Aquaresis

Aquaresis is a term used to describe the selective excretion of water by the kidneys without a proportional loss of electrolytes, particularly sodium. The word derives from the Latin aqua, meaning water, and the English suffix –resis, denoting resistance or rejection. It is most commonly contrasted with diuresis, which entails the removal of both water and solutes, and with natriuresis, which refers specifically to sodium excretion. Aquaresis is mediated by changes in the activity of aquaporin water channels in the collecting ducts, largely under the influence of vasopressin (antidiuretic hormone) and, to a lesser extent, other hormonal factors such as calcitonin.

In clinical settings, aquaresis may occur in conditions that lead to inappropriate secretion of vasopressin, such as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), certain types of head injury, or in the presence of pulmonary or post‑renal obstruction. It can also be induced by specific medications, for example selective serotonin reuptake inhibitors, carbamazepine, or the antidiuretic hormone relative, desmopressin in reverse. Because water retention can dilute serum sodium, aquaresis is a recognized cause of hyponatremia.

Diagnosis involves measuring urine output, serum sodium and osmolality, and urine osmolality. A low serum osmolality with a high urine osmolality indicates that water is being retained relative to solute. Imaging studies and endocrine work‑up may identify underlying etiologies. Management focuses on correcting the fluid imbalance and addressing the causative disorder. Fluid restriction is the first line of therapy, while hypertonic saline is reserved for severe hyponatremia. In SIADH, the use of vasopressin receptor antagonists (vaptans) can normalize water excretion.

Research into aquaresis has implications for the broader understanding of water balance disorders and the development of targeted therapies for hyponatremia and related conditions.