fluid, the fluid that bathes the tissues between one cell and the next, and one quarter is plasma, the liquid part of blood.
What decides how much water goes to each side is osmolarity, the concentration of dissolved particles. Water moves on its own until every compartment has the same osmolarity as the body as a whole. The compartments work like two connected tanks: if one gets concentrated, water moves until they even out. Between the interstitium and the plasma, several other factors are at play as well, among them hydrostatic pressure and oncotic pressure, which is the pressure exerted by dissolved proteins.
In brief
In an average 70 kilogram man, 60% of body weight is water, about 42 liters, in two compartments: intracellular and extracellular.
Two thirds of it sits inside cells and one third outside; of that extracellular water, 75% is interstitial and 25% is plasma.
Water redistributes until the osmolarity of every compartment matches total body osmolarity; between the interstitial and plasma spaces, factors such as hydrostatic and oncotic pressure are at play.
Two connected tanks: if one gets concentrated, water moves until they even out.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Water Balance, StatPearlsPublished
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