Osmotic diuretics

Diuretics are medications that act on the kidneys to increase production of urine, and therefore, elimination of water from the body.

There are 5 main types of diuretics: carbonic anhydrase inhibitors; loop diuretics; thiazide and thiazide-like diuretics; potassium sparing diuretics; and last but not least, osmotic diuretics - which we’ll get intimately acquainted with during this video.

Now, the basic unit of the kidney is called a nephron, and each nephron is made up of a glomerulus, which filters the blood. The filtered content goes through the renal tubule, where excess waste, and molecules (such as ions and water), are removed or filtered through an exchange between the tubule and the peritubular capillaries. So the renal tubule plays a huge role in secretion and reabsorption of fluid and ions - such as sodium, potassium, and chloride - in order to maintain homeostasis - or the the balance of fluid and ions in our body.

The renal tubule has a few segments of its own: the proximal convoluted tubule; the U-shaped loop of Henle, with a thin descending, a thin ascending, and a thick ascending limb; and finally, the distal convoluted tubule, which empties into the collecting duct, which collects the urine.

The prototypal osmotic diuretic is mannitol; other osmotic diuretics like glycerin and isosorbide are rarely used.

Following IV administration, mannitol travels through the bloodstream and acts like you’d expect an osmotically active molecule to act: it sucks water out of the cells it encounters along the way, and all that extra water reaches the kidneys as increased renal blood flow.

Once at the kidneys, it gets secreted by the glomerulus into the renal tubule. Some of the segments of the renal tubule, like the proximal convoluted tubule and the thin descending limb, are freely permeable to water. So any osmotic agent that ends up here will tend to make water stay inside the tubule, rather than be reabsorbed.