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A concentration gradient across Post Date: Fri, 17 Oct 2008 3:50:14 +0000 This has two consequences. First, there is a fall in the concentration of sodium and chloride in the tubular fluid, reaching a minimal value usually in the first portion of the distal convolu-tion; second, the sodium and chloride concentration becomes elevated in the interstitial fluid. A concentration gradient across the tubular epithelium is thus established by active transportat the site of low water permeability. Autor of the post: Undefined The third sodium transport mechanism Post Date: Fri, 17 Oct 2008 3:38:45 +0000 This gradient then becomes multiplied in a longitudinal direction by the countercurrent mechanism, so that within the interstitial fluid a large osmotic gradient becomes established between the isosmotic renal cortex and the hyperosmotic medulla and papilla. The osmotic gradients are partly maintained by the relatively meager blood flow to the medullary region, which acts as a passive countercurrent exchanger. The third sodium transport mechanism (1-C) is probably less significant than the others in terms of the total amount of sodium reabsorbed, but is of unique importance in being associated with the area of the nephron susceptible to the antidiuretic hormone (ADH). Autor of the post: Undefined In the absence of ADH Post Date: Fri, 17 Oct 2008 3:19:11 +0000 In the presence of ADH, there is a high permeability to water in this segment. As a result, the tubular fluid, particularly within the collecting ducts, equilibrates with the hyperosmotic interstitium and is then discharged at the end of the collecting duct as a hypertonic or concentrated solution. In the absence of ADH, this portion of the nephron is relatively impermeable to water. Autor of the post: Undefined Thus, in the absence Post Date: Fri, 17 Oct 2008 3:07:41 +0000 Thus, the reabsorption of sodium chloride, as indicated by reactions 1-B and 1-C, progressively lowers the osmolality of the tubular fluid. Under this condition, the tubular fluid does not reach osmotic equilibration with the adjacent interstitium. Thus, in the absence of ADH, the voided urine is characteristically hypoosmotic, or dilute. Autor of the post: Undefined This fraction may vary over Post Date: Fri, 17 Oct 2008 2:53:30 +0000 There are two other features that characterize distal sodium reabsorption. First, the absolute amount reabsorbed in this area is determined not only by the amount filtered but also by the proportion of the filtrate that has already undergone reabsorption at more proximal sites. This fraction may vary over a wide range, particularly in pathological conditions associated with edema formation or oliguria. Autor of the post: Undefined By definition, this term refers Post Date: Fri, 17 Oct 2008 2:39:32 +0000 Second, the distal mechanisms may have discrete sensitivities to the action of some drugs, including the adrenocortical hormones.Free-Water Production. By definition, this term refers to the amount of distilled water (ie, water free of solute) that would have to be added to, or subtracted from, the urine voided over a period of time (usually calculated on a minute basis) in order to render that urine specimen isosmotic with a simultaneous plasma sample. Autor of the post: Undefined When the urine is more Post Date: Fri, 17 Oct 2008 2:27:07 +0000 In arithmetical terms, free-water production equals urine volume (V) minus the osmolal clearance (C0SM). The latter term has the usual dimensions of clearance (UV/P) and refers to the sum of the concentrations of all osmotically active solutes in plasma and urine. When the urine is more dilute than plasma, free-water production is positive; when the urine is more concentrated, free-water production is negative. Autor of the post: Undefined Free-water production is an operational concept Post Date: Fri, 17 Oct 2008 2:09:50 +0000 In the first instance, solute-free water is actually excreted as part of the voided urine; in the latter instance, solute-free water can be considered as being returned to the body from the kidney. When the urine has the same osmolality as plasma, free-water production is zero regardless of the rate of urine flow.Free-water production is an operational concept. Autor of the post: Undefined By introducing the dimensions Post Date: Fri, 17 Oct 2008 1:58:10 +0000 At no time, and in no place, does distilled, or solute-free, water exist as such within the kidney. The concept is important in that it takes into account more than just the concentration of osmotically active solute in the urine. By introducing the dimensions of volume per unit time, the net rate at which either the concentrating or diluting mechanism is operating may be accurately described. Autor of the post: Undefined Some of the most potent Post Date: Fri, 17 Oct 2008 1:39:34 +0000 This has become a useful technic in the analysis of diuretic action.It should be emphasized that free-water production is not synonymous with diuresis. Some of the most potent diuretics may produce a massive diuresis of almost isosmotic urine, and hence with a minimal rate of free-water production. 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