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Body water is distributed mainly between intracellular and extracellular compartments. Hypothalamic osmoreceptors detect changes in plasma osmolality and help trigger thirst and ADH release. ADH acts at V₂ receptors in the kidney’s collecting ducts, promoting aquaporin-2 insertion and water reabsorption; reduced ADH favors dilute urine and water excretion. Volume signals also influence this system: the renin–angiotensin–aldosterone system supports sodium and water conservation, while atrial natriuretic peptide promotes their excretion. Baroreceptors and other factors can modify ADH secretion, and the kidney’s countercurrent mechanism enables a wide range of urine concentrations. The lesson also connects these controls to water intake, losses, and the distinction between osmolality and volume regulation.