water balance

Water balance is how the body keeps the right amount of water relative to its salt, like keeping soup neither too watery nor too thick. It is managed separately from total salt: water balance protects the concentration of the blood, while salt balance protects its volume.

Two arms keep this in check, both driven by osmolality sensed by hypothalamic osmoreceptors. When the blood becomes too concentrated, vasopressin rises and the kidney reabsorbs more water, and thirst makes you drink. When the blood is too dilute, vasopressin falls and the kidney pours out extra water as dilute urine. Together these adjustments hold blood concentration in a narrow window, with the kidney able to make urine ranging from very dilute to very concentrated as needed.

When this control fails, blood sodium concentration goes wrong. Losing too much water relative to salt — as in untreated diabetes insipidus — raises blood sodium, while retaining too much water — as in the syndrome of inappropriate vasopressin secretion — dilutes it and lowers blood sodium. Notably, a low blood sodium usually signals a water problem, not a salt deficiency.