Figure 112

Decreased plasma osmolality or increased arterial circulating blood volume

Decreased thirst iz

Decreased ADH release

Decreased water intake

Decreased water excretion

Water excretion

Increased plasma osmolality and decreased arterial circulating volume

Increased ADH release and thirst

Pathways of water balance (conservation, A, and excretion, B). In humans and other terrestrial animals, the thirst mechanism plays an important role in water (H2O) balance. Hypertonicity is the most potent stimulus for thirst: only 2% to 3 % changes in plasma osmolality produce a strong desire to drink water. This absolute level of osmolality at which the sensation of thirst arises in healthy persons, called the osmotic threshold for thirst, usually averages about 290 to 295 mOsm/kg H2O (approximately 10 mOsm/kg H2O above that of antidiuretic hormone [ADH] release). The so-called thirst center is located close to the osmoreceptors but is anatomically distinct. Between the limits imposed by the osmotic thresholds for thirst and ADH release, plasma osmolality may be regulated still more precisely by small osmoregulated adjustments in urine flow and water intake. The exact level at which balance occurs depends on various factors such as insensible losses through skin and lungs, and the gains incurred from eating, normal drinking, and fat metabolism. In general, overall intake and output come into balance at a plasma osmolality of 288 mOsm/kg, roughly halfway between the thresholds for ADH release and thirst [10].

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