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When the rate of glomerular Post Date: Thu, 16 Oct 2008 9:16:13 +0000 In general, there are three separate ways by which the rate of solute excre-tion may be augmented: the inhibition of electrolyte reabsorption by specific drugs, an increase in the filtered load of electrolytes that are poorly or incompletely reabsorbed, and an increase in the filtered load of the nonelectrolyte osmotic agents.It is important to appreciate the circum-stances under which the osmotic diuretics may play a unique role in either maintaining or increasing urine volume. When the rate of glomerular filtration is acutely reduced, either experimentally in the laboratory or clinically as a result of hypovolemic shock, dehydration, or trauma, the solutes of the glomerular filtrate undergo more complete reabsorption so that there is a disproportionately large fall in the rate of urine flow and solute excretion. Autor of the post: Undefined Under these same conditions, those Post Date: Thu, 16 Oct 2008 9:05:33 +0000 As indicated previously, the administration of a normal solute, such as sodium chloride, may restore renal excretory function, but only if there is improvement in renal hemodynamics. If the rate of glomerular filtration remains severely reduced, sodium chloride administration fails to augment urine flow because of the virtually complete tubular reabsorption of this normal electrolyte. Under these same conditions, those diuretics that normally act by inhibit-ing tubular transport may also be ineffective because the extent to which they reduce tubular reabsorptive capacity is not of sufficient magnitude to compensate for the greatly diminished filtered load. Autor of the post: Undefined The tubular impermeability to mannitol Post Date: Thu, 16 Oct 2008 8:46:05 +0000 However, under the same conditions, that is, when the renal circulation is acutely com-promised, the osmotic diuretics retain their efficacy. To take mannitol as an example —even though the filtration rate is reduced, mannitol is still filtered at the glomerulus. The tubular impermeability to mannitol is not altered by acute renal ischemia of short duration. Autor of the post: Undefined As a consequence, urine volume Post Date: Thu, 16 Oct 2008 8:28:36 +0000 Hence, the amount of mannitol that is filtered is also excreted in the voided urine. Unreabsorbed solute limits the back diffusion of water. As a consequence, urine volume can be maintained even in the presence of decreased glomerular function. Autor of the post: Undefined The tubule is then no Post Date: Thu, 16 Oct 2008 8:08:49 +0000 As a first approximation, urine volume is proportional to the rate of solute excretion, which under these circumstances may be composed largely of the agent administered as the osmotic diuretic. Nephrotoxic agents and prolonged, severe renal ischemia may damage the tubular epithelium and produce acute tubular necrosis with oliguria. The tubule is then no longer selectively impermeable, and osmotic diuretics become ineffective. Autor of the post: Undefined From a practical point Post Date: Thu, 16 Oct 2008 7:49:57 +0000 An additional action of the osmotic diuretics is to increase the rate of electrolyte excretion, particularly sodium, chloride, and potassium. However, this occurs only with large doses. From a practical point of view, this action should be clearly distinguished from the effect on the rate of urine formation itself. Autor of the post: Undefined Thus, the sodium concentration Post Date: Thu, 16 Oct 2008 7:32:28 +0000 The action on electrolyte reabsorption is related to the isosmotic characteristics of the proximal tubule. Under normal conditions, sodium chloride is the major solute in the proximal fluid and, as it is reabsorbed, water diffuses passively so that the tubular fluid remains isosmotic. Thus, the sodium concentration in the tubular fluid is virtually unaltered, despite the fact that sodium itself is undergoing active reabsorption. Autor of the post: Undefined It should be reemphasized Post Date: Thu, 16 Oct 2008 7:14:08 +0000 However, in the presence of non-reabsorbable solute, the diffusion of water is reduced relative to the movement of sodium in order to maintain an isotonic fluid in the tubule. As a consequence, the concentration of sodium in the tubular fluid decreases, as does the total amount of sodium that is reabsorbed, proba-bly due to the progressively increasing con-centration gradient of sodium between the tubular urine and extracellular fluid.It should be reemphasized that the natri-uretic action of osmotic diuretics is largely of experimental interest and is of little im-portance as a practical means of increasing sodium excretion or mobilizing edema fluid. Autor of the post: Undefined Sucrose is excreted Post Date: Thu, 16 Oct 2008 7:03:28 +0000 The same general considerations apply to all osmotic diuretics, even though they may be handled slightly differently by the renal tubule. Mannitol undergoes very little reab-sorption, and for many practical purposes the tubule may be considered to be impermeable to it. Sucrose is excreted in a similar manner. Autor of the post: Undefined In the case of glucose Post Date: Thu, 16 Oct 2008 6:46:54 +0000 In the case of urea, about 50% of the amount filtered at the glomerulus is not reabsorbed. With elevated urea loads, this fraction tends to increase. In the case of glucose, at normal plasma concentrations the amount filtered is virtually completely reabsorbed by an active process in the proximal tubule. 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