Osmotic diuresis, or solute diuresis, refers to increased urination accompanied by daily solute excretion of more than 750 milliosmols. An increase in the filtration of solutes that could not be reabsorbed by the kidneys, such as urea or glucose, can lead to impaired reabsorption of sodium and water. This leads to osmotic diuresis. Osmotic diuresis leads to water loss from the renal system, thus it can cause hypovolemia or an abnormally low blood volume. It can occur in patients who use pharmacological diuretics or in patients who have poorly controlled diabetes mellitus.
Diuresis refers to the increase in urine volume output or urination. It is an important concept in nephrology, particularly because it can be used to reduce blood volume in hypervolemic states such as hypertension and edema. In osmotic diuresis, substances that are not easily reabsorbed by the renal tubules are retained in the lumen, causing an increase in osmotic pressure. With the phenomenon of osmosis, wherein water goes through a semipermeable membrane into a solution of high solute concentration, water then goes to the lumen. This leads to a reduction in water reabsorption, thereby resulting in increased urine output.
Osmotic diuretics are substances that are not easily reabsorbed by the renal tubules. These substances include urea, sucrose, and mannitol. Mannitol mainly acts on the proximal tubules and inhibits both water and solute reabsorption in the kidney tubules by increasing the osmolarity of the renal tubular fluid. It is used in medical conditions wherein there is an increase in the amount of body fluids such as hypertension, cerebral edema, kidney failure, and glaucoma. Sometimes, it is used to treat a drug overdose with aspirin, bromides, and barbiturates.
Disease states associated with failure of the kidneys to reabsorb excess solutes from the tubular fluid may also result in osmotic diuresis. In diabetes mellitus, for example, the blood glucose concentration levels become abnormally elevated. When the concentration of glucose in the blood reaches about 250 milligrams per deciliter, only a very small amount of glucose is reabsorbed by the tubules. At this point, the “transport maximum” of the tubules is exceeded.
The increased load of glucose in the kidneys exceeds the capacity of the tubules to reabsorb glucose. Excess glucose in the tubules serves as an osmotic diuretic, leading to rapid loss of fluid and frequent urination or polyuria. The polyuria of diabetes is accompanied by polydipsia, or increased frequency of drinking. Polydipsia occurs due to the detection of high urine output and activation of the thirst mechanism.