Water and electrolytes for production of sweat by sweat glands come primarily from extracellular sources. These sources comprise plasma water and interstitial tissue water. There is a rapid exchange of water between plasma water and extravascular sources. Even if total body water declines in work, it would seem that the level of plasma water is retained to the very last, which means, in turn, that water losses in absolute figures are primarily made up from intracellular water.

The result is intracellular dehydratiom with impaired cellular metabolism. Because perspiration is hypotonic, extracellular water tends to become hypertonic, a circumstance that contributes to intracellular dehydration. On the other hand, the osmalality in the exercising muscle cell may be enhanced, which partly counteracts the flux of the water from the cell.