plasma contributes (3.5:14 as 1:4) 2,100 ml × 1/4 = 525 ml. If we add the 175 ml from the pure water portion (525 ml + 175 ml = 700 ml), we see that the plasma has lost 20% of its volume (700 ml × 100/3,500 ml) which, as seen above, is close to the shock threshold. Thus, a 4.2 liter sweat loss has theoretically as much impact on the plasma (volume −20%) as a much greater volume (7.4 liters) of pure water loss (−18%).
If the subject drank pure water until the plasma osmolality reached the thirst threshold (295 mosmol/kg), he or she would reach a thirst TBW of:
(dehydrated TBW) × (dehydrated Posmol) = (thirst TBW) × (thirst Posmol).
Assume the dehydrated condition to be a TBW of 37.8 liters and a salt-depleted, dehydrated plasma osmolality of 295.6 mosmol/kg of water. Assume the plasma osmolality to be 295 mosmol/kg at the thirst threshold.
(37.8 liters) × (295.6 mosmol/kg) = (? liters) × (295 mosmol/kg),
(11,172 mosmol/295 mosmol/kg) = 37.88 liters of TBW.
The subject would increase his or her TBW by only 80 ml (37.88 liters – 37.8 liters) before the thirst threshold was reached. This is equivalent to only 1.9% of the initial water deficit (80 ml x 100/4,200 ml). In contrast to a similar volume of pure water loss from a hydrated starting point, a hypotonic deficit reduces the expected percent rehydration from 50% to 1.9%. This example serves to indicate the impact of solute loss on rehydration. Under these conditions, thirst is not inadequate. The problem is the missing solute. Any fluid intake under these conditions would probably be stimulated by the volume deficit.
Assume that a subject was producing a sweat of minimum sodium concentration (a very hypotonic sweat; 0.17% NaCl = 0.2 isotonic saline) due to heat acclimation and a low-salt diet (high aldosterone levels). He subsequently loses 6% of his body weight (4.2 liters) after beginning work in the heat, fully hydrated.