Osmotic diuretic
An osmotic diuretic is a type of diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...

 that inhibits reabsorption of water and sodium. They are pharmacologically inert substances that are given intravenously. They increase the osmolarity of blood and renal filtrate.

Two examples are mannitol
Mannitol is a white, crystalline organic compound with the formula . This polyol is used as an osmotic diuretic agent and a weak renal vasodilator...

 and isosorbide.

In the nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

, osmotic diuretics act at the portions of the nephron that are water-permeable.

Osmotic diuretics works by expanding extracellular fluid and plasma volume, therefore increasing blood flow to the kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

. This washes out the cortical medullary gradient in the kidney. This stops the loop of Henle
Loop of Henle
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

 from concentrating urine, which usually uses the high osmotic and solute gradient to transport solutes and water.

These agents can also act at other parts of the body. For example, they can be used to reduce intracranial and intra-ocular pressure.

Physiological working of osmotic diuretics

The renal proximal tubule is the primary site of action of osmotic diuretics.

Normally, water molecules follow Na+ out of the proximal tubule, resulting in Na+ and water reabsorption. When osmotic diuretics are introduced, they hold onto water molecules in the tubule. Since the luminal membrane is quite leaky to Na+, this causes a high back leak of Na+ into the tubule.

Na+ is normally followed by K+ and Cl- out of the proximal tubule. When there is high back leak of Na+, these electrolytes stay in the tubule and are lost through urine.

Note: Glucose is completely reabsorbed by the kidneys but not Mannitol.
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