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It has now been shown Post Date: Fri, 17 Oct 2008 6:16:17 +0000 In addition, there are related but separate mechanisms for the reabsorption of calcium, magnesium, phosphate, and sulfate; in the glomerular filtrate these ions are present in low concentrations, and diuretic drugs may influence their transport. It was previously thought that primarily sodium was actively reabsorbed against an electrochemical gradient throughout the nephron, accompanied by the back diffusion of an equivalent amount of fixed anion, mostly chloride. It has now been shown that in the ascending limb of the loop of Henle it is the chloride anion that is transported actively. Autor of the post: Undefined Nevertheless, in terms Post Date: Fri, 17 Oct 2008 6:02:08 +0000 The secretion of hydrogen and potassium ions has often been described in terms of a tubular-cell exchange mechanism by which the reabsorption of sodium is visualized as being coupled with the transport of either hydrogen or potassium in the opposite direction. Newer studies, however, cast doubt on the validity of an exchange reaction as an actual cellular mechanism and emphasize that reabsorption occurs by one process and secretion by another. Nevertheless, in terms of the overall events, significant cation secretion (either H+ or K+) must be accompanied by Na+ reabsorption in order to preserve cation-anion balance. Autor of the post: Undefined All but a very small fraction Post Date: Fri, 17 Oct 2008 5:50:52 +0000 From this point of view, therefore, it may still be proper to consider the secretion of either H+ or K+ as quantitatively equivalent to a component in the reabsorption of Na+. It is for this reason that it is at times convenient to refer to exchange reactions involving Na+-K+ and Na+-H+.All but a very small fraction of the filtered sodium and water is normally reabsorbed, sodium by active transport and water by diffusion. Autor of the post: Undefined As indicated in Figure VIII-1 Post Date: Fri, 17 Oct 2008 5:36:24 +0000 On the basis of the quantitative relationship between these two processes, it is possible to identify three segments of the nephron on both an anatomical and functional basis. These are intimately related to the processes of concentration and dilution of the urine. As indicated in Figure VIII-1, the reabsorption of sodium and chloride, the major solutes of the glomerular filtrate, can be considered in terms of three different mechanisms: 1-A, 1-B, and 1~C. Autor of the post: Undefined Schematic representation Post Date: Fri, 17 Oct 2008 5:22:27 +0000 There are at least three criteria by which each of these can be distinguished: anatomical localization, concomitant water permeability, and sensitivity to diuretic drugs. The reabsorptive mechanism labeled 1-A is in the proximal convolution; 1~B is in the ascending limb of the loop of Henle. Schematic representation of the major electrolyte transport systems in the kidney that are susceptible to drug action. Autor of the post: Undefined Interstitial osmolality is given Post Date: Fri, 17 Oct 2008 5:08:53 +0000 Numbers coincide with the description in the text.The functional organization of the nephron in relation to the countercurrent system is depicted on the vertical axis to show interstitial osmolality (isosmotic cortex; hypertonic medulla), and also on the horizontal axis to show the water permeability characteristics of the nephron segments. Interstitial osmolality is given in milliosmoles per kilogram of water. Autor of the post: Undefined In zone II, chloride Post Date: Fri, 17 Oct 2008 4:52:45 +0000 In zone I, osmolality of tubular fluid (TF) is approximately equal to that of interstitial fluid (ISF), both being isosmotic to plasma in the cortex but increasingly hypertonic to it in the medulla. The blood vessels are also isosmotic to the adjacent ISF. In zone II, chloride and sodium reabsorptive transport is relatively greater than outward diffusion of water. Autor of the post: Undefined Three types of tubular epithelium Post Date: Fri, 17 Oct 2008 4:35:40 +0000 Hence, osmolality of TF is less than that of adjacent ISF. In zone III, water permeability is under the control of the antidiuretic hormone (ADH). Three types of tubular epithelium are schematically shown to indicate differences in water permeability; the associated mechanisms of sodium reabsorption are indicated by 1-A and l-C, and of chloride reabsorption by 1-Á. Autor of the post: Undefined For a full explanation Post Date: Fri, 17 Oct 2008 4:22:26 +0000 The beginning and the end of epithelium in zone II (mostly the ascending limb of the loop of Henle) are left indeterminate because of uncertainty concerning exact functional boundaries.Mechanisms are also shown for acidification of the urine (2) and secretion of potassium (3). For a full explanation of these tubular functions, see text. Autor of the post: Undefined In the ascending limb (1-B) Post Date: Fri, 17 Oct 2008 4:10:00 +0000 is in the distal nephron, in both the distal convolution and the collecting duct. At 1-A, there is a high water permeability so that, as solute (Na+ÑÃ") is reabsorbed, water diffuses along the osmotic gradient with such rapidity that the tubular fluid and the adjacent peritubular or interstitial fluid maintain approximately the same osmolality. In the ascending limb (1-B) the tubule is relatively impermeable to water despite the active reabsorption of chloride. 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