An osmotic diuretic is a type of
diuretic
A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...
that inhibits reabsorption of water and
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
(Na). They are pharmacologically inert substances that are given intravenously. They increase the
osmolarity
Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...
of blood and renal filtrate.
Two examples are
mannitol
Mannitol is a type of sugar alcohol used as a sweetener and medication. It is used as a low calorie sweetener as it is poorly absorbed by the intestines. As a medication, it is used to decrease pressure in the eyes, as in glaucoma, and to lo ...
and
isosorbide
Isosorbide is a bicyclic chemical compound from the group of diols and the oxygen-containing heterocycles, containing two fused furan rings. The starting material for isosorbide is D-sorbitol, which is obtained by catalytic hydrogenation of ...
.
In the
nephron
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ca ...
, osmotic diuretics act at the portions of the nephron that are water-permeable.
Osmotic diuretics work by expanding extracellular fluid and plasma volume, therefore increasing blood flow to the
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
. This washes out the
cortical medullary gradient in the kidney. This stops the
loop of Henle
In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin language, Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. ...
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
The cranial cavity, also known as intracranial space, is the space within the skull that accommodates the brain. The skull minus the mandible is called the ''cranium''. The cavity is formed by eight cranial bones known as the neurocranium that in ...
and
intra-ocular pressure. Osmotic diuretics increase plasma volume, but because they do not cross the blood-brain barrier, this does not affect the nervous system. In effect, this is the cause of their action reducing locally the plasma volume in the nervous system.
Mechanism of action
Osmotic diuretics have their major effect in the proximal convoluted tubule and the descending limb of the Loop of Henle. These sites are freely permeable to water. Through osmotic effects, they also oppose the action of ADH in the collecting tubule. 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 rate decreases the contact time between fluid and the tubular epithelium, thus reducing sodium as well as water reabsorption. The resulting natriuresis is of lesser magnitude than the water diuresis, leading eventually to excessive water loss and hypernatremia.
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. Such agents can be used to reduce intracranial pressure and to promote prompt removal of renal toxins. The prototypical osmotic diuretic is mannitol.
Mannitol lowers the intra cranial pressure through two effects in the brain. The first,
rheological effect, reduces blood viscosity, and promotes plasma expansion and cerebral oxygen delivery. In response, cerebral vasoconstriction occurs due to
autoregulation
Autoregulation is a process within many biological systems, resulting from an internal adaptive mechanism that works to adjust (or mitigate) that system's response to stimuli. While most systems of the body show some degree of autoregulation, it ...
, and cerebral blood volume is decreased. The second effect occurs through creation of an osmotic gradient across the blood-brain barrier, leading to the movement of water from the parenchyma to the intravascular space. Brain tissue volume is decreased and, therefore, ICP is lowered.
See also
*
Osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...
References
External links
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