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(The mechanism that maintains Post Date: Mon, 13 Oct 2008 8:10:49 +0000 In either case, since the cortical interstitium is isosmotic with plasma, the graded ADH-induced increases in water permeability permit passive diffu-sion of water out of the hypotonic luminal fluid toward the point of isosmolarity as this fluid approaches the medullary portion of the collecting system. The final concentration of the urine is then determined in the collect-ing ducts as they pass through the hypertonic interstitium of the renal papillae. (The mechanism that maintains the hypertonicity of this region is described below. Autor of the post: Undefined The amount of water abstracted Post Date: Mon, 13 Oct 2008 7:55:14 +0000 ) During dehydration and maximal antidiuresis, water is passively abstracted from the lumen of the collecting duct into the hypertonic peri-tubular fluid so that the two fluids are in osmotic equilibrium as the urine emerges from the tip of the papilla. Since, under these conditions, papillary osmotic pressure may reach 1400 milliosmoles or more per liter, urine having four to five times the osmotic pressure of plasma may be excreted. The amount of water abstracted and conserved in concentrating the urine to an osmotic pressure greater than that of plasma is known as the negative free-water clearance. Autor of the post: Undefined Thus, at the usual levels Post Date: Mon, 13 Oct 2008 7:39:26 +0000 The action of ADH in reducing the output of urine to a small volume of fluid having a markedly elevated osmotic pressure is striking when the concurrent rate of solute excretion is normal or low, that is, about 2% or less of the filtered load of solutes. For example, 0,6 milliosmole of solute in 2 ml of water (300 milliosmoles per liter) could be concen-trated to 0,44 ml of urine (1400 milliosmoles per liter) each minute in the papillary col-lecting ducts, a conservation or negative clearance of 1,56 ml of free water. Thus, at the usual levels of solute excretion, free water can be conserved at a far lower rate than it can be excreted. Autor of the post: Undefined During extreme osmotic diuresis, the os-motic Post Date: Mon, 13 Oct 2008 7:23:58 +0000 However, when large amounts of solute escape reabsorption, as occurs, for example, during mannitol, glu-cose, or sodium salt diuresis, the volume of isotonic fluid reaching the concentrating seg-ment is elevated proportionately. The ab-straction of a few milliliters of free water from a large volume of isotonic urine then causes a much less striking elevation of os-motic pressure (see Orloff et at, 1958).During extreme osmotic diuresis, the os-motic pressure gradient of the papilla ismuch reduced {see below) and the ability to reabsorb free water is thereby impaired. Autor of the post: Undefined During diu-resis induced by agents Post Date: Mon, 13 Oct 2008 7:04:51 +0000 Under these conditions, the urine approaches isotonicity despite the presence of ADH. In the absence of ADH, however, a large, posi-tive free-water clearance may still be super-imposed on an osmotic diuresis. During diu-resis induced by agents such as the thiazides and furosemide, the renal handling of free water may be impaired by alterations in electrolyte reabsorption in the diluting and concentrating segments of the nephron {see Chapter 39). Autor of the post: Undefined However, reabsorption of sodium Post Date: Mon, 13 Oct 2008 6:51:42 +0000 The Renal Countercurrent Osmotic Multiplier. The net electrolyte reabsorption that occurs as tubular urine traverses the ascending limb of the loop of Henle accounts for only a small portion of the total renal reabsorption of sodium chloride (roughly half of that which escapes proximal reabsorption). However, reabsorption of sodium and chloride in this segment plays an important role in establishing and maintaining the hypertonic milieu in the medulla and papilla that is necessary for the formation of concentrated urine in the collecting ducts. Autor of the post: Undefined Our present understanding Post Date: Mon, 13 Oct 2008 6:33:44 +0000 The anatomical arrangement of those loops of Henle that descend deep into the medulla is such that, over a long course, the descending and ascending limbs are in close proximity to each other and to the parallel capillary loops. Movements of solute and water be-tween one and another of these elements can occur readily across the intervening narrow interstitial space. Our present understanding of the events that take place in the loop is as follows. Autor of the post: Undefined Microperfusion studies of iso-lated segments Post Date: Mon, 13 Oct 2008 6:23:15 +0000 In the descending limb, the low epithelium allows the free passive diffusion of water and, to a lesser extent, urea and other solutes. Thus, at any point along its descent, the luminal fluid approaches osmotic equilibrium with interstitial fluid. Microperfusion studies of iso-lated segments of the loop indicate that in the as-cending thin limb permeability to water decreases and permeability to sodium and chloride increases. Autor of the post: Undefined The continuous transfer of sodium Post Date: Mon, 13 Oct 2008 6:05:37 +0000 Therefore, passive outward movement of these ions could occur as the thin segment returns through areas of progressively decreasing hypertonicity. These studies have confirmed the very low permeability of the thick ascending limb to water and urea and the active reabsorption of chloride accompanied by sodium in this segment {see Kokko, 1974). The continuous transfer of sodium and chloride from ascending limbs to interstitial fluid surrounding the parallel permeable structures results in a continous increase in the osmotic pressure within these struc-tures, which reaches its peak at the apex of the papilla. Autor of the post: Undefined Those nephrons having juxtamedullary glo-meruli Post Date: Mon, 13 Oct 2008 5:48:39 +0000 Thus, tubular fluid of very high sodium content and osmotic pressure reaches the bend of the loop of Henle. Then, as sodium reabsorption from the ascending limb proceeds, the tubular fluid again becomes progressively more dilute {see Figure 40-2).Those nephrons having juxtamedullary glo-meruli have long loops of Henle that approach the tip of the papilla and are responsible for the countercurrent multiplication of osmotic pres-sure in the medulla and papilla. 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