Diuretics are medications designed to increase the amount of water expelled from the body as urine.There are 5 main types of diuretics: carbonic anhydrase inhibitors; loop diuretics; thiazide and thiazide-like diuretics; potassium sparing diuretics; and last, osmotic diuretics.
In this video we are going to talk about osmotic diuretics.
Now, the basic unit of the kidney is called a nephron.It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and an encompassing Bowman's capsule.
The glomerulus is the network known as a tuft, of filtering capillaries located at the vascular pole of the renal corpuscle in Bowman's capsule. Each glomerulus receives its blood supply from an afferent arteriole of the renal circulation. The glomerular blood pressure provides the driving force for water and solutes to be filtered out of the blood plasma, and into the interior of Bowman's capsule, called Bowman's space. The filtrate next moves to 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 prototypic osmotic diuretic is mannitol. Mannitol is poorly absorbed by the gastrointestinal tract, and when administered orally, it causes osmotic diarrhea rather than diuresis. For systemic effect, mannitol must be given
intravenously.
Osmotic diuretics have their major effect in the proximal tubule and the descending limb of Henle’s loop. The proximal tubule and descending limb of Henle’s loop are freely permeable to water. Any osmotically active agent that is filtered by the glomerulus but not reabsorbed causes water to be retained in these segments and promotes a water diuresis. The presence of a nonreabsorbable solute such as mannitol prevents the normal absorption of water by interposing a countervailing osmotic force.
As a result, urine volume increases. The increase in urine flow decreases the contact time between fluid and the tubular epithelium, thus reducing sodium as well as water reabsorption. The resulting natriuresis, excretion of sodium in the urine, is of lesser magnitude than the water diuresis, leading eventually to excessive water loss and hypernatremia so mannitol is considered an aquaretic diuretic, meaning it promotes water excretion without increasing the loss of electrolytes.
Osmotic diuretics alter Starling forces so that water leaves cells and reduces intracellular volume. This effect is used to reduce intracranial pressure in neurologic conditions and to reduce intraocular pressure before ophthalmologic procedures.
Since mannitol also increases renal blood flow, it can be used to flush away harmful substances that builds up in the kidney. Examples include myoglobin from rhabdomyolysis, which is the break down of muscles, and hemoglobin from hemolysis, which is the breakdown of red blood cells.
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