Objective: To study the physiological mechanisms of plasma hypoosmolality in patients with pneumonia and on this basis to elaborate principles of therapy for this condition.

Methods: 52 individuals of different age, including 26 patients with pneumonia, were examined. Osmolality, the concentrations of ions of sodium, potassium, magnesium, and creatinine were measured in the serum.

Results: The patients with pneumonia were found to have osmolality, hyponatremia in combination with severe hypodiuresis, high urinary osmotic pressure and intensive reabsorption of osmotically free water in the kidney, which leads to blood dilution. As hypoosmolality usually causes higher diuresis and decreased urinary osmolality; hypodiuresis with high urinary osmolality in pneumonia is indicative of effective renal performance and its altered regulation evidently due to the hypersecretion of vasopressin or to the decreased formation of a number of autacoids in the kidney.

Conclusions: Blood hypoosmolality and hyponatremia in the examined patients result from inadequate blood osmolality and high urinary osmotic concentrating. The principles of this condition in pneumonia are discussed and aquaretics are proposed for use as pathogenetic therapy.